absolute intensity measurements in the extreme ultraviolet spectrum of solar radiation

A B S O L U T E I N T E N S I T Y M E A S U R E M E N T S

I N T H E E X T R E M E U L T R A V I O L E T S P E C T R U M O F

S O L A R R A D I A T I O N

H. E. H I N T E R E G G E R

Air Force Cambridge Research Laboratories, BedJbrd, ~VIassachusetts, U.S.A.

(Received January 15, 1965)

Abstract. Experimental results and problems of absolute intensity measurements of solar electromagnetic radiation in the extreme ultraviolet (EUV) and soft X-ray region of the spectrum (designated cumulatively as "XUV" for brevity) are reviewed. The numerous practical problems are divided in two major areas, (a) general problems of heterochromatic absolute XUV spectrophotometry in the laboratory and (b) specific problems characteristic of requirements of solar physics, the physics of planetary atmospheres, and existing restrictions of space technology. Within the first area (a) emphasis is placed on recent progress toward justified reliance on ionization detectors without necessary connection to source standards. For the second area (b), emphasis is placed on the immediate need to have existing exploratory observations followed by a new phase of more systematic experiments of increased accuracy.

1. Methods of XUV Speetrophotometry

1.1 ABSOLUTE DETECTOR CALIBRATION

Measurements o f radiation intensities are most often carried out with so-called

" seconda ry" detectors. This designation indicates that the conversion of the actual

detector response into absolute values o f intensity requires a calibration based on

either absolute source standards or absolute detection standards.

The ideal method of "absolu te" detection would be represented by calorinmtric

measurements of the heat produced by complete absorpt ion o f the radiant energy

under conditions where all forms of non-thermal energies resulting f rom the pho ton

absorpt ion may be neglected. Unfor tunate ly the experimental realization of this

theoretically most attractive method is extremely difficult. It is practicable only for

source intensities much higher than those usually available even for work in the

visible or near ultraviolet region of the spectrum. Therefore, it is not surprising that in

practical work the absolute calibration o f detectors is usually based on other methods.

Thermoelectric detectors (see Section 1.3) have been used primarily to establish

absolute sensitivities for secondary detectors such as photographic emulsions or

photoelectric devices. In this connection thermoelectric detectors are often called

" p r i m a r y " or "absolu te" detectors. However, the expression "absolute" refers es-

sentially only to the wavelength-independence o f the detector 's response per unit radiant energy received. The actual value of the absolute sensitivity in terms of micro-

volts per microwatt , on the other hand, is usually not based on any absolute detection s tandard but is determined by a calibration against some standard lamp.

Since thermoelectric detection techniques are practicable only for relatively in-

tense radiat ion fluxes, the actual intensity measurements in most o f the existing

Space Science Reviews 4 (1965) 461-497; �9 D. Reidel Publishing Compalo,, Dordrecht-Holland

462 H . E . HINTEREGGER

laboratory XUV work as well as XUV measurements from space vehicles have been carried out with other more sensitive and more convenient detectors. The most popular types have been the following: photographic emulsions; sealed-off photomultipliers requiring conversion of XUV radiation into fluorescent light of longer wavelengths; direct photoelectric emission detectors requiring the use of open structures of photo- diodes or multipliers; photoionization detectors, such as ion chambers, proportional counters and GM counters (sealed-off or flow-type) with various types of windows; or devices using differentially pumped small aperture slits instead of material windows.

Only certain types of photoionization detectors have been qualified per se to provide truly absolute data on intensities (see Section 1.4). However, for most detectors the absolute calibration factors, if determined experimentally at all, have been based mainly on various comparisons with some thermoelectric "primary" detector which in turn has been calibrated against some standard lamp (see Section 1.3). Even these experimental comparisons were carried out only rarely and were not made at all for the shorter wavelengths of the XUV region. For instance, the tungsten targets used as reference detectors in the calibration of solar EUV spectrophotometers (HINTEREGGER, 1961 a) had been previously calibrated against thermoelectric detectors only for wavelengths down to 584 A (WATANABE, MAR~IO and INN, 1953; HINTER- EGGER and WATANABE, 1953; HINTEREGGER, 1962) and similar photoelectric yield measurements by WEISSLER (1956) and his associates were not carried to wavelengths below 300/~.

Since photoionization techniques did not appear to qualify as calibration standards for absolute intensity measurements in the 60-1000 ~ region until most recently (see Section 1.4), it is not surprising that absolute solar XUV flux data published so far are the least reliable for the region of wavelengths ranging from 300/~ down to about 60 ~ or even 20/k. At still shorter wavelengths, reasonably accurate absolute calibrations had again been obtained on the basis of well-established knowledge of the ionizing action of X-rays.

Both thermoelectric measurements and photoionization techniques have been and still are of outstanding importance in the experimental determination of absolute data on XUV intensities. This is true for most of the past work as well as contemporary research and it will probably remain true for much future work. Therefore, these two techniques are discussed separately in more detail in Sections 1.3 and 1.4 of this review.

1.2 ABSOLUTE SPECTROPHOTOMETER CALIBRATION

The absolute calibration of an XUV spectrophotometer may be accomplished either by (a) certain procedures based ultimately on the calibration of suitable detectors (see Section 1.3); or by (b) exposing the entire instrument to standard sources.

The practical realization of the first method generally represents a far more complicated problem than that of merely calibrating one or more detectors. The second method, on the other hand, is practicable only if standard XUV sources of the required properties are indeed available. Even where a sufficiently powerful

ABSOLUTE INTENSITY MEASUREMENTS 463

source of known absolute intensity distribution, e.g. a well-understood synchrotron radiation source (ScHWlNGER, 1949; TOMBOULTAN and HARTIVlAN, 1956), can be made available for the spectral range of interest, a reliable absolute calibration of a given XUV spectrophotometer for a given type of research objective would still remain a fairly complex experimental task. The reasons for this are the following:

The practical performance of XUV spectrophotometer calibrations requires a consideration of numerous details. These details depend on the actual research objective, design of the spectrophotometer, specific properties of its major com- ponents, and characteristics of the available laboratory source standards. In general, the main complications may be listed as follows: scattered light; other spectral impurities or overlapping orders of diffracted radiation in the spectrophotometer or the laboratory standard (which itself may also consist of a grating spectrometer); geometrical conditions of the entrance beam, range of intensity levels, and different degrees of polarization of the radiation for the calibration and for solar observations, respectively. Further complications may arise from temporal variations and differences in characteristic time constants (signal variations, instrumental response times, duty cycles, scanning speeds, fatigue effects, intermittency effects). If significant operational electronic noise or environmental disturbances exist and if their levels occurring during the calibration are different from those during subsequent application of the instrument, such differences may also deteriorate the accurate applicability of absolute calibration factors. Therefore, the absolute calibration of different types of XUV spectrophotometers appears to demand different sets of experimental procedures. Their choice not only depends on the specific research application but also may be restricted inadvertently by the specific construction of a particular instrument.

Whenever an XUV spectrophotometer is designed for applications requiring absolute flux measurements, the design-criteria and specifications of construction should therefore include a specific plan of anticipated procedures of absolute calibration. The inclusion of special provisions aiding calibration becomes extremely important as one progresses from exploratory research satisfied by relatively crude measurements to more refined studies demanding much better accuracies.

The practical resolution of certain conflicts between absolute calibration requirements and all other requirements including various non-scientific ones is known to be rather difficult even for the design of laboratory XUV spectrophotometers. These difficulties are augmented substantially, if an instrument has to be designed for operation from space vehicles. Besides the obvious difficulties due to limitations in volume, weight, and power, the most serious problems are probably those of re- calibration and appropriate re-adjustments of satellite instruments after launch. It should be noted, however, that even the procedures of the pre-launch calibration of solar XUV spectrophotometers practiced so far leave much to be desired.

1.3 THERMOELECTRIC DETECTION STANDARDS

If the radiation target of a thermoelectric detector is sufficiently "black" over the

464 ~. E. HINTEREGGER

entire range of wavelengths extending from the near infrared into the XUV region, the absolute calibration factor (microwatts/microvolt) can be determined from an exposure to a known flux of radiant energy from any standard lamp regardless of its spectral distribution. One may then use a lamp radiating mainly in the visible part of the spectrum to calibrate such a detector for use in the XUV region. This possibility is very convenient, since absolute XUV source standards have not yet been developed satisfactorily.

The actual calibration of thermoelectric detectors for the XUV region is usually accomplished somewhat indirectly because the given data of radiant energy output of a standard lamp for the visible spectrum are applicable accurately only for well controlled atmospheric conditions, whereas measurements in the XUV region usually require operation of the detector in a vacuum. A reliable overall calibration may therefore have to consist of the following three phases of experiments: (a) calibration against a standard lamp at atmospheric pressure; (b) careful determination of the ratio of thermoelectric sensitivity in vacuum relative to that under standard atmospheric conditions; (c) experimental determination of certain recognized corrections which may be necessary to account for expected departures from the ideal 100~ blackness of the target. The latter should be obtained not only for the radiation from the standard lamp but of course also for the XUV region.

The accuracy obtained for the absolute calibration factors of thermoelectric detectors for the XUV region is usually no better than about 20 ~o in spite ot following rather laborious procedures. Even if one tries to correct for all possible experimental deviations from ideal conditions, a reduction of the overall error below about 5 ~ o seems to be hardly attainable. For instance, if one wishes to obtain accuracies ap- proaching 5 o~, departures from ideal target blackness in the XUV alone are generally not negligible.

Measurements of such departures from ideal target blackness have been carried out recently by JO~NSVON and MADDrN (1964) and by SAMSON (1964). Their results show that a loss in the effective thermal conversion efficiency of targets of gold-black of about 2-3 o~ around 584 A is due to the kinetic energy carried away by the emitted photo-electrons. This loss may be prevented, however, by forcing the emitted photo- electrons to re-enter the target. Another important loss appears to be that due to diffuse scattering of the incident radiation. JOHNSTON and MADDEN (1964) have found that this loss integrated over the solid angle 2rr may range from 2-7 o~ even though the specular reflection loss was found to be indeed as small as expected, i.e. much smaller than 1%. Since the sum of these two losses alone, ifleR uncorrected, may produce systematic errors up to about 10~ it is clear that the achievement of an overall accuracy of better than 10~ represents a truly difficult and laborious experimental task.

1.4 PHOTOIONIZATION DETECTION STANDARDS

The ionizing action of X-rays passing through a gas has been used as the basic mechanism for practical measurements of X-ray dose and intensity (dose rate) for a


long time. The qualification of this method for quantitative measurements of absolute intensities has been verified most satisfactorily for the X-ray region of wavelengths up to about 10/~. These photoionization techniques allow a quantitative conversion of detector response into absolute intensity based on the following aspects of well- established knowledge of photoionization by X-rays:

(a) The average number of ion-electron pairs formed per unit number of photons absorbed in a gas is directly proportional to the photon energy;

(b) The actual value of this proportionality factor, e.g. one ion pair per 30 electron volts for argon (and not too different for other gases), is known reliably from numerous experiments;

(c) The fractional number of photon absorption processes not leading to any ion formation (i.e. processes including scattering) is practically negligible.

