systematic photometry of xuv solar images

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

C. W. A L L E N

University of London Observatory, London, U.K.

(Received 17 December, 1968)

Abstract. Units and methods have been devised to express the photometry of solar XUV images. The source and limb-brightened fluxes are given in terms of the sun's quiet central intensity. Measurements made on this system can be meaningfully compared with solar data and with theoretical predictions. Calculations have been made of the XUV distribution for optically thin solar models and results have been converted onto the measurement system. Photometric measurements have been made from four films lent by the Culham and Leicester Laboratories. Certain inconsistencies suggest that the measurement accuracy is not yet good enough for definitive results. However, there is evidence that: (a) the X-ray emission sources are brighter, relative to the quiet sun, than the longer wave EUV sources; (b) X-ray limb photons all escape (i.e. limb optically thin) but EUV limb emission is affected by absorption; and (c) the observed image diameter fits an emission scale height of 0.05 ~|

1. Introduction

Each successful X-ray or far ultraviolet (XUV) image of the sun gives direct in-

format ion on the solar emission at one time only. In order that this information

shall have relevance to solar activity in general it is necessary that each particular result be standardized by comparison with regular and systematic solar observations.

This requirement presents two coordinating problems: (1) What observations should be made on each X U V image in order that useful

quantitative information might accumulate? (2) What are the most suitable solar data for standardizing purposes?

The present paper deals with the first question. The pr imary advantage to be obtained by observing the sun's X U V in the form

of images is that the various sources of emission can be segregated and measured

independently. The image data required for physical studies are the details of flux,

intensity, size, shape, spectrum, etc., of each X U V feature. Such measurements are naturally at tempted by those who have first access to new images. Comparabi l i ty

with other images is not likely to be their main concern. Systematic studies, on the

other hand, are concerned with the comparison of data obtainable on the majori ty o f images. For such purposes measurements and units must be standardized as far

as possible. Systematic photomet ry calls for a preview of what may need measurement.

It has long been known that the solar ionizing radiation can be segregated into a quiet and an active component . The same two components must be identified and

measured on the X U V image. The quiet component may be expected to cover the whole disk fairly smoothly and be brighter at the limb. The active components should be associated with centres of activity (CA's) characterized by sunspots,

Solar Physics 8 (1969) 72-87; �9 D. Reidel Publishing Company, Dordrecht-Holland

SYSTEMATIC PHOTOMETRY OF XUV SOLAR IMAGES 73

plages, flares, coronal emissions in certain lines, etc. Of these the flare emissions can best be studied by the time variations in total XUV and they do not need solar images. Consequently our systematic image studies may be restricted to the slowly varying phenomena with a time-scale of days or weeks.

There are some advantages in having various images photometered and compared at one laboratory and this is now being attempted at the University of London Observatory. Solar images loaned from the Culham Laboratories and from the Leicester University have been photometered in accordance with a consistent procedure.

2. Photometry of Solar Images

The features to be photometered are S, the contribution from the CA sources, L, the limb brightening, D, the quiet disk. It is necessary to be able to separate S, L, and D from one another and to express

the results in a form that is quantitative but not too dependent on observational hazards.

The measurement to be made from each feature is the flux of radiation (directed towards the earth) since this is not greatly affected by instrumental distortion. As a standardized reference intensity we use (if possible) the disk intensity, ID, near the sun's centre. The numerical data for various images become comparable when this unit is used for intensity and the sun's visible radius No is used for unit distance. Measurements of the flux ~- and other quantities based on these units are said to be in 'disk units'.

The standardization through disk units meets a difficulty in the shorter X-ray regions where the disk and limb may be too faint to be observed. In that case the images allow only the relative fluxes for the various CA sources to be evaluated. The fluxes may be expressed in absolute units, if known, but the accumulated am- biguities in spectral intensity, response, filtering and atmospheric loss, make such data of doubtful value for comparing one image with another. We will find that provided a limb brightening can be detected and measured this can usually be converted into a disk intensity thus making disk units available.

The following observations are to be attempted:

S. Centre of activity sources: As many CA emission sources as can be detected and resolved should be identified

with the corresponding MeMath plage numbers (from Solar-Geophysical Data) or perhaps some other identifying numbers. The flux for each source should be determined in the form ~-s = ~ Is dA, where I s aims at representing the intensity in excess of the otherwise existing quiet disk or limb (see Figure 1). The area element dA should be in terms of ~ . The flux from a particular CA numbered, say, 1234 would be written ~-s(1234). The flux for all CA's would be ~ ~ s .

74

Fig. 1.

