observational, reference and model data on solar euv, from measurements on ae-e

GEOPHYSICAL RESEARCH LETTERS, VOL. 8, NO. 11, PAGES 1147-1150, NOVEMBER 1981

OBSERVATIONAL, REFERENCE AND MODEL DATA ON SOLAR EUV, FROM MEASUREMENTS ON AE-E

Hans E. Hinteregger, Katsura Fukui

Air Force Geophysics Laboratory, Hanscom Air Force Base, Massachusetts 01731

and

Bruce R. Gilson

Computer Sciences Corporation, Silver Spring, Maryland 20910

Abstract. Information on solar irradiance at reduced to that of supporting the Ly-• data from wavelengths below 185 nm, observed by the EUVS ex- the fixed-t monochromator 22. periment on the AE-E satellite over the entire de- The various monochromators used to scan par- velopment of the present sunspot cycle 21, is im- tially overlapping sections of the total wave- portant to many investigations of planetary therm- length range had reached good photometric stabil- ospheres and ionospheres. $•7vic•y oõA•ua•iona• in{0•o•e•0m is generally lacking in both the completeness and the spectral detail required by the more advanced study programs. Therefore, it has been necessary also to develop computer m0d• in connection with fully detailed compilations of an appropriate re•ef•emce Apee•t•. our selection of appropriate forms of effective publication has been difficult for various reasons. Recognizing

ity by July of 1976, with the only exception of the measurement of Ly-• (Hinteregger, 1980). Therefore we were able to select the period of 13- 28 July as an observationally reliable AE-E dat• re•ere•e period reflecting Solar conditions of minimum activity (R z = O; F10.7 = 68) for solar cycle 21. The selected set of observational reference data, consisting of the wavelength scans of all of the irradiance-monitoring EUVS monochroma-

that full reproduction of our various lists of ob- tors averaged for all useful experiment turnohs of servational, reference, and model data in scien- the aforementioned period, is characterized by tific journals would be impracticable, we started high statistical significance even for relatively a more or less informal procedure of timely re- weak solar emission lines for which the count sam- lease of information to a limited number of par- ples from a single turnon would be rather poor. ticularly interested colleagues. With the present The averaging is certainly justified, •as 27-day or letter, we hope to mitigate at least some of the day-to-day variations in this period were neither dissatisfactory aspects of this procedure. expected nor indeed found.

This observational AE-E data reference has ba-

History of Observations and Data Releases sically nothing to do with the absolute values of irradiance assigned to this period. The EUVS in-

Nearly identical EUVS instruments (Hinteregger strument had no provisions for any on-board cali- et al., 1973) were flown on the Atmosphere Explor- bration, and the pre-launch calibration was not er satellites, AE-C, D, and E. but only the AE-E only crude but is indeed inapplicable to data ac- observations are discussed here. The AE-C instru- quired during or after the initial period of ob- ment was only partially successful (Hinteregger, viously degrading sensitivities. Therefore, our 1976, 1977; Hinteregger et al., 1977), and the assignments of absolute irradiance-reference val- AE-D instrument had not yet reached stability by ues have been drawn from all available sources of the time of spacecraft failure. The attainment of information other than that provided by AE-E. good stability in the AE-E experiment took from 3 to 18 months depending on both the type of detec- tor and the part of the spectrum for the various monochromators involved in the acquisition of irradiance data in the total instrumental range of 14.2 - 185 nm.

The only mishap in the performance of the EUVS instrument on the AE-E satellite was a failure of

the HV supply for the sealed-off photomultipliers used for the longer wavelengths. The resulting

A reference spectrum for the conditions of minimum activity for cycle 21 is intended to list absolute fluxes for many more wavelengths than those actually resolved by the EUVS instrument, including extensions to both shorter and longer wavelengths outside the instrumental range, to provide the completeness required by some advanced aeronomical computer models. Our preliminary version, F76REF (Hinteregger, 1981a), was based on merging AE-E experiences on relative ratios of

loss •'of observational capability for all dates af- various solar emissions observed simultaneously ter 30 March 1979 is most regrettable because of the rather marginal response of monochromator 8 which •cans the nominal range of 123 - 170 nm but uses open channel electron multipliers exhibiting a drastic drop of spectral sensitivity for wavelengths beyond 135 nm. This causes the actual response in the scan range of I•145 nm to be prac-

and on ratios for different phases in the solar cycle with some estimated adjustments to observed variations •in the period fro TM 23 April 1974 (AFGL rocket experiment for "calibration" of AE-C satellite instrument) through July 1976. This period included the chromospheric minimum of April•1975 (Hinteregger, 1977) in the complicated transition

tically completely due to scattered H Ly-• (Hint- from cycle 20 to 21 (Hinteregger, 1979). The con- eregger, 1980; Hinteregger et al., 1981a). There- nection of F76REF to t•e reference spectrum for fore the value of this data return is essentially 23 April 1974, F74113 (Heroux and Hinteregger,