Since the detector gas is usually exposed to the incident radiation through some material window such as a metal foil, the spectral transmission characteristics of the whTdow must also be known before the absolute calibration factor, at least that for nearly monochromatic radiation, can be determined from the basic aspects (a), (b) and (c) above.

The absolute energyflux calibration factor of an ionization detector, defined as the incident X-ray power per unit signal response, may be expressed in units of incident X-ray power per unit saturation current, e.g. 60 watts/ampere if one assumes 50 window transmission and complete absorption within the sensitive volume of gas in an ion chamber or GM counter.

The absolute photon flux calibration factor, defined as the number of incident photons per unit detector response, is connected with the absolute intensity calibration factor in a unique relation only for nearly monochromatic radiation of known wavelength. For heterochromatic X-rays the quantitative relationship between the calibration fiactors for energy and photon fluxes respectively can be determined only if the shape of spectral distribution of the incident X-ray flux, at least that within the spectral sensitivity limits of the detector, is known too. The physical meaning of the reciprocal value of the absolute photon flux calibration factor is simply that of a photon comTth~g effieieno,.

The techniques of X-ray photometry of the type practiced for measurements of solar radiation and the appropriate use of filters have been reviewed recently (FRIED- MAY, 1963). Therefore, it is sufficient here to summarize these developments by saying that the techniques of absolute photometry of X-rays have been established satisfactorily for some time. This is true at least for monochromatic radiation and for wavelengths up to 10 • or, with somewhat poorer accuracy, even up to considerably longer wavelengths, say 60 ~.

Until recently, ionization detection techniques of intensity measurements in the important region from about 60 ~ to 1000 A did not appear to qualify as truly "absolute". The use of thermoelectric detection standards, on the other hand, was known to be extremely cumbersome and increasingly difficult below 500 A (see Section 1.3). Therefore, improved techniques were needed particularly for the wave

466 H.E. HINTEREGGER

length range from about 60 A to 500 A. It is most gratifying that substantial progress in this direction is now being achieved (EDERER and TOMBOULIAN, 1964; SA~SON, 1964). The most recent results of detailed experimental studies of the photoionization of the rare gases and the advanced theoretical evaluation using improved compu- tational techniques are clearly converging toward a most valuable improvement in the state of the art of absolute XUV photometry.

In general, some of the simple rules of ionization of gases by X-rays, designated as (a), (b) and (c) above, become questionable as one goes from hard X-rays to soft X-rays and EUV radiation of much longer wavelengths. However, if the ionization detection is based on the use of a rare gas in the spectral region of its ionization continuum, it is generally well justified to assume that the most important rule (c) above is still applicable. However, the more specific rules (a) and (b) can be retained, only as approximations. This means that there is no generally accurate formula for the wavelength-dependence of the average measured number of ion-electron pairs per unit number of EUV photons absorbed by or incident upon the ionization detector. Therefore, the technique of measuring the saturated ion current leads to accurate absolute intensity data for the EUV region of the spectrum only if one acquires additional quantitative knowledge of the fractional probabilities of all types of multiple ion production per photon absorbed in the ion chamber.

Fortunately the need for this additional knowledge can be avoided by the adoption of GM counting techniques if the latter fulfil the basic condition of unity count rate per unit photon absorption rate regardless of wavelength and if practically all photons transmitted through the window (of known transmission) are absorbed within the sensitive volume of the counter. For wavelengths longer than about 300 ~ the use of windowless or special thin-windowed ion chambers with helium, neon, argon or xenon as detector gas appears to represent the simplest technique because conditions can be chosen so that the effective ionization effciency is practically equal to unity.

Although much work toward improved techniques of XUV photoionization detection remains to be done, the following conclusions seem justified:

Ionization detection techniques have now reached a state of development where measurements of this type may give absolute intensities with better accuracy than that attainable from the most painstaking experimental work with standard lamps and thermoelectric devices (see Section 1.3).

From a comparison of the two approaches, the designation "absolute" detector seems to be more truly justified for ionization detectors.

The determination of absolute calibration factors for appropriate ionization detectors can be based on the realization of certahz absolute detection principles without any reference to a standard lamp, whereas the same is not true for thermoelectric detectors.

Therefore, certain appropriate methods of ionization detection deserve to be placed in the same general category with the calorimetric method (see Section 1.1). All other methods are characterized by the need to rely on absolute source standards. Since exclusive reliance on the latter presents considerable difficulties, it is perhaps


advisable to define a second major category of XUV photometry somewhat less

rigorously, i.e. as a mixed type. This mixed type of XUVphotometry may then refer

to all methods for which the availability of some standard source of known radiant

energy output is a necessary but generally not a sufficient basis of the overall calibration. A typical example of such a method is the use of thermoelectric detection

standards as outlined in Section 1.3.

PHOTON ENERGY (eV)- - *

,~ ,~ ,~ ~0 o . . . . . ?,,-,~i~,,,,? ? ? j 7C / -4

. . . . .

F-- ~ O Z Z

12,2 0 (D

. . . . . i

w ~p i~ ~ 0 0 z CO

- ~ F- -

13213- ,~.: Oz

7 9 U ,90 c o 43

'~-- WAVE LENGTH (A)

Fig. 1. Photoionization crosssections of the rare gases which are suitable for ion chamber measurements of absolute EUV radiation intensities (SAMSON and KELLY, 1964).

12,0

IOO

8.0

e- (Mb) 6 0

4.0

2 0

0 leO 200

PHOTOIONIZATION CROSS SECTION OF NEON

.~ , . ~l : - "~ . '~ ' - " .... "-'-----~. ~ Present data fr~/ . . . . . : . .

- - ~ ~ �9 L2,.~

..i. / /J ~ '~ /m / ~ Ditchburn

/ / t ,~

, , , : " /a

i . f , , . ~ / D e r s h e m e, Sehelrn

I , I , r , I , I ,

500 400 500 X (A)

I

600

Fig. 2. Comparison of recent measurements of photoionization cross sections of Ne, marked as "Present data", with results of earlier investigation. The accuracy of the new measurements is

about 5 % (from EDERER and TOMBOUL~AN, 1964).


The spectral ranges over which different rare gases may be used for absolute EUV intensity detection with suitable ion chambers are summarized in Figure 1, which has been reproduced from a report by SAMSON and KELLY (1964). Figure 2 shows the data on cross-sections of XUV photoionization of neon measured with an accuracy of about 5 o/ ~o over the wide range of wavelengths from 575 A down to 80 ~ by EDERER and TOMBOULIAN (marked "Present Data") together with results of earlier investigations (Figure 2 has been reproduced from the paper by EDERER and TOM- BOUL[AN, 1964).

1.5 SOURCE STANDARDS

A standard source is supposed to generate radiation of certain qualitative and quantitative properties. These properties must be reproducible and are supposed to be known with a certain accuracy whenever the source is operated within specified tolerances of certain experimental conditions and whenever certain procedures of measurement and evaluation are followed.

Unfortunately, standard sources of XUV radiation of similarly well-established properties as those of standard lamps for the visible or near infrared are nonexistent. At present, one should therefore not insist on any rigorous definition of the meaning of the expression "standard" in reference to various existing types of laboratory sources used in spectrophotometric calibration work in the XUV region. The same is true, of course, for designations such as "absolute", "primary" or "secondary" used occasionally in reference to various experimental XUV sources. Within the next few years of further experimental developments, however, it will be desirable to adopt a more rigorously defined terminology.

A general review of the various types of existing laboratory XUV sources is not within the scope of this paper. Therefore, the present section will merely highlight the most promising approach of work toward the establishment of an absolute XUV source. The availability of such a source seems to be most desirable for the wavelength range extending from about 50 ~ to 500 A (or 1000 A). For wavelengths above 500 A and below 50 A, the development of absolute source standards for photometer calibrations is probably less important because of the established reliance on thermoelectric detection standards and ionization detection standards respectively.

It is obviously of great fundamental importance to establish radiation source standards of the type for which the spectral intensity distribution and other source characteristics may be determined accurately on the basis of well-established theory. The use of ordinary blackbody radiation as an ideal absolute source standard is not practicable, since the attainable intensity in the XUV region would be unacceptably low. Searching for other sources of similar fundamental character, i.e. sources accurately describable from first principles, one appears to be forced to the conclusion that nearly all existing sources such as spark sources, gaseous discharge sources (including hollow cathode sources, condensed discharges through capillaries, gas discharges in a Penning tube, etc.) and conventional X-ray sources do not qualify for this category. The only XUV source which is tractable theoretically without too

ABSOLUTE INTENSITY MEASUREIvlENTS 469

many reservations and at the same time can deliver the necessary radiant power over the entire wavelength range of interest appears to be the electron synchrotron.

A detailed study of this "classical acceleration radiation" for a 300 MeV synchrotron has been carried out at Cornell University (To~BoUUAN and HARTMAN, 1956). This experimental study not only confirmed the general results of the theoretical treatment of the radiation problem (ScHx~qNGER, 1949), but also laid the foundation for well-justified hopes that this synchrotron radiation may serve eventually as an "absolute" standard for the entire region of the spectrum extending from the visible through the near and far ultraviolet into the X-ray region. At the same time it was clear, however, that various instrumental and procedural difficulties would have to be surmounted before this most desirable application could become a reality.

Following the first experiments with the Cornell synchrotron, efforts of further exploration remained largely restricted to essentially theoretical computations of the detailed characteristics of the radiation emitted by electrons accelerated in a synchrotron. These studies greatly clarified details of the spectral characteristics (To~BoUUAN and BEDO, 1958) as well as the angular distribution and polarization (BEDO, TO~X- BOULIAN and REGERT, 1960). Experimental work was apparently not resumed until most recently. The lack of activity in this direction was probably caused by the fact that electron synchrotrons had been built to serve research objectives other than XUV spectroscopy. To the author's best knowledge, the only machine in the United States officially assigned to XUV work, at least for a part of its operating time, appears to be the 180 MeV synchrotron at the National Bureau of Standards in Washington. This recent resumption of experimental XUV research with a synchrotron as the radiation source has already led to most significant scientific contributions (MADDEN and CODLING, 1963 and 1964; CODLING and MADDEN, 1964). In these studies, however, the synchrotron was treated mainly as a convenient source of an otherwise unattainable XUV radiation continuum without any specific orientation toward the application of this source for future absolute intensity calibration.

Therefore, most of the present knowledge of the potential applicability of electron synchrotrons as standard sources of XUV radiation is still based only on the work of TOMBOULIAN and his associates. An extensive revitalization of similar work with synchrotrons appears to be most desirable. Until results of such work become available, the merits and disadvantages of using a synchrotron as a standard XUV source can probably not be evaluated in the form of any definitive judgment.

The successful adaptation of one of the existing electron synchrotrons as a primary source standard would certainly not eliminate the future need for secondary standard sources and various types of other sources of XUV radiation for the practical pursuit of experimental XUV research in many different laboratories. At least for the present time, absolute calibrations of these laboratory sources, if accomplished at all, are based essentially on the thermoelectric method or the ionization method discussed in the Sections 1.3 and 1.4 above.