C. W . ALLEN

I ',

i / k /

21 I\ Illustrating an intensity scan along a solar diameter and defining Is, IL, ID, r~. rD, and L.

L. Limb brightening: If the disk is scanned radially the limb brightening could be expressed

oo

= I- IL dr, (1) L

0

where (see Figure 1) IL is in excess of I D and dr is an element of radius. If L(c0 rep- resents the variation of L with orientation ~ (measured from the equator) the flux becomes

~ L = rE j L (c 0 dc~, (2)

where rE is the effective radius of the limb (perhaps a few percent less than No). However, it may be more normal to make E-W (East-West) scans and N-S displacements. The data could then be used to determine

= f IL dA. (3) ~ L

Usually, the limb brightening vanishes at the poles and therefore ~'~L can be segregated into ~ L (E) on the East limb, ~-L (W) on the West limb, and total o~- L = ~ L (E + W).

Whether J e is determined by radial or E-W scans it must be expected (see Figure 1) that image diffusion will have the effect of sending some L flux beyond the limb where it will be added to the disk D flux (by increasing rD). A correction of flux from D back to L should be made if thought necessary (see below).

D. The quiet disk: The quiet disk near the centre is to be used as our reference intensity. The main

difficulties of evaluating ID for this purpose are (1) it is sometimes too faint, and (2) it is sometimes quite uneven. In the latter case the best procedure is to extract the active centres and then average the intensity along the solar meridian (see Section 3 for justification).


The disk radiation flux is represented with reasonable accuracy by

~ D = 7Ep2DID, (4)

where r D is the �89 radius (Figure 1). The expected value of r D is related to the scale height and may be derived from Table I. If the measured radius r m is greater than rD, the flux represented by ~ZlD(r2m--rzD) should be transferred from D to L. This correction may be difficult to estimate but should not be neglected.

S, L, D. The total f lux: The total flux is expressed

2 ~ = Z -~"s n t- ~-L(E q- W) q- ~-D"

It is directly related to the absolute measurements of XUV radiation and to XUV ionizing effects. The quiet emission .,W(quiet) is represented by the terms ~L(E + W) + + YD, and the active emission by ~ ~-s.

3. Interpretation of Limb Brightening and Polar Darkening

Although we attempt to segregate the measurements of the bright limb L from the quiet disk D it is understood that both come from the same quiet sun source. The need to determine ~ L and ~-D separately is in order to interpret the quiet emission and also to derive I o when the disk is too faint to observe. For both of these purposes it is necessary to be able to convert a model of solar emission into observable values of L, rD, YL, etc.

We adopt an idealized quiet model in which (1) the quiet XUV source is optically thin, (2) the emission is superimposed upon photospheric radiation which can be neglected, (3) the emission rate is characterized by a scale height H and a base value e, and (4) the base coincides with the visible limb. At first we will take the solar image to have circular symmetry and later we investigate polar darkening by using the form e=eo cos~b, where ~b is heliocentric latitude.

(1) Case o f circular symmetry (e = %)

The notation is illustrated in Figure 2, where it is seen that x and y differ in meaning for the rays emitted from 'within the disk' and those 'beyond the limb'. At the limb the notations agree. All distances x, y, h, R are expressed in terms of the solar radius ~ o , i.e. in disk units.

Within the disk we use No sin 0 = sin0 = R as the projected radial variable. Then

(1 + x) 2 = (sin 0) 2 + (cos 0 + y)2

x 2 + 2x = 2y cos0 + y2

y = (COS 2 0 ~- X 2 q- 2x) a/z - cos 0

(x + 1) dx = (y + cos0) dy.

76 C.W. ALLEN

Fig. 2.

centre

I within timbbey~

disk limb

to the Earth

Illustrating XUV emission from within the sun's disk and beyond the limb, and defining 0, x, y, h and R.

The XUV intensity I is o o

I = ~ e0e-X/H (x + 1) (y + cos 0) -1 dx qY

0

O0

= eo f e - X m ( x + 1)(cos2 0 + x 2 + 2x) -1/2 dx. (5) I/

0

Thus the 'within disk' intensity is available in terms of eo (the emission per steradian and per unit volume at the base of the XUV emitting atmosphere), H, and 0 (or R).

At the centre of the disk (cos 0 = 1, R = 0)

I D = ~ 0 H = the disk unit. (6)

When H is small (and therefore all emission comes from small x)

I (0) = eo H sec 0 = eoH/(1 - R2) 1/2 . (7)

This formula will apply over most of the sun (R<0.9) when H<0.1 . Beyond the limb we use R = 1 + h as the projected radial variable.