'• ' 1978) still suffered from the lack of applicable This paper is not subject to U.S. copyright. Pub- rocket data for the new solar cycle. lished in 1981 by the American Geophysical Union. The most recentlyissue• reference spectrum,

Paper number 1L1323. 1147

1148 Hinteregger et al.: Data on Solar EUV

Iab]e 1. Lists of Detailed Reference Spectrum @ of EUV Irradiance for wavelengths below 200 nm -2 -1 Period

Reference Date Number of Fluxes/ 1013photons m s of Issue ID or Wavelengths •/nm •/nm •/nm Class C•-avg.

lis ted References File Name (YYDDD) Period 1 - 15 15 - 37 37 - 103 "K"

F74113 78115 23 April 1974 1957'

F76REF 79135 13 - 28 July 1972 1976

SC#21REF 81099 -"- 1659

SC#21REFW 81101 - "- 1659

0.58 15.90 26.89 N.A. N.A. Heroux and Hinteregger

(1978) 0.64 15.37 31.77 0,1•,2 Feb 79 Hinteregger

{1981a)

0.55 11.99 30.99 1,2 Jul 77 Hinteregger -Sep 80 et al. (1981a)

0.55 12.01 30.99 1,2 - "- "in work"

@) including many wavelength entries outside the instrumental range of the EUVS experiment and using separation of listed lines far beyond the modest spectral resolving power of that instrument, with continuum fluxes expressed by entries 1 • apart (wavelengths are listed in units of 1 • = 0.1 nm)

*) subsequent relatively minor revisions of'F74113 were issued with ID = 78234, 79040, and 79109, with an increase of the number of wavelength records from 1957 to 1972 in f•na.1 version (ID--79107)

SC•21REF, reflects some significant revisions of with a preliminary supplement for 1980 ($EUVFLXA), F76REF, including (a) various corrections accumu- to the •.most recently issued list (sc#210BS). The !ated in an occasionally edited working copy of latter covers 15 'wavelength groups' i e 9 in- F76REF, but more importantly, (b) results of a more dividual lines and 6 sections of the spectrum, extensive analysis of AE-E data on various ratios, and (c) the invaluable use of the most recently completed final evaluation of an AFGL rocket experiment of 14 August 1979 kindly provided by L. Heroux prior to publication.

Lists of any reference spectrum of the typed e- scribed above, summarized in Table 1, are too extensive for complete reproduction in a scientific journal. Therefore, complete lists were distrib- uted only by informal mailing to a limited number of colleagues. Those engaged in computer-aided aeronomical studies obviously preferred the data in the form of disk files, either on the then ex-

tabulated for •he observing perio d from 1 July of 1977 through the end of 1980 in the last version.

Regarding the use of both observational and reference data listings of the type discussed above, we should emphasSze that proper identifica- tion•of issue dates is recommended to facilitate the tracing of discrepancies such as could appear in the results from various independent studies.

"EUV-Class" and "F10.?-Association"Models

Any observable and available quantity expres- sing some measure of solar'activity may be desig-

clusively AE-mission-dedicated computer at ,Goddard nated as ke• Par•abZe to be used in some scheme Space Flight Center or on some other computers of •calculating •Z value•s of daily irradiance (after transportation on magnetic tape). The same by certain •Z•• of solar EUV emissions (K-- 1, comment applies to lists of observational data .... ,N) after defining a suitable mathematical de-

Lists of observational, date-ordere• data, for pendence on the key variable for each class, e.g. certain assortments of selected solar emissioms

have been issued at various stages (see Table 2), I• = I•REF + I•REF-(RK•- 1)-C%; RK-=VK/VKREF (1) progressing from a rather preliminary list for 9 wavelengths (EUVFLUX), a 7-wavelength list for the where the subscript REF identifies the values ad- longer period of data to the eng•of 1979 ($EUVF•X) optred-Is. •reference for minimum activity. For I%,

lab]e •. Lists of Observational EUV Data issued at various stages of progressing work

Date of

File Name Issue Observations Number of July 1976

for sep. Lines; Irradiance Data Period (Intervals) Reference

Un.its used for References Data Listing

EUVFLUX 12 Sep 79 77154-79226

$EUVFLX 5 Feb 80 77154-79364

$Eb•FLXA 16 Jan 81 80001-80269

SC#21OBS 2 Apr 81 77182-80269 SC#21OBSX 22 Apr 81 80270-80365

SC#210BS* 7 May 81 77182-80365

9; (0) F76REF -2 -1 1010 Phot.m s ß Hinteregger(1981a)