Since the presently established experimental procedures of determining absolute intensity calibrations for either XUV sources or XUV detectors are generally saris-

470 H.E. HINrEREGGER

factory only within certain limited ranges of wavelengths and since suitable electron synchrotrons will hardly become routinely available as XUV source standards for many laboratories even in the future, one can hardly overrate the importance of theoretical and experimental studies of methods yielding at least the ratio of absolute intensities at two widely separated wavelengths with the desired degree of precision. The perfection of methods of reliable ratio-determinations, e.g. the "branching ratio technique" (GRIFFIN and McWHIRTER, 1962; H~NNOV and ]7IoFMANN, 1963), and various practicable improvements of absolute source and detector standardization over certain limited parts of the spectrum, combined, represent a most promising approach to the overall problem of absolute intensity measurements throughout the XUV region of the electromagnetic spectrum.

Whereas ordinary sources of blackbody radiation appear to be quite impractical for wavelengths much shorter than about 2000 A_, BOLOT (1961) has demonstrated the practicability of a method producing a quasi-thermal radiation spectrum at least within a limited range of wavelengths around the center of certain resonance lines such as hydrogen Lyman alpha excited in a well thermalized argon arc operated under appropriate experimental conditions.

This brief review on XUV source standards is concluded with the hopeful outlook of expecting significant improvements in the state of the art to be accomplished within the next few years.

2. Existing Data on Solar XUV Intensities

2.1 BRIEF HISTORY

The history of solar XUV spectroscopy and spectrophotometry had its beginning in 1946 when the first direct observations above the most strongly absorbing part of the earth's atmosphere were made. The first observations were accomplished with a spectrograph mounted without any solar pointing control atop a V-2 rocket over White Sands in New Mexico. The progress from this early beginning to the more recent developments of spectrographs and telemetering XUV monochromators mounted on improved rockets and orbiting space vehicles has been discussed in various general review articles (FRIEDMAN, 1961 and 1963; TOUSEY, 1961 and 1963; RENSE, 1961 ; HINT~REGG~R, 1961a and 1964). Since these published reviews describe the general scientific and technological situation essentially up to date, the addition of another general review in the literature is probably not warranted at this time. On the other hand, since much work has been done and more work is currently planned to be undertaken by various growing groups of researchers interested in many different objectives, a more detailed survey of the specific topic of the absolute intensity distribution in the solar XUV spectrum is probably in order.

The emphasis on the specific subject area of absolute intensity measurements in the present review is justified firstly, because its thorough consideration is mandatory for the effective pursuit of many different specialty-fields of science, and secondly, because there might be a regrettable though understandable tendency to neglect this


topic and to consider much of the work in this field as a rather unattractive distraction from work in other fields which may qualify as clearly defined scientific disciplines offering nmch greater incentives for most scientists in academic careers.

Information on absolute intensities of solar emissions in the XUV region of the spectrum has been acquired with the aid of experimental techniques which may be divided in two major groups, namely

(a) Dispersive techniques including conventional photographic spectrophotometry as well as spectrophotometry conducted with the aid of photoelectric and photoionization detectors and

(b) Non-dispersive techniques comprising the use of various types of ionization techniques and appropriately selected filters as well as some exploratory measurements based on photoelectron retardation analysis.

The most significant progress of measurements with dispersive techniques began only following the development of an appropriate biaxial solar point#lg control. This pointing control was designed and constructed at the University of Colorado and was first flown successfully in 1952 by Rense and Pietenpol (RENSE, 1953). Biaxial pointing controls of similar construction built at the University of Colorado and later also at Ball Brothers Research Company, have been instrumental in most of the solar XUV radiation measurements with spectrographs and monochromators conducted so far.

Measurements based on the use of non-dispersive techniques have a much longer history. This is understandable, since these measurements in general do not require solar po#~ting of the equipment. An exception exists, of course, for X-ray measurements requiring solar image formation (e.g. see FRIEDMAN, 1961, 1963).

Non-dispersive measurements conducted rather simply and inexpensively have led to a wealth of new information ever since they were introduced well over a decade ago by the pioneer work of Friedman's group of the Naval Research Labora- tory. The use of non-dispersive XUV measurements in contemporary XUV research is still most fruitful. Such measurements have been providing more or less continuous monitoring not only of the solar hydrogen Lyman alpha intensity but also of various fluxes contained in several wavelength bands of the solar X-ray spectrum. Perhaps the greatest merit is the fact that these observations have been carried out under many different types of conditions of solar activity. In spite of the great fundamental importance of these data for solar physics, it seems appropriate to restrict the present review to a consideration of solar XUV spectrophotometry by means of dispersive techniques of spectral resolution.

Dispersive measurements of absolute intensities in the solar XUV spectrum accomplished so far will be discussed in detail in the Sections 2.3-2.5 in chronological order. The history of these measurements is still characterized mainly by the following aspects :

(a) The actual beginning of these measurements is marked by rather recent dates in general.

(b) For the shorter wavelengths, say below 300 A, as well as for any continuous

472 ~.E. HINTEREGGER

monitoring of temporal variations, it is indeed too early to speak of any history at all. These facts will continue to represent a most serious deficiency in the body oL

experimental knowledge on solar XUV radiation for many years, say at least until the next solar maximum.

2.2 NOTES ON TECHNIQUES

Since this review is intended to emphasize the scientific aspects of the subject, a review of technological developments and instrumental details is not attempted here except in the form of a brief summary including essential references, some more detailed notes on specific aspects not covered in the literature, and comments on aspects treated in the literature but requiring reexamination. The latter will be rnainly contained in the concluding part of this review. As indicated in Section 2.1 above, the present summary will cover dispersive techniques of solar XUV spectrophotometry only.

The use of photographic techniques for measurements of solar XUV radiation has been discussed in sufficient detail in recent review articles (e.g. TousgY, 1963; EDLEN, 1963). Therefore, we summarize here only what appears to be a general con- sensus of opinions as follows: ~ (a) In spite of considerable progress in photoelectric recording techniques, the photographic plate appears to remain superior for any precise wavelength measure- merits as well as for well-defined and simple image formation such as that required in XUV spectroheliographs;

(b) For the determination of absolute XUV photon fluxes or intensities, however, the use of photographic techniques has been largely abandoned in favor of adopting modern photoelectric techniques for XUV detection. Photoelectric detectors have a far greater dynamic range of linear response, a most valuable insensitivity to radiation of longer wavelengths, and generally allow more precise intensity measurements.

The use of photoelectric detection in XUV radiation measurements from space vehicles also has been covered in recent reviews (FRIEDMAN, 1961 and 1963; HINTER- E6GER, 1961a; TOUSEY, 1963). General distinctions of photoelectric detection tech. niques in comparison with photographic recording already are reflected in the conclusions (a) and (b) above. It should be noted, however, that photographic techniques are not practicable for XUV experiments on orbiting space vehicles unless one provides for either recovery or on-board processing of the exposed photographs and protection for stock of film.

Neither photographic emulsions nor photoelectric XUV sensors such as open structure photomultipliers are usually qualified as primary standards for absolute XUV flux measurements. However, the reproducibility of the absolute spectral sensitivities of these convenience detectors has been found to be adequate at least within certain modest limits of required accuracy.

In many instances it is very difficult to obtain an accurate determination of the maximum possible total error which comprises departures from reproducibility as well as errors in the actual calibration of the convenience detector against some


suitable standard. Overall accuracies better than about 20 ~ have been claimed only rarely and most of the accuracies quoted in the literature reflect usually the experi- menter's estimation, of a somewhat intuitive nature, rather than the result of detailed error calculations which are often nearly impracticable.

A third group of convenience detectors of solar XUV radiation is represented by the family of gaseous photoionization detectors. The best known ones among these are the nitric oxide filled ion chambers with lithium fluoride windows (used extensively for measurements of the solar hydrogen Lyman alpha flux) and various types of X-ray ion chambers and X-ray photon counters (used equally extensively for measurements of solar X-ray fluxes ever since these detectors were first introduced by Friedman and his collaborators (e.g. see FRIEDMAN, 1961, 1963). Some of the detectors within this third group may indeed qualify not only as convenient detectors but, within a modest accuracy, as the actual absolute detection standard (see Section 1.4). In spite of this most favorable aspect, photoionization detectors have not been used in conjunction with dispersive techniques of solar XUV spectroscopy so far. One reason for the actual choice of photoelectric detectors was the desire to avoid gas-filled devices with thin windows whenever other devices may be used. Not all reasons for this choice are clearly justified, however, and certainly not for all the regions of the XUV spectrum. It would be quite wrong at present to claim the past choice as a permanent one for the future.

For the period up to mid-1960, the technological and instrumental aspects of the problem of telemetering monochromator measurements of solar XUV radiation have been reviewed in considerable detail in Part I of the author's article in Space Astrophysics (HINTEREGGER, 1961a). During the four years passed since then, the development of the experimental program has provided many additional valuable results. On the other hand, this development has apparently not uncovered any basically new technological aspects. Therefore, the appraisal of the most important experimental problems as given in the 1961 article can be repeated now in essentially unchanged form but perhaps with a more securely established emphasis.

The instrumental calibration factors, required to convert the signals received fi'om a telemetering XUV spectrophotometer into absolute values of solar radiation intensities or photon fluxes at the place of observation, are most conveniently described in terms of suitably defined spectralphoton counting efficiencies. The latter have only crudely the same meaning as the photon counting efficiencies of a detector, because it is generally also necessary to consider the various complicating aspects which have been summarized in Section 1.2. In particular, it is most important to use a geometry of the bundle of incident rays during the calibration with laboratory test sources of XUV radiation which should be sufficiently similar to the conditions during the actual solar measurements aloft the space vehicle.

The most reliable experimental determination of absolute calibration factors for solar spectrophotometers appears to be that based on the exposure of the rocket instrument to suflTeiently monochromatic test radiation fluxes, qiT(2v; A). T ~A2), to be produced by a suitable arrangement of laboratory XUV sources and monochromators

474 I-I. E. HINTEREGGER

and to be measured by a reference detector, i.e. one of known photon counting efficiency, ~ID(2T). This reference detector is best mounted in a manner allowing its reproducible insertion between the entrance slit (representing a certain area A for radiation acceptance) and the grating of the solar spectrophotometer. The necessary arrangements for a valid calibration also include a suitable type of angular aperture stop or collimator for the monochromatic XUV test radiation in the laboratory to ensure adequate similarity with the actual solar exposure. Since photoelectric spectrophotometers for measurements of solar XUV radiation intensities have been built only for relatively modest resolution, it is not too difficult to approach the condition A2T,~A2 where A,~ designates the instrumental spectral width of the solar spectrophotometer (to be discussed in detail below) and A2 T should be understood as the practically complete spectral width of the nearly monochromatic XUV test radiation produced in the laboratory.

The use of various laboratory XUV sources producing strong and reasonably narrow line emissions without any significant continuous background offers a great convenience, since the condition Aihr~ A2 can then be approached without requiring a laboratory monochromator of very high resolving power. A high instrumental resolving power of the laboratory monochromator would, of course, entail a rather poor luminosity, i.e. a most unattractive aspect for the overall calibration procedure involving two instrumental dispersions in tandem.