Then (R + x) 2 = R 2 + y2

2 R x + x 2 = y2

(R + x) dx = y dy

SYSTEMATIC P H O T O M E T R Y OF X U V S O L A R I M A G E S 77

and oo

I = 2eo e-h/u ~ e-Xm(R + x)y -~ dx 0

oo

= 2eo e-h/H ( e-Xm(R + x)(2Rx + X2) -1/2 dx o ]

0

= 2eoe-h/nReR/nK1 (R/H) (8)

(GRADSHTEYN and RYZHIK, 1965, p. 316). The last two terms are tabulated as Modified Bessel Functions (ABRAMOWITZ and STEGUN, 1965, p. 417). Thus the 'beyond limb' intensity is available in terms of e o, H, and R ( = 1 + h).

The expressions (5) and (8) are similar when c o s 0 = 0 , R = 1.0, h = 0 , but there is an intensity jump of a factor 2 at the limb because of the sudden penetration to the far side of the atmosphere.

Figure 3 shows the calculated distributions of the intensities I (sin 0) within the

7

6

~5

4

~3

2

1

Fig. 3.

H = 0"00

H+o,ol / H=o,o

0.2 04+ 0"6 0.8 1.0 1.2 1,/, Projected radial distance

Illustrating calculated intensity along a solar radius for H = 0.00, 0.01 and 0.10. Data agree with Table I representing a circular symmetrical image. Disk units.

9

8

7

6

5~ J

>.

~~

2

1

disk, and 1 (R) beyond the limb, for the values H = 0.00, 0.01, and 0.10. The factor 2 at the limb can be illustrated only for H=0 .10 .

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The calculated results for a circularly symmetrical sun are summarized in Table I. The intensity at the limb, I N, refers to a point just within the disk; the intensity just beyond the limb is 2 I M. Values of L, defined in Equation (1), are subdivided into: within disk, beyond limb, and total. The flux ~-L is from Equation (2) integrated around the whole circumference, The calculations are not precise.

The density scale height of the sun ranges from about 1000 km in the chro- mosphere to 100000 km in the corona. Halving these to get the emission scale height we find our range of interest of H to be from 0.001 to 0.1 (• ~| It can be seen from Table I that the observable L, ~-L, rL and r D are not very sensitive to H until this exceeds 0.10.

(2) Case o f polar darkening (e = eo cos qS). We have so far considered only the circularly symmetrical solar image. However,

this is very unrealistic in that the observed limb brightening is much greater at the equator than at the pole. In order to make allowance for this polar darkening we now take the emission to vary with heliocentric latitude ~b in accordance with

z ~0 COS ~ .

We take H to be independent of ~b and the earth's heliocentric latitude to be zero. The data of Table I, excepting O~L, will still apply at the equatorial diameter (c~ =0). Moreover, it should be sufficiently accurate to apply r L from Table I to all diameter orientations. The simplified expression (7) applicable for most of the sun's disk becomes

I (0, qS) = e0 H see 0 cos q5 (9)

On the meridian, where 0 = r (because we have taken the earth's heliocentric latitude to be zero), the intensity becomes

Io = eoH = the disk unit as in Equation (6).

We notice that in the polar darkened model Io is a constant along the meridian. This can be checked by observation and provides a satisfactory method of averaging along the meridian to derive the disk unit I D.

When Equation (9) is converted to represent the intensity along a radius-oriented ct from the equator (using sine s in0= sine) it becomes

I ( R , ~) = ~o H [(1 - R 2 sin 2 ~)/(1 - R 2 ) ] 1 / 2 . (10)

To obtain the limb intensities I M ( ,) and 2 I M (,) we can use either Equation (5) or (8) with R = sin 0 = 1 and with eo replaced by e 0 cos q5 (= e o cos,) . One can plot I (R, , ) from Equation (10)against R and then join this to I M ( ,) at the limb. The curves so obtained are illustrated for H = 0.02 in Figure 4. Numerical integrations of such curves along the radii using Equation (1) give L (~), and integrations around the whole circumference using Equation (2) give O~L(E+W ). The resulting calculated values are given in Tables II and III. It can be seen in Table III that the calculated value of L ( , ) is negative for regions within a few degrees of the pole. However, only positive measurements are made and negative ones would be recorded as zero.

80 C.W.ALLEN

~5

>,

1

0 0

I

0 ~

10 ~

20 ~

30 ~

t.O ~

5 0 *

60 ~

70 ~

80 ~

9 0 ~

o12 o'.' o16 0.8 1.o 1.~ Projected radiat distance

Fig. 4. Illustrating calculated intensity along a solar radius for varying orientation c~. Data represent polar darkening and H = 0.02. Disk units are used. Each