7; (0) F76REF

7; (0) F76REF

same Hinteregger (1981a; 1981b)

same informal distribution

9; (6) 9; (6)

SC#21REF Ratios/(July 76) informal distribution same; supplement giving extension to end of year

9; (6) SC#21REF Ratios/(July 76) Hinteregger et al. (1981a; • 19816)

replacing combination of previous SC#21OB$ and supplement

Hinteregger et al.: Data on Solar EUV 1149

these values must be provided by an appropriate ability model of Cook et al. (1980) offered for list such as SC#21REF for cycle 21. If a key var- 11117.5 - 210 nm with similar warnings regarding iable other than an EUV flux is used, the corres- ponding REF-value should of course also apply to solar minimum. The quantity C1 is a wavelength- peculiar adjustment parameter which ties the variation of I% to that of the key variable, VK, expressed above as ratio (RK) relative to solar minimum. Writing R% for the ratio I%/IIREF for any listed %, we see from equ. (1) that the ratio of (R! - 1)/(R 2 - 1) for some pair of different wavelengths, %1 and %2, can be regarded either as the C%-ratio of the two wavelengths placed in a common class K with %K different from both, or as the value of C(%1) with the flux at 12 defined as key variable, implying C(% 2) -- 1.

The S-class model (K=0 keyed to %%175-185 nm, K=i keyed to He I 58.4 nm, and K--2 keyed to Fe XVI 33.5 nm) attached to the preliminary reference spectrum, F76REF, treated the 'quasi-continuum' in the range %%120-200 nm as well as the H Lyman continuum with special formulations replacing that given by equ. (1) above which was used in all other cases (Hinteregger, 1951a,b). The

the use of "typical" contrast factors as noted above with respect to C%. Since both R z and the calcium plage index are but crudely quantified parameters, the use of F10.? has been most widely adopted as an indirect measure of the solar input for the most successful empirical models of atmospheric structure. These models actually by-pass the problems of representing solar EUV, by formu- lating relationships only for the association of F10.? with atmospheric model parameters directly.

Limitations similar to those discussed for our

class-variability mode.•_s above also exist for Fro ' ? - association models (Hinteregger, 1981a,b; Hinteregger et al., 1951a,b). It suffices here to mention the two-variable formulation

* ?) + I% = A%.F10.7 + B%.(F10.7 - .

where F10.7 and F•0.7 are the daily and $1-day mean values, respectively. The l-dependent values of A%, B%, and C% above are obtained from least- square fits of the expression of equ. (2) to the

values of C1 listed in F76REF are adjusted to re- actually observed values of I% (OBS) for some spe- flect the observed relationships between the "pre- cific period of existing AE-E measurements. If maximum" of January-February 1979 and the refer- this observation-fitting period is rather long, ence period of July 1976, i.e. these values were not based on any long-term averaging. Experiences acquired in various computer studies accessing an improved and greatly extended AE-E data base, the lack of observations of the key flux of class K=0 for all dates after 30 March 1979 (HV failure mentioned above), and the variously expressed desire for simplicity, combined to motivate the development of the new model outlined below.

The 2-class model (K--l, now keyed to H Ly-B, and K--2, again keyed to Fe XVI 33.5 nm) attached to the most recently issued reference spectrum, SC#21REF, eliminated the previous quasi-continuum class K--0 by absorbing its representation as a part of the class K--1 and also abandoned the sep- arate treatment of the Lyman continuum (Hintereg-

say Jan 1977 - Sep 1980, the coefficients of de- termination (square of correlation coefficients) are impressively high, e.g. typically % 92-95%. However, this highly correlated long-term association is in a remarkable contrast to the very notable differences between the model-calculated

and the actually observed fluxes for some specific date or even for some time spans as long as 6 months. For instance, the correlation for certain 6-month data fits was indeed poor enough to render the simple mean value an actually better representation than that of equ. (2) with any non-zero values of A% and B•. Since a conversion from typ- i• to d•e-ad•ted model representation is of course practicable only through some EUV-class variability model and only if detailed observa-

ger et al., 1951a,b). The values of the wave- tional EUV data are actually available, the appli- length-peculiar adjustment parameters C% listed in cation of the F10.7- association model to periods SC#21REF are not adjusted for the best possible after the termination of AE-E observations will be representation of any specific date but represent apema•e palues established for the long period of July 1977- September 1980. This means that this set of C%-values can be regarded as tropical for some simplified picture of this part of cycle 21, but represents only an app•o•mate representation for any given specific •te, since actually date- adjusted values of C1 may differ from the listed mean values more or less significantly, i.e. by percentages not conducive to any reliable estima- tion without truly detailed evaluation of all available observational data for that date. Typi-

obviously valuable, but clearly restricted to est- imates of extrapolated typical values of EUV irradiance. The designation "extrapolated" indicates our expectation that we will have no experimental basis for identifying possibly significant differences between the observationally covered behavior of the first part of this solar cycle and its forthcoming phase of declining activity.