With this introduction the spectral photon counting efficiency of a telemetering solar XUV spectrophotometer can be defined formally as

8(~-r) =- Smax().)/A~T(~T) for A), T ~ AJ~ (1)

where A is the instrumental area of radiation collection under conditions of solar pointing. The quantity Aq~r is the suitably coUimated nearly monochromatic test flux [ph sec-~] measured by the reference detector, 2 T is the known wavelength of this test radiation, and Smax is the maximum signal (count rate) observed in the course of a sufficiently slow scan of 2, i.e. the wavelength setting of the solar spectrophotometer. Under the same experimental conditions, an observation of the signal count rate S (2) as a function of instrumental wavelength departures from the peak, 62 ~ 2 - 2T, reveals the #lstrumental window function,

w(62) -= S(62)/Sm~;Sm~ ~ = S(62 = 0) for A2 T < A2 (2)

A knowledge of the shape of this window function, w(62), is required for an experimental determination of the instrumental width A2. The latter has a quantitative meaning only within a certain agreement about the specific percentage of peak signal at the level where A2 is supposed to be measured. The most commonly adopted definition is that of the so-called half width (more precisely designated as full width at the signal level of 50 ,%0 of the maximum) i.e.

for the case of sufficiently monochromatic test radiation, A2T~A2, assumed here.


The experimental procedure required for this determination at the same time checks or even establishes the instrumental wavelength scale by identification of the instrumentally indicated wavelength for the peak with the known test wavelength A T.

Raw data of solar observations in the form of count rates as a function of the wavelength setting, can be translated into absolute spectrophotometric information rigorously only for two limiting cases.

The first and simplest case exists where the actual solar spectrum within the instrumental width A2 consists essentially of an isolated solar line of sufficient intensity, i.e.

X+A2 2+A2

f ~z( l ine)d2> f ~z(other l ines+cont inuum)d2. (4)

2--A2 2--A2

It is of course also assumed here that the actual shape of this solar line of center wavelength 2@ is indeed not resolved by the instrument, i.e. A2@ ~A2. Within these restrictions the absolute photon flux density of sufficiently isolated and strong solar emission lines can be determined reliably from the relation

# ( 2 o , line) = [Ae~T~o] - 1 S . . . . ~,,~,~ ~@ ( 5 )

where A is the area of radiation acceptance of the instrument and Sma x is the observed signal at the peak.

The quantity [Ae]-i represents the calibration factor for the conversion of the observed signal peaks, (e.g. Smax in counts per second) into absolute solar photon flux densities ( [#]= ph cm-2sec - 1). The quantity g is the spectral photon counting efficiency determined fi'om relation (1) for some test wavelength 2 z reasonably close to the solar emission wavelength 2@. If the instrumental efficiency ~(2) is a slowly variable function of the instrumental wavelength setting 2, the use of a reasonably small number ot test wavelengths is sufficient for an adequate calibration.

The second ideal case is approached where the actual solar spectrum within the instrumental width A2 is indeed dominated by a continuum, i.e.

,~ + A). 2 + A 2

f ~z(continuum)d2>> f ~a(lines)d2 (6) -~--A2 2--A2

and where the differential flux distribution of the continuum, d~/d2 = ~ , as well as the instrumental efficiency e are essentially constant over the instrumental width A2. In this case, the signal is given in the form

S = Ae~x S w (fi2) dfi). (7)

where w(fi2) is the instrumental window function (2) as defined above. For an ideally triangular function defined as w~-1-[fi2/B] for [d2[<B and w=-0 for lfi2[ >B, the quantity B is known to be equal to the instrumental half width, A2vwHM.

Within these restrictions the conversion of the observed signals into spectral

476 It. E. HINTEREGGER

(differential) solar continuum flux densities ( [~a]=ph cm-2sec -a A -a) is then accomplished in the form

q~a (continuum) = [AeA,~FWHM ] - 1 N (8)

where the quantity e is still that defined by relation (1) even though the latter has been established for isolated line radiations above.

Since the integral over the instrumental window function appearing in the equation (7) is exactly equal to A)~FWHM not only for triangular shapes of w(d2) but also for square, trapezoidal, and various combination shapes, the simple formula (8) can be applied to nearly all practical measurements, if not exactly, then at least as a good approximation.

An inspection of any recent solar XUV spectrophotometry record (see Sections 2.4, 2.5) shows immediately that neither the ideal case (4) nor the case (6) could be �9 for the entire solar spectrum. The case of relatively strong and well isolated ines dominating the small part of the solar spectrum which falls within the instru-

mental band width A2, however, is obviously much more common. On the other hand, it is equally evident that many sections of the solar XUV spectrum will have to be subjected to more accurate and more highly resolved spectrophotometry than that reflected in the existing data. Therefore it may be necessary in future work to abandon the present experimental emphasis on the conveniently simple case (4) and to bring most efforts to bear on high resolution spectrophotometry to be accomplished with instrumental widths much smaller than the actual widths of most solar XUV emission lines. For a discussion of the existing data of solar XUV spectrophotometry, however, it is certainly appropriate to emphasize the fact that the shapes of lines apparent in various published traces are clearly not solar but instrumental and that integral line intensities usually refer to the observed peaks, S . . . . rather than the area under the curve of signal versus instrumentally indicated wavelength, S(2),

around 2 = 2o. The foregoing notes on the basic aspects of the experimental determination of

absolute intensities in the solar spectrum have been included to serve primarily as a guide to students of various overlapping fields in the space sciences who are interested in the results without being actively engaged in the technological processes of ac- quiring these results. Details about the latter are not within the scope of this review. Readers interested in these aspects may be referred to existing original papers of specialized nature published not only in the literature on space science, astrophysics, and geophysics, but also in journals covering general physics, optics, electronics (incl. solid state), and scientific instrumentation. Various unpublished instrumental details of the AFCRL monochromators developed by the author's group primarily for atmospheric rocket sounding, exploratory solar XUV observations, and solar monitoring experiments now scheduled for the U. S. satellites OSO-C and OGO-CD have been the subject of a comprehensive smnmary, compiled by SULLIVAN et al.

(1964) only in the form of a contractual report, since these details do generally not hold enough interest for formal publication.


Whenever the actual solar spectrum within the instrumental width, A2, contains more than one line or a continuum, except for the special case fulfilling condition (1), the observed spectrum of instrumental count rate versus instrumental wavelength position, S(2), can be translated into quantitative information on the actual solar spectral intensity distribution only approximately and only if a rather complex procedure of data reduction is followed. To make such a reduction possible it is usually necessary to obtain much more laboratory test data besides the afore- mentioned spectral photon counting efficiencies, e(2), and the instrumental width,

d2. Disturbing effects of difJerences & polarization of solar and laboratory test il-

lumination respectively are probably not serious for grating monochromators designed for a sufficiently grazing incidence such as have been used exclusively in the photoelectric spectrophotometry of solar XUV fluxes published so far. To neglect this effect it is apparently not necessary to require that the monochromatic test radiation in the laboratory should be unpolarized or even that the degree of polarization should be known. Of course, this is justified only as long as the rocket instrument itself can be considered as a spectrophotometer that is essentially insensitive to the degree of polarization. The foregoing assertion of polarization-independent response of grazing incidence monochromators is probably only approximately correct and a quantitative experimental determination should be accomplished before claiming accuracies better than those quoted for the existing measurements. The most serious errors would have to be expected, if a rocket instrument with inter- mediate angles of incidence at the grating, say around 45 ~ , were attempted to be calibrated with monochromatic test radiation of an unknown degree of polarization. It is important to note, however, that partially polarized radiation and even most strongly polarized radiation such as that from an electron synchrotron are readily applicable to spectrophotometric calibrations, whenever the latter are actually carried out for various orientations of the rocket instrument's plane of dispersion relative to the plane of polarization of the test radiation (in principle, measurements for two mutually perpendicular directions would be sufficient).

2.3 INTENSITY DATA UP TO 1960

The body of knowledge concerning the absolute intensities in the solar spectrum below 2200 ~ acquired during the period from 1946 to 1958 is based mainly on a considerable amount of measurements with nondispersive techniques (not reviewed here at all) and some limited densitometry of photographic records from a few spectrograph flights. In these early rocket spectrograms, scattered light and other experimental difficulties were still too serious to allow any determination of solar emission intensities for wavelengths below about 1000 fit. It should be noted, however, that spectrophotographic data obtained for the solar vacuum ultraviolet spectrum of longer wavelengths (say above 1000 di and particularly above about 1800 A) were relatively satisfactory for both qualitative and quantitative purposes even before 1958.

478 I-I. E. HINTEREGGER

Absolute intensities based on the evaluation of photographic spectrograms from a rocket flight of August 6, 1957 (ABouo, BEHRING and RINSE, 1959) are reproduced

here as Table I in the same form as given in the review paper by RENS~Z in Space Astrophysics (RENSE, 1961). However, these absolute intensity data, particularly

those for the shorter wavelengths, appear to be accurate only within limits remaining too uncertain for a meaningful quantitative comparison with more recent data. Similar uncertainties probably exist also for much of the intensity data obtained

spectrophotographically from the earlier rocket experiments conducted by the U. S.

TABLE I

SOLAR EMISSION LINE INTENSITIES OF AUGUST 6 7 1957'

Identification Wavelength Intensity Identification Wavelength Intensity (A) (ergs/cmZ/sec) (A) (ergs/cm2/sec)

Sire 1206.5 0.23 Sill 1526.7 0.03 HLa 1215.7 3.43 SiIt 1533.4 0.06 Hell 1215.1 0.17 Civ 1548.2 0.55 SiII 1265.0 0.02 C iv 1550.8 0.3 l O I 1302.2 0.04 C~ 1560.3 0.04 O i 1304.9 0,04 CI 1561.4 0.08 O I 1306.0 0.04 Hen 1640.4 0.27 Sin 1309.3 0.04 C I 1656.3 0.28 CII 1334.5 0.31 CI 1658.1 0.20 CII 1335.7 0.33 Fell 1670.8 0.21 Silv 1393.7 0.24 SiII 1808.0 1.05 SiIv 1402.7 0.17 Sill 1816.0 1.65

* Obtained from photographic records of University of Colorado Spectrograph (RENsE, 1961).

Naval Research Laboratory reviewed by TOUSEY (1961, 1963) and FRIEDMAN (1961, 1963). To appreciate the newness of existing infolmation on the XUV part of the solar spectrum a few remarks about solar spectrophotometry for longer wavelengths

are included below. The first representative determination of absolute intensities of the solar spectrum

from 3400 A to 2200 A (not included under the designation EUV here) obtained by the Naval Research Laboratory group piior even to 1953 certainly represents a remarkable experimental accomplishment (TOusEY, 1953). Subsequent improvements in the accuracy of these early intensity measurements above 2200 A have been relatively minor (JOHNSON, 1956 and 1959; WILSON et al., 1954; MALITSON et al., 1960; DETWlEER et al., 1961). This apparent lack in progress may be due in part to the fact that spectrograph developments and rocket measurements (including the 4000-2000 A region) were oriented primarily toward improved spectral resolution rather than toward improved measurements of absolute intensity.

For wavelengths below 2200 A, the progress in photographic spectrophotometry achieved during the period from 1953 to 1960 by Tousey's group at the Naval Research Laboratory (NRL) appears to be much more impressive. Results of these


NRL studies, published by DETWILER et al. (1961), are reproduced here as Table Ila and Table l ib respectively showing absolute intensities integrated over adjoining 50/k intervals from 2600/k to 850 A (continuum and lines) and the intensities of the strongest lines (for the 1892 A-835 k region). DETWILER e t al. (1961) mention that their values of intensities have been corrected for absorption effects so that they are representative of solar fluxes incident upon the earth's atmosphere. They also note that the intensity o17 5.1 erg cm- 2sec- 1 listed for Lyman alpha does not stem from

T A B L E II

RESULTS OF PHOTOGRAPHIC SOLAR SPECTROPHOFOMETRY"I'~:~

(A) THE INTENSITY OF THE SOLAR SPECTRUM

(CONTINUUM AND LINES), IN 50 X INTERVALS AT A DISTANCE OF 1 AU AND INCLUDING

BOTH CONTINUUM AND LINES FROM 2 6 0 0 ~, TO 8 5 0 ~,

Wavelength Ranges Energy Fhtx Deltsit), [2] = /~ [AIl = erg cm-Zsec -1

2625-2575 7OO 2575-2525 560 2525-2475 380 2475-2425 390 2425-2375 340 2375-2325 320 2325-2275 360 2275-2225 350 2225-2175 310 2175-2125 240 2125-2075 145 2075-2025 90 2025-1975 70 1975-1925 55 1925-1875 41 1875-1825 28 1825-1775 19 1775-1725 12 1725-1675 8.2 1675-1625 5.0 1625-1575 3.2 1575-1525 1.7 1525-1475 0.95 1475-1425 0.50 1425-1375 0.26 1375-1325 0.26 1325-1275 0.18 1275-1225 0.15 1225-1175 5.7 1175-1125 0.08 1125-1075 0.06 1075-1025 0.10 1025-975 0.18

975-925 0.15 925-875 0.25 875-825 0.11

480 rI. E. HINTEREGGER

TABLE II (eonthmed)

(B) INTENSITY AT ONE ASTRONOIvIICAL UNIT PRODUCED BY THE STRONGEST SOLAR EMISSION LINES OBSERVED BELOW 2000

B (]~) Identification erg cm-Zsee -1

1892.03 Sim 0.10 1817.42 * Si i][ 0.45 1808.01 Sili 0.15 1670.81 Altt 0.08 1657.00" CI 0.16 1640.47 He tt 0.07 1561.40 * C t 0.09 1550.77 CIv 0.06 1548.19 CIv 0.11 1533.44 Sin 0.041 1526.70 Sin 0.038 1402.73 Silv 0.013 1393.73 Si Iv 0.030 1335.68 CII 0.050 1334.51 C n 0.050 1306.02 O I 0.025 1304.86 O I 0.020 1302.17 OI 0.013 1265.04 Silt 0.020 1260.66 * Si tt 0.010 1242.78 * N v 0.003 1238.80 N v 0.004 1215.67 HLy-a 5.1 1206.52 Silii 0.030 1175.70 * C nl 0.010 1139.89 * C t 0.003 1085.70' Ni t 0.006 1037.61 * OvI 0.025 1031.91 Ovt 0.020 1025.72 HLy-fl 0.060 991.58 * Ntli 0.010 989.79 * N III 0.006 977.03 C in 0.050 949.74 HLy-d 0.010 937.80 H Ly-e 0.005 835 * On, lit 0.010

* Asterisks indicate blends. ** Results obtained by Tousey's group at the Naval Research Laboratory, as published by DETWlLER et al. (1961).

p h o t o g r a p h i c d e n s i t o m e t r y b u t h a s b e e n i n f e r r e d f r o m ion c h a m b e r m e a s u r e m e n t s

a c t u a l l y r e c o r d i n g 6.0 e rg c m - 2 s e c - 1 , o f w h i c h 0.9 e rg c m - 2 s e c - t was s u b t r a c t e d fo r

t h e w i n g s o f L y m a n a l p h a a n d t he s u m o f o t h e r e m i s s i o n l ines w i t h i n t h e s p e c t r a l

r e s p o n s e r a n g e o f t he i on c h a m b e r , i.e. a n i t r i c ox ide cell r e s p o n d i n g to t he s p e c t r u m

p r i n c i p a l l y b e t w e e n 1075 a n d 1275 ~ . T h e a u t h o r s e m p h a s i z e t h a t t he a c c u r a c y o f

t h e i n t e n s i t y d a t a p r e s e n t e d in these t a b l e s is dif f icul t to e s t i m a t e . F o r t he r a n g e f r o m

2000 A - 1 4 0 0 A they be l ieve t h a t t h e a c c u r a c y is b e t t e r t h a n +__20~ a n d are fa i r ly


sure that there are no errors greater than a factor of 1.5, whereas below 1300 A errors as great as a factor of two or more may be possible. They also emphasize that the question of solar variability is, of course, still open.

Different parts of the data in Table II were obtained from rocket measurements conducted at different times, e.g. March 13, 1959 for the 2000-1700 A range and April 19, 1960 for all wavelengths below 1520 A (for more details, see original paper by DETWILER et al. [1961]). This mixing of different dates of actual data acquisition, however, is probably not too important since it is unlikely that the corresponding true solar intensity differences for most of the wavelength range of the tabulated data would be significantly greater than the spread of possible errors admitted to exist in these data.

Opinions on the question of true solar variability, unfortunately, are based on extremely meager experimental evidence only. For instance, the 1959/1960 data of Table IIb for Ly-e and shorter wavelengths may be compared with 1960/1961 data from photoelectric records (HtNTEREGGER, 196lb; HALL et al., 1963) obtained with telemetering monochromators. This comparison, for the strongest lines only, is presented in Table IIIa, which shows that the apparent differences in the intensities for all of these lines fall within the possible experimental errors.

A comparison of the August 6, 1957 University of Colorado data (Table I) with the April 19, 1960 NRL data (Table IIb) is presented here in Table IIIb, restricted to strong and clearly identified lines in the range from 1206 A to 1533 A. This comparison (Table IIIb) might suggest a much greater true solar variability than that apparent in Table lIIa. However, a serious objection against accepting the comparison shown in Table IIIh for any quantitative purpose exists in the nature of the apparent differences. The latter do not seem to follow any consistent pattern of any reasonably expected true variation of solar emission conditions.

During the period from 1953 to 1963 photographic techniques of solar XUV spectroscopy have become extremely successful with respect to identification of wave-

TABLE III COMPARISON OF SOLAR EMISSION LINE INTENSITIES FROM

DIFFERENT MEASUREMENTS

(A) A COMPARISON OF NRL DATA QUOTED FROM 1 9 6 0 W I T h AFCRL DATA QUOTED FOR 1 9 6 0 AND 1961

Lille Identification Ly-a Ly-fl Ly-d Si nI C HI OII, III Wavelength (/~) 1216 1026 950 1206 977 833/35

April 19, 1960 NRL 5.1 0.06 0.01 0.03 0.05 0.01

Quoted Intensity

August 23, 1960 AFCRL 3.3 (6.0) 0.03 0.006 0.07 0.08 0.017

Quoted Intensity

August 23, 1961 AFCRL 5.0 0.05 0.007 0.08 0.08 0.01

Quoted Intensity

482

(8)

H. E. HINTEREGGER

TABLE III (continued) A COMPARISON OF THE NRL DATA QUOTED FOR 1960 W~TH THE U~VERSlTV OV COLORADO DATA FOR

1957 GIVEN BY RENSE (1961)

Line Identification Ly-a OI Sin Sire Silv Cn

1302 Wavelength(/~) 1216 1305 1265 1527 1533 1206 1393 1403 1335 1336

1306

August 6, 1957 0.rJ4 U. of C. 3.4 0.04 0.02 0.03 0 .06 0.23 0.24 0.17 0.31 0.33

Quoted Intensity 0.04

April 19, 1960 0.013 NRL 5.1 0.020 0.02 0.038 0.041 0.03 0.030 0.013 0,050 0.050

Quoted Intensity 0.025

lengths and the achievement of good spectral resolution. The same is not true for the application of photographic techniques for absolute intensity measurements, particularly in the part of the XUV spectrum of shorter wavelengths. Fairly accurate photographic data on absolute intensities have been obtained essentially only for wavelengths above 800/~.

For wavelengths below 800 • and particularly below 500 ,~, the results of photographic techniques of spectrophotometry have not yielded sufficiently representative data on absolute intensities. Therefore, the following summary of intensity data for the period 1960-1963 will refer mostly to solar XUV spectrophotometry based on the use of photoelectric detectors.

2.4. INTENSITY DATA fOR 1960-1962

During the period from 1960 to 1962 substantial parts of the solar XUV spectrum ranging from 2000 A to shortest wavelengths of about 60 ,A, have been observed successfully with the aid of both photographic and photoelectric detection of the dispersed radiation from diffraction gratings (see reviews by TOUSEY (1961, 1963), FRIEDMAN (1961-1963), and HINTEREGGER (1961a, 1964). Single-dispersion and double-dispersion techniques using gratings in normal incidence have provided solar EUV observations to a p~actical short-wavelength limit of about 500 ~ and single dispersion with grazing incidence spectrographs were used for measurements below 500 ,A,. All photoelectric recordings accomplished in published experiments with telemetering monochromators have used single-dispersion with gratings in grazing incidence.

Valuable absolute intensity data were derived from photographic records of April 19, 1960. They were accomplished with the new double-dispersion normal incidence spectrograph of greatly improved performance characteristics described by DETWlLER et al. (1961). Representative results are summarized in Table IIb (for the range from 850-1530 ~). For a more complete description of the absolute intensity distribution of the entire spectrum recorded in this April 1960 flight and for details of the photo-


metric evaluation the reader is referred to the original paper by DETWILER et al. (1961) or to the recent review by TOUSEY (1963). A comparison of these NRL densitometer records with photoelectric records from telemetering monochromator experiments conducted by the author's group of the Air Force Cambridge Research Laboratories (AFCRL) is shown in Table Ilia, which includes only the strongest lines in the wavelength range from 835-1216 A. Unfortunately any separation of possibly existing true variations in the absolute intensity distribution of the sun from apparent variations due to experimental and procedural errors would be hardly justified.

Still largely exploratory photoelectric records were obtained with two monochromators of the early stage of development launched on January 19, 1960 (1300 A- 300 A) and January 29, 1960 (310 A-60 A) respectively (HINTEREGGER, 1961a). Both instruments had rather poor spectral resolution, a disturbingly high background and a strong variation of instrumental sensitivity as a function of the position in the wavelength scan. The accuracy of the data on absolute photon fluxes derived from these exploratory measurements is apparently not sufficient to attach any quantitative significance to the differences between the published values of January 1960 flux data (HINTEREGGER, 1960, 1961a) and various sets of data derived from measurements with a considerably improved version of AFCRL monochromator. The first measurements with the improved type of AFCRL rocket monochromator were carried out on August 23, 1960 (HINTEREGGER, 1961b).

It is important to remember that the history of reasonably accurate absolute intensity determinations with XUV rocket monochromators begins with the launch date of August 23, 1960, but only for wavelengths ranging from 1300 A-250 A. For measurements at wavelengths below 250 A, the corresponding date is May 2, 1963. Considering this situation one must admit that we have indeed no convincing experimental basis for conclusions concerning the long-term variability of the absolute solar XUV intensities as a function of the 11-year cycle of solar activity.