Use of "Model" versus "Observational Data"

The type of solar EUV irradiance data consider- cal RMS departures of date-peculiar C%-values from ed most useful to ionospheric or other aeronomical the listed mean are in the range of 5% - 35%, i.e. studies probably varies greatly, depending on the notably different for different %. This is hardly nature of a specific study. A most important dis- surprising if one considers the greatly diversi- tinction appears to be that of studying either fied nature of active regions of many different typ• or actually •g-pg•• situations. The types, different ages, and different locations on latter will clearly profit from the most extensive the solar disk. possible use of observational data, whereas the

The key variable used to model the solar-cycle former type of studies may be much clearer if variability of some class of EUV emissions does based on a suitable variability model, even with- not necessarily have to be the flux in some EUV out considering the size of the effort required emission. It could be any other suitable index to accomplish detailed date-adjustments of the of solar activity such as F10.7, RZ, or the calci- entire spectrum for some large number of days. um plage index. The latter was used in the vari- Since our lists of observational and of associ-

1150 Hinteregger et al.: Data on Solar EUV

ated reference or model data will continue to re-

flect intermediate stages of progressing work, any investigators planning computer studies requiring solar EUV inputs for cycle 21 are encouraged to consult us before investing any major effort.

Discussion

Iterative improvements of both the observational and the associated reference- and model-data

sets will probably require the continuation of our present level of effort for another two years until •• Aev•6•o• of these data sets may be issued. To serve an obvious interest in the earl-

ier release of more or less preliminary data, we intend to continue the informal dissemination to

particularly interested colleagues as well as the occasional deposition of intermediary data sets on magnetic tapes with the National Space Science Data Center (NSSDC). All of the EUV data files mentioned in this report are indeed included on a magnetic tape presently prepared for transfer to the NSSDC.

A•kn0wœedgem•t6. The EUVS experiment was spon- sored by the National Aeronautics and Space Admin- istration (NASA DPR S50030-AG). The authors wish to than M. Gross and B.E. Jeffe of Computer Sci- ences Corporation for the excellent implementation of difficult program requirements.

References

Cook, J.W., G.E. Brueckner and M.E. VanHousier, Variability of the solar flux in the far ultraviolet 1175-2100 •, J. Geophys. Res., 85, 225L 1980.

Heroux, L., H.E. Hinteregger, Aeronomical reference spectrum for solar UV below 2000 •, J. Geophys. Res., 83, 5305, 1978.

Hinteregger, H.E., D.E. Bedo and J.E. Manson, The EUV spectrophotometer on Atmosphere Ex- plorer, Radio Science, 8, No. 4, 349, 1973.

Hinteregger, H.E., EUV fluxes in the solar spectrum below 2000 •, J. Atm. & Terr. Phys., 38, 791, 1976.

Hinteregger, H.E.D.E. Bedo, J.E. Manson and D.R. Skillman, EUV flux variations with solar rotation observed during 1974-1976 from the AE-C satellite, COSPAR Space Research XVII, 533, 1977.

Hinteregger, H.E., EUV flux variation during end of solar cycle 20 and beginning cycle 21, observed from AE-C satellite, Geophys. Res. Let- ters, •, 231, 1977.

Hinteregger, H.E., Development of solar cycle 21 observed in EUV spectrum and atmospheric ab- sorption, J. Geo.phy. Res., 84, 1933, 1979.

Hinteregger, H.E., AE-E experiences of solar irradiance monitoring for 1200-1850 •, in ?ro- ceedin•s of Workshop on Solar UVMonitorinõ held 31 July - 1 August 1980 in Boulder• Colo- rado, pp. 9-25, 1980.

Hinteregger, H.E., Representations of solar EUV fluxes for aeronomical applications, Adv. Spa. ce Res., •, 39, 1981.

Hinteregger, H.E., The solar ultraviolet source of the ionosphere and its variation, in NATO AGARD Conference Proceedin•s• No. 295, I-i--- I-11, 1981b.

Hinteregger, H.E., K. Fukui and B.R. Gilson, AE-E observations and associated models of solar ir-

radiance, (1) Wavelength range 120-185 nm, to be submitted to J. Geophys. Res. 1981a.

Hinteregger, H.E., K. Fukui and B.R. Gilson, AE-E observations and associated models of solar ir-

radiance, (2) Wavelengths below 120 nm, to be submitted to J. Geophys. Res. 1981b.

(Received May 18, 1981; accepted August 6, 1981.)

observational, reference and model data on solar euv, from measurements on ae-e

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