It is true that the absolute data published for January 1960 (H~NTEREGGER, 1960, 196 la) indicate generally somewhat larger fluxes than the values published for August 23, 1960 (HINTEREGGER, 1961b) or for August 23, 1961 (HAZE et al., 1963). However, the possibility cannot be ruled out that the nominal differences (generally no more than a factor of 1.5) could be laid to errors (e.g. an accidental overrating of most of the January 1960 fluxes, say by some 30-40 ~ combined inadvertently with a slight underrating of the fluxes of August 23, 1960, say by 10-20 ~ From a conservative experimental viewpoint it is therefore not appropriate to speak of an "observed" decline of solar XUV intensities from January 1960 to August 23, 1960.

The interpretation of apparent changes in the values of intensities of most of the observed solar emission lines in the 250-1300 A region derived from subsequent rocket experiments meets with similar difficulties. In general, the differences in the data on fluxes for any pair of flight dates beginning with August 23, 1960 are smaller than the differences with respect to January 1960 indicated above. On the other hand the estimated accuracy of the absolute calibration factors for any one of these flights still was no better than 20~. Because of the very small number of


monochromator flights (each of which represents only a glimpse at the sun of a useful duration of only about two minutes) an actual reduction of the experimental uncertainty of results could have been obtained only by a major improvement in absolute calibration accuracies. Since this was indeed not accomplished, one cannot exclude the possibility that certain "observed" changes of 20, 30, or even close to 40 70 may be an accidental effect of the measurement.

2.5 INTENSITY DATA FOR 1962-1964

The year 1962 marks the beginning of a new era of solar XUV spectrophotometry by the successful launch and operation of the first orbiting solar observatory (OSO-A). The solar pointed section of this satellite was equipped with a telemetering monochromator provided by Lindsay's group of Goddard Space Flight Center (GSFC) which had been designed for the nominal range of wavelengths from 400 ~ to 10 A (BEHRINa et al., 1963; NVUPERT et al., 1964).

From the viewpoint of studying temporal variations in the intensity distribution of the solar XUV spectrum, the most significant progress achieved by the OSO-A measurements was undoubtedly that of providing for the first time experimental evidence of the variability of sevelal solar EUV line intensities over a time longer than the period of solar rotation. The GSFC instrument was a grazing incidence grating monochromator of a type similar to the AFCRL instrument (HINTEREGGER, 1961a) except for the use of a different mechanism for the motion of the exit slit, a smaller radius of the Rowland circle, and a grating ruled directly in glass (all AFRCL monochromators have been equipped with gratings ruled on blanks first coated with evaporated metal (originally with aluminium and more recently with gold). The data obtained from the OSO-A satellite monochromator left no doubt about the much greater amplitude of temporal variations in the intensities of those solar emission lines which require a high energy for their production in the solar atmosphere (e.g. the coronal lines Fe XV and Fe XVI observed at 284 A and 335 A) in comparison with the He II kyman alpha line at 304 A.

The OSO-A observations as well as all those made with the AFCRL rocket monochromators refer to measurements of the spatially unresolved radiation from all XUV-visible parts of the solar atmosphere. This is a consequence of the fact that grazing incidence grating monochromators for the XUV exhibit no significant image formation or variability of the instrumental photon counting efficiency as a function of the relatively small departures of the incident rays from the central solar direction (+_ 16 minutes of arc). Naturally this is true only if imaging optical elements (colli- mators) between the sun and the entrance are avoided and gratings of sufficiently homogeneous diffraction efficiency (at least over the part actually exposed to solar rays) are chosen. The first condition has been fulfilled by both types of instruments and certain expected departures from the second condition have been investigated experimentally, at least crudely, for various gratings mounted in AFCRL instruments. During solar exposure of the AFCRL instrument with routinely accomplished accuracies of solar pointing of the instrumental entrance axis (i.e. within less than


two minutes of arc with respect to the central solar ray) only one-third of the ruled

width and about one-third of the height of the gratings (centered at the pole) are

effective. The results of these tests suggest that inhomogeneit ies exceeding about

15 % are apparent ly not common. It is impor tan t to note, however, that these checks

were made with test radiat ion which was not collimated much better than the natura l

beam from the whole solar disk and with a relatively wide entrance slit of 50 microns.

For narrower entrance slits and for very strongly localized sources of X U V radia t ion

TABLE IV

INTENSITIES IN SOLAR XUV-SPECTRUM AT A DISTANCE OF 1 A.U. (JULY 1963)*

Wavelength or Range in ]~ Identification r 10~ ph , ] erg cm 2 sec l~ /

1775-1325 see Table II (a) 2700 32

1325-1275 11.8 0.18 1275-1220 26 0.41

1215.7 H Ly-a 270 4.4 1206.5 Siu~ 4.3 0.071

1220-1200 excl. HLy-a, Sim 7.4 0.121 1200-1180 5.5 0.092

1175.7 Cm 2.5 0.042 1180-1130 excluding CIII 5.8 0.100 1130-1090 4.4 0.079

1085.7 Nil 0.48 0.009 1090-1040 excluding Nu 4.2 0.078

1037.6 OvI 1.33 0.025 1031.9 Ovl 1.89 0.036

1040-1027 excluding OvI 0.69 0.013

1325-1027 total 350 5.7

1025.7 H Ly-fl 2.3 0.045 991.5 Nut 0.33 0.007

1027- 990 excl. HLy-fl, Nm 2.4 0.049 977.0 Cm 4.0 0.081 972.5 H Ly-), 0.55 0.011

990- 950 excl. CnI, HLy-y 0.97 0.021 949.7 HEy-3 0.25 0.005 937.8 H Ly-e 0.17 0.004

950- 920 excl. HLy-3, e 1.07 0.022 920- 911 1.25 0.028

1027- 911 total 13.4 0.027

911-890 H Ly-continuum 4.0 0.089 890-860 H Icy-continuum 4.2 0.096 860-840 H Ly-continuum 2.0 0.047 832-835 On, Iii 0.54 0.013 840-810 excluding 0 II, III 2.0 0.048 810-796 0.7 0.017

911-796 total 13.4 0.31

486 H . E. HINTEREGGER

TABLE IV (continued)

Wavelength or Range bl l~ Identification q~o[ 109 ph T ] erg cm 2 sec /~ /

790.1 OIv 0.36 0.003 787.7 OIv 0.32 0.008 780.3 Nevm 0.15 0.004

796-780 excl. Orv, N e v m 0.75 0.019 770.4 Ne viii 0.41 0.011 765.1 Nrv 0.21 0.006

780-760 excl. NevuI, NIv 0.73 0.019 760-740 0.48 0.013 740-732 0.18 0.005

703.8 O m 0.25 0.007 732-700 excluding OnI 0.53 0.015 700-665 0.68 0.020 665-630 0.54 0.017

796-630 total 5.6 0.153

629.7; 625 O r ; Mg• 1.77 0.056 630-600 excluding O v, Mgx 1.2 0.039

584.3 He1 1.56 0.053 600-580 excluding HeI 0.40 0.013 580-540 1.41 0.050 540-510 0.48 0.018 510-500 1.04 0.041 500-480 including SixlI 1.02 0.042 480-460 0.70 0.030

630-460 total 9.6 0.34

460--435 0.49 0.022 435--400 0.99 0.047 400-370 0.56 0.029

460-370 total 2.0 0.098

368.1 Mglx 0.58 0.031 370-355 excluding Mgix 0.93 0.050

(including Fexv0 355-340 0.77 0.044 340-325 including Fe xvI 0.75 0.045 325-310 0.74 0.047

303.8 Hell Ly-a 3.8 0.25 31 0-280 (including Fe xv)

excl. HeiI Ly-a 1.65 0.113

370-280 total 9.2 0.58

280-260 0.84 0.062 257; 256.3 Six; Hell 0.30 0.023

260-240 excl. Six, N e n 0.81 0.064 240-220 0.94 0.081 220-205 0.63 0.059

280-205 total 3.5 0.29

ABSOLUTE INTENSITY MEASUREMENTS

TABLE IV (continued)

487

]photons / c m ~-erg Wave~ngth or Range in ~, Identification ~ o [ ~ Io ~e~

205-190 1.6 • 109 0.163 190-180 2.3 x 109 0.250 180-165 3.2 x 10 ~ 0.371

205-165 total 7.2 x 109 0.78

165-138 7.0 • 108 0.092 138-103 6.0 • lO s 0.099 103- 83 7.0 x lO s 0.149 83- 62 5.0 • 10 s 0.137 62- 41 3.5 • 108 0.135 41- 31 1.5 • 10 ~ 0.083 31- 22.8 5 • 106 0.004

22.8- 15 3 • 10 o 0.003 15- 10 ~ 5 • 105 ~ 0.001 10- 5 < 1.5 • 105 < 0.001 5- 3 < 2 • 103 < 10 -~ 3- 1 < 102 < 10 -7

165- 1 total < 3 • 10 ~ ~< 0.70

* The data shown, taken from a paper by HINTEREGGER et aL (1965), represent fluxes essentially reduced to zero optical depth in the earth's atmosphere. The fluxes listed for wavelengths below 60 A represent the author's estimate of currently applicable upper limits for generally quiet conditions of solar activity, based on a consideration of various published data. All other fluxes represent values determined from AFCRL measurements with telemetering EUV monochromators (HINrE~O6ER et al., 1965).

in the solar a tmosphere , the poss ibi l i ty o f much more serious ins t rumenta l var ia t ions

can not be ruled out wi thout fur ther study.

Since a review of existing da ta on solar X U V spec t ropho tomet ry as of mid-1963

was presented at the sympos ium 'The Solar Spec t rum' in Ut rech t (H~NTERmGER,

1965) and since a summary o f da ta on the absolute intensi ty d is t r ibut ion in the solar

X U V spec t rum based on measurements up to M a r c h 1964 was presented at the 1964

C O S P A R meeting (H~NTEREGCER et al., 1964), it is certainly sufficient for the purpose

o f the present review, to present here (see Table IV) the recent X U V intensi ty da ta

given by HINTEREOGER et al. (1961a, 1965) (wi thout inclusion o f any da ta on the

var ious cross sections) and to include a compar i son (Table V) of earl ier solar X U V

intensi ty da ta with results f rom an A F C R L rocke t m o n o c h r o m a t o r exper iment

conduc ted on December 12, 1963 (HALt et al., 1965). This table (V) reveals only two

significant aspects of our present knowledge o f long- term var ia t ions in the absolu te

intensit ies o f solar X U V emission lines. First ly, it is clear that for most lines a safe

ident i f icat ion o f t ru ly solar var ia t ion is not war ran ted experimental ly . Secondly, the

decrease in the observed intensi ty values for the high-exci ta t ion lines such as Mg x,

SixII, or F e x v , xvI is quite cer ta inly real. A m o n g some puzzl ing aspects of these

observa t ions it suffices to men t ion the change in the indica ted value o f the He t

488 H. E. H1NTEREGGER

TABLE V

FLUXES OF SOLAR EUV EMISSION LINES IN UNITS OF [ ~ ] : 109 ph cm-Zsec -1.

Wavelength Identification (A) Aug. 23 (B) Jtd. 10 (C) Dec. 12 Ratio in t~ 1961 1963 1963 (A) / (C)

1215.7 H Ly-c~ 310 270 1.1 1206.5 Sini 4.0 2.1 3.3 1.2 1025.7 H Ly-fl 2.5 2.4 1.8 1.4 977.0 Cnl 4.0 3.7 3.0 1.3 949.7 H Ly-3 0.35 0.28 1.3 834 OIi, m blend 0.40 0.34 0.28 1.4 790.2 O iv 0.31 0.32 0.23 1.4 770.4 Nevm 0.41 0.22 1.9 629.7 Ov 0.85 0.75 0.61 1.4 609.8 Mgx 0.46 0.20 2.3 584.3 HeI 0.89 0.67 0.42 2.1 554 O~v 0.30 0.20 1.5 499.3 Sixn 0.25 0.09 2.7 465.2 NevIi 0.18 0.12 1.5 368.1 Mglx 0.39 0.36 0.24 1.6 335.0 FexvI 0.33 0.07 4.5 303.8 He II Ly-a 2.5 3.7 2.3 1.1 284.2 Fexv 0.43 0.08 5.4

* Comparison of Data on Solar Emission Lines Acquired at Different Dates. All values shown are based on measurements with similar telemetering rocket monochromators conducted by AFCRL. The Dec. 12, 1963 data have been provided by HALL et al. (1965a).

(584 A~) intensity, for which a reasonable explanation cannot be offered on either theoretical or experimental grounds (584 A_ is one of those wavelengths for which the absolute laboratory calibrations have been established rather securely).

3. Conclusions and Outlook

3.1 SUMMARY OF ACCOMPLISHMENTS

Since the t ime when the first comprehens ive review of te lemeter ing m o n o c h r o m a t o r

measurements of solar X U V rad ia t ion was publ i shed abou t four years ago (HINTER-

EGGER, 1961 a), many re levant exper imenta l and theoret ical developments have evolved

bo th within the Uni ted States and ab road . The overal l progress may be considered

ei ther as extremely grat ifying or as somewhat d i sappoin t ing . Both viewpoints could

indeed be just i f ied by long lists of appl icab le examples.

In general , it appears to be true tha t the au thor ' s ear l ier t r ea tment o f the subject

o f absolu te spec t ropho tomet ry of solar X U V rad ia t i on in the 1961 review does no t

yet require any majo r correct ions on either scientific or technologica l grounds. M a n y

desi rable deve lopments ant ic ipated merely by wishful th inking in 1960 have now

mater ia l ized. Examples may be l isted as fol lows: The first orb i t ing solar X U V

spec t rometer was launched successfully; diffract ion grat ings of improved proper t ies

have become ava i l ab l : the use of solar grat ing spect rometers has been improved


and extended toward shorter wavelengths (HALL et al., 1963; HINTEREGGER et al., 1964; TOUSEY, 1964) nearly overlapping with recently accomplished solar X-ray crystal spectrometry (BLAKE e t aI., 1963); special monochromators for aeronomical applications have been built and operated successfully; the state of the art of laboratory measurements of XUV intensities has been improved (see Section 1) including most valuable work on high temperature plasmas such as ZETA (HEROUX, 1964) and Scylla (HousE, 1964); the use of electron synchrotrons as a powerful source of polarized and rather intense XUV radiation continuum has been revived (see Section 1.5); intensity calibrations of solar XUV spectrophotometers in the laboratory have been extended from 584 A to shorter wavelengths, reliably at least to 256 A; more elaborate laboratory sources, monochromators, and auxiliary equipment have been constructed; a larger number of scientists are now taking an active part in the experimental program; the peculiar interdisciplinary position of absolute XUV spectrophotometry as a field of science probably no longer represents any significant disadvantages.

On the other hand, those most desirable developments which still remain to be accomplished are unfortunately equally numerous as the positive accomplishments listed above. It is not within the scope of this review, however, to belabour these deficiencies in any detail.

Nearly all knowledge of the absolute intensity distribution in the solar XUV radiation spectrum is still based on rocket experiments conducted below about 240 km in the earth's atmosphere. The optical depth of the earth's atmosphere remained significant at various wavelengths even near the peak of the flight trajectories. The vehicles used, i.e., Aerobee Hi rockets with solar pointing control, usually reached altitudes from 210 to 240 km. It is therefore impossible to determine the intensities of the solar XUV fluxes incident upon the outermost part of the earth's atmosphere, i.e. for zero optical depth, without an adequate knowledge of the optical depth of the part of the upper atmosphere which remains in the line of sight from the point of observation to the sun. Consequently, the accuracy with which the incident fluxes 4o can be determined from the observed fluxes ~(h, Z) depends not only on the accuracy of the measurement of q~ but also on the accuracy of the absorption correction. If the objective of a rocket experiment is solar physics rather than atmospheric absorption studies, it is therefore most desirable to launch under conditions of small solar zenith angles, say Z~<30 ~ Even in this case, however, the relative correction (q~o-~b)/~o may be greater than 10 %. The magnitude of this correction obviously depends also on the wavelength, i.e. the correction may be practically vanishing (e.g. for H Ly-~) or in some exceptional case may be greater than 100% (e.g. for H Ly-,,,). If the objective of an experiment is the determination of the optical depth in the atmosphere with a desired accuracy of 10 %, it would be necessary to require accuracies of at least 10 o/for the adopted data on absolute absorption cross sections as well as the absolute concentrations (and scale heights) of all absorbing constituents of the atmospheric gas. Under the conditions of most rocket experiments with solar XUV spectrophotometers, these parameters were indeed known only with consider-


ably poorer accuracy. Therefore, the actual values of optical depths may have been uncertain with possible errors up to about 50 O/,o in some cases. However, in experiments conducted under conditions of relatively small solar zenith angles the optical depth around 240 km for most of the solar emission lines is much smaller than unity, say no greater than 0.2. For these cases a relatively unreliable determination of the actual optical depth, say one with an error of +_50%0, would produce an error of no more than 10 % in the calculated data on the incident fluxes (i.e. the extra- polated values calculated for the " top of the atmosphere").

From the viewpoint of exploring the true variability of the absolute fluxes in the solar XUV spectrum it is most regrettable that most of the existing solar XUV flux data are based on rocket experiments, each of a useful duration of only a few minutes. The only continuous observations covering at least the part of the solar EUV spectrum from 400 • to about 170 ]k with a good signal to noise ratio was accomplished from OSO-A (NEuPFRT, BEHRING and LINDSAY, 1964). Unfortunately the absolute spectral intensity calibration factors of this OSO-A monochromator were more or less unknown before launch and had to be established rather indirectly by subsequent comparison with other measurements (BoURDEAU et al., 1964). The most significant progress achieved by the OSO-A monochromator measurements is therefore that of allowing for the first time direct observations of relative intensity variations over several solar rotations. The result of these observations is a convincing proof that the intensities of certain high-excitation lines vary much more strongly than that of the Ly-~ line of He II (304 A). This observation is in most satisfactory accord with the evidence of similar changes over the much longer period of time from August 1961 to December 1963 apparent in Table V of this review (Section 2.5).

3.2 VARIOUS NEEDS OF MONITORING

The development of a reliable system of monitoring the absolute spectral intensity distribution of solar J(UV radiation must be recognized as a most desirable objective for many speciaIO,-fields of space science. Reasonably continuous and quantitatively accurate monitoring of solar XUV radiation intensities with adequate spectral resolution seems to be required most urgently by solar physics and the physics of planetary atmospheres, with about the same general emphasis for both disciplines, even though the importance of special objectives may vary rather widely.

The most clearly evident interest in solar XUV spectrophotometry is of course presented by the physics of the solar chromosphere, the transition region, and the corona, since these regions are now recognized as the primary sources of radiation in the XUV part of the spectrum. Recognizing existing difficulties of accurate spectrophotometric monitoring of the integrated XUV flux from the entire solar disk (i.e. the total flux stemming from all XUV-visible parts of the solar atmosphere), one might tend to overrate the difficulties to be expected for solar XUV spectrophotometry requiring experimental isolation of small emission regions on the sun having areas very much smaller than the whole disk. However, the additional requirement of substantial spatial resolution besides adequate spectral dispersion presents no basic


experimental problem (i.e. requires no improvements beyond the present state of technology). Naturally this is true only as long as one is satisfied with an XUV spectrohcliophotometer (i.e. a rather long name for the photoelectric telemetering counterpart to a "spectroheliograph") limited to work only for certain reasonably selected wavelengths, i.e. primarily for solar emission lines of rather outstanding intensity such as HeII (304 ~), whereas operation for substantially weaker lines would be practical only if these lines originate from strongly localized sources on the sun and if their wavelengths are not too close to strong lines of less localized solar radiations. A discussion of measurements involving spatial resolution (image formation) is not within the scope of the present review. However, it is appropriate to emphasize that this "second generation" of solar XUV spectrophotometers would hardly eliminate the need of continued monitoiing with the presently established "first generation" of solar XUV spectrophotometers, i.e. instruments responding essentially to the integrated radiation fi'om the whole solar disk. The latter is indeed most important as a primary source of photodissociation and photoionization in planetary atmospheres. At any rate, the development and continuation of systematic XUV spectrophotometry of the spatially unresolved solar disk will probably hold rather permanent interest even for the "pure" discipline of solar physics.

Although the foregoing conclusions are nearly self-evident, it seems impossible to avoid a considerable amount of scientific-technological-economical debate for the present as well as the near future. This apparently peculiar situation is probably caused by the need to establish carefully weighed priorities among many desirable projects of contemporary space research. It would be virtually impossible to pursue all of the scientifically sound objectives efficiently at the same time, even if economical considerations could be discarded. Since many parts of the current and near future international programs of space research are related to the exploration of p]anetat T atmospheres and to the improvement of our present knowledge of the earth's upper atmosphere (thermosphere, ionosphere), the need to establish a systematic monitoring system of solar XUV spectrophotometry as seen from the viewpoints of these scientific fields is discussed in more detail below.

The absorption of solar XUV radiation fluxes represents the most common major cause of heating and ionizing the earth's upper atmosphere (thermosphere, ionosphere). Therefore, the knowledge of the incident XUV radiation intensities throughout the most important part of the spectrum of photon energies from about 10 eV to 1000 eV is an obvious requirement for the current pursuit of ionospheric physics, aeronomy, the physics of planetary atmospheres, and, more specifically, solar terrestrial relationships. Since the required quantitative knowledge can be acquired and maintained only by actual measurements of these fluxes, the general question, "Do we or don't we need solar XUV flux monitoring?", hardly requires any further discussion. Consequently, we may restrict our discussion to specific problems of monitoring. These may be summarized tentatively as follows:

(a) Should we monitor continuously or with some limited duty cycle? (b) Should we cover the entire XUV spectrum, or restrict measurements to a

492 H . E . HINTEREGGER

limited number of lines and/or wavelength bands; in the latter case, how many lines should be picked and at which specific wavelengths?

(c) How accurate should the measurements be with respect to absolute intensity and with respect to relative variations?

(d) What degrees of spectral resolution would seem inadequate, reasonably adequate, desirable, or extravagant?

(e) How important is additional monitoring with spatial resolution of localized solar emission regions on the solar disk besides the basic monitoring of the radiation from the entire XUV-visible solar disk?

Existing knowledge does not warrant definitive answers to all questions (a) through (e) in all desirable details. However, preliminary appraisals of each of these problems can be worked out on sufficiently well established grounds to aid the planning of space science experiments. Such preliminary appraisals are offered below.

Before commenting on the specific questions in the order (a) through (e) above, it should be explained why this discussion considers only the XUV range of the solar electromagnetic emission spectrum, say from 1750 • to about 15 ~ (omitting the shorter wavelengths of X-rays, gamma-rays, as well as the longer wavelengths of near ultraviolet, visible, infrared, and radio emissions) and why no reference is made to corpuscular fluxes which are also known to be incident upon the upper atmosphere (solar wind, cosmic or solar high energy particles, charged particles from the earth's radiation belts and "auroral" particle fluxes).

The author's main reasons for these restrictions and omissions are some inevitable subjective bias, a preference to comment mainly within the area of his own working experience, and the fact that the merits of additional monitoring of the other types of fluxes are analyzed much more appropriately by other scientists.

Another justification for the present emphasis on solar XUV radiation is given by the rather objectively obtainable assessment of the approximate minimum levels of absolute intensities required for any other fluxes to be of comparable or dominant importance for the upper atmosphere in comparison with the heating and ionizing action of solar XUV radiation. For the latter we know not only the order of magnitude of the spectral intensities on top of the atmosphere, but also considerable details of the different vertical distributions of the dissipation rates at the different wavelengths throughout the earth's thermosphere (ionosphere). This experimental knowledge together with the most commonly accepted part of theoretical and experimental aeronomical information such as the various atomic, molecular and ionic cross sections (including various secondary reactions), heat conduction, diffusion, and mixing characteristics can be combined into a reasonably well established semiquantitative picture. Within the present accuracy of this picture it follows only that XUV absorption alone is certainly a major and possible the dominant normal source of global thermospheric heating and ionization. It would be quite unwarranted, however, to try to deduce from this picture any specific percentile contributions by any additional agents such as the corpuscular heat source. Attempts at deducing the nonex- istence of such additional sources on this basis would of course be equally unjustified.


After these remarks about the author's general attitude, the tentative answers to the questions (a) through (e) above are offered not as conclusions but essentially as personal opinions as follows:

The solar XUV fluxes should be monitored with a reasonably high duty cycle, say at least about once per hour; continuous monitoring, and the use of short response times, say 0.1 second, would certainly be very desirable (at least occasionally upon ground command).

A rather complete wavelength coverage of the entire XUV spectrum is obviously most desirable. A nearly complete spectral coverage may be accomplished, at least for some time, by two monochromators (by GSFC and AFCRL) scheduled to be flown on the NASA satellite OSO-C. It should be mentioned, however, that the OSO series of NASA satellites is not oriented specifically toward objectives of monitoring.

Even if all OSO experiments are launched successfully, there will be many periods of time, during which some aeronomically important parts of the XUV spectrum are indeed not measured. Furthermore, one can hardly rule out the possible occurrence of other periods of time~ when no applicable OSO experiments or simply no OSO vehicle may be operational at all. Therefore, it seems to be very desirable to prepare spectral XUV flux monitoring experiments designed to serve this purpose exclusively, designed for a high duty cycle and capable of flying on a sufficient number of different vehicles, so that the risk of data loss for the future may be minimized.

Since fast time response, nearly or fully continuous duty cycle, completeness of spectral range, and high spectral resolution are not readily compatible technically, it is very difficult to suggest any specific compromise as an optimum even for a given set of conditions of currently produced space vehicles. Any such choice suggested or adopted at this time, e.g. that realized by the XUV spectrophotometer now completed for flight on the OGO-C and OGO-D satellites in the near future (Br:DO and HINTER- EGOER, 1965) should be allowed to be revised (if necessary several times) before adopting definitively a certain instrumental solution together with some specific system of communicating results to the interested part of the scientific community.

Completeness of wavelength coverage without any significant spectral resolution is certainly inadequate. The reasons for this inadequacy are, firstly, the diverse character of the solar sources of the various fluxes at the different wavelengths and, secondly, the strong wavelength dependence of the absorption cross sections of the constituents of the earth's upper atmosphere. To illustrate these difficulties, we may, for the moment, divide the range from about 1750 ~ to 15/~ into a certain number (16) of sub-ranges agreeing to choose some of the limits so as to match characteristic discon- tinuities in the absorption cross sections of the three major atmospheric constituents (N2, 02, and O). As an example, let us consider the following ranges: 1750-1027 A; 1027-911 A ; 911-796 A ; 796-700 A; 700-630/~; 630-465 A; 465-368 A; 368-250 .~; 250-200A; 200-170/~ ;170-140 A; 140-80 A; 80-50/~; 50-31 A; 31-23 A; 23-15 A.

At various occasions it has been argued that the monitoring of some very small number of lines or wavelength bands would be adequate for most aeronomical

494 ~q. E. HINTEREGGER

purposes. However, the fallacy of such arguments can be illustrated heie by showing that even the choice of 16 groups as indicated above is indeed hardly adequate. Nearly each of the 16 tentative sub-ranges listed above contains line emissions of drastically different solar source characteristics, e.g. Mgx (,610 A) next to HeI (584 A), FexvI (335 A) and Fexv (284 A) near Hen (304 A). Rather substantial variations of atmospheric cross sections within each sub-range together with the possible variability of the spectral intensity distribution of the solar emissions within each sub-range therefore place serious limitations on the scientific significance of monitoring relatively wide spectral bands, even if one agreed to monitor an apparently large number of such bands.

Our knowledge of the physics of the temporal and spatial variability of the solar XUV source spectra is indeed far from the desirable stage where we could hope to infer the variations of the intensities at all wavelengths from results obtahTed by monitoring the intensities of just a few properly chosen emission lines.

Weighing the known experimental aspects as well as the scientific objectives, the author cannot escape the conclusion that the most logical selection of specifications for a solar XUV monitor at present is still that of requirhN both spectral completeness and adequate spectral resolution to separate at least the stronger solar emission lines.

The realization of this requirement is not too difficult experimentally, since instrumental spectral band widths of about 1 A to 20 A (depending on the position in the spectrum) appear to be quite satisfactory. As a result of conducting this type of spectrally nearly complete though cumbersome solar XUV monitoring for a reasonable length of time, it should be possible to accumulate the necessary additional knowledge which may eventually eliminate the need to monitor always the intensities for all wavelengths in the future. Thus the number of XUV lines for which continuous monitoring is considered necessary should, eventually, drop to some more conveniently cataloguable level. For various reasons it seems most probable, however, that one would always wish to catalogue XUV intensities for at least about 30 wavelengths (some lines, some bands) and preferably for about twice as many. However small this number may be estimated to become eventually, there is indeed no reason at present for failing to start with nearly complete monitoring as soon as possible. To discover at some later date that one needs data on fluxes that were indeed never measured would be rather frustrating.

After this strong plea for an early start of nearly complete spectral XUV intensity monitoring, it should be emphasized that one must not underestimate the value of various currently conducted (or planned) solar monitoring experiments merely because they may cover only a small spectral range. At least for the present and near future, monitoring of absolute XUV intensities at any number of wavelengths, no matter how few, or even for a single line or band, will almost certainly enrich our presently rather meager knowledge. Another clarification is perhaps in order with respect to the AFCRL development of a rocket monochromator operating at only nine fixed wavelengths (HALL et al., 1964). The author has often stated (e.g. H[~TEReG6ER, 1962) that this specialization presents a considerable progress beyond


the type of measurements conducted with the old type of AFCRL multi-purpose monochromators, even though the latter covered the XUV spectrum incomparably more completely. The explanation is simply that the fixed-wavelength XUV monochromator is used for vertical probing in the earth's atmosphere whereas the discussion above refers to instruments to be used on orbiting vehicles above the regions of significant atmospheric absorption, designed to monitor the temporal variations in the spectrum of the incident solar XUV fluxes.

The accuracy of data on absolute intensities of certain XUV fluxes is still rather crude even for the times of actual measurements. Our knowledge of most fluxes for all other times is, of course, that much more uncertain. Consequently, any experiments conducted soon may be extremely valuable even if the guaranteed absolute accuracy per se may have to be rated as relatively modest or poor. To improve the absolute accuracies to better than say 20~ may be very difficult at least for certain sub-ranges of the XUV spectrum. However, it is much less difficult to improve the reproducibility of instrumental sensitivity, i.e. the precision of de- determining relative changes to accomplish relative accuracies far better than those claimed at present. The attainment of such "relative accuracies" of the order of 1 ~o is apparently not too difficult. Certain realistic goals for current developments may be stated by requiring accuracies of about 10 ~ o for absolute values and 1% or better for the diagnosis of relative temporal variations.

An appraisal of the importance of additional monitoring of specific XUV emission regions with spatial resolution of the location of sources in the solar atmosphere would certainly exceed the scope of this review. There should be no doubt, however, that such XUV spectroheliographic observations will provide invaluable information on many aspects of solar physics and may become an absolute necessity for purposes of forecasting.

3.3 REMARKS ABOUT FUTURE WORK

Various aspects of importance to future work in the field of absolute spectrophotometry of solar XUV radiation have been discussed in the preceding sections of this review. The pursuit of work in this field will certainly maintain an interdisciplinary character, probably in an increasingly pronounced form for the future. In particular, many fundamental and practical problems associated with solar XUV spectrophotometry may be attacked most successfully on the basis of results of laboratory studies of XUV radiation produced by various powerful sources such as electron synchrotrons and all sorts of experimental plasma machines. The state of the art of XUV intensity measurements in the laboratory appears to be ready for significant further advances in the near future. Obviously, these advances will have to be accomplished before one may hope to achieve corresponding improvements in the state of the art of solar or stellar measurements.

Among the reliably predictable advances, the adoption of absolute detection standards (see Section 1.4) is probably particularly noteworthy. As soon as significantly improved accuracies will have been realized in the pre-launch calibration of

496 ft. E. HINTEREGGER

solar XUV photometers in the laboratory, one will, of course, be faced with a new problem, i.e. that of maintaining and checking these accuracies in the subsequent measurements from space vehicles. At present, this problem is not too serious only

because absolute calibration factors are commonly no more accurate than within about 20 ~ even in the laboratory.

The problem of reestablishing absolute calibration factors of instruments in orbiting observatories will become increasingly important in the future. The need to attack this problem will result f rom anticipated progress of the overall state of the physics of the solar atmosphere toward a more quantitatively significant picture. When this is accomplished, errors of 20 ~ may be more serious than certain errors of a factor of two or more at present.

A similar situation is obviously characteristic of aeronomical applications of

solar XUV intensity data (e.g. see the recent analysis of ionospheric conditions by NICOLET and SWtDER (1963). The progress in aeronomy accomplished up to this date clearly points at a rapidly developing need for considerably improved experimental ilformation on solar XUVfluxes. Existing semiquantitative studies concerned with most interesting solar-terrestrial relationships such as the recently discussed corre- lation with thermospheric temperatures (BOURDEAU et al., 1964) can hardly be ex-

pected to progress any further unless solar XUV fluxes are indeed monitored continuously and with accuracies better than those of most of the existing data.

A consideration of all developments of solar XUV spectrophotometry up to this

date together with an appreciation of existing experimental capabilities suggest to conclude this review with the general remark that the most important step to be

taken at present appears to be that of bringing much effort to bear on the transition fi'om scattered exploratory observations in solar XUV spectrophotometry to a phase of DIOl'e systematic and more accurate llleasut'enletlts.

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absolute intensity measurements in the extreme ultraviolet spectrum of solar radiation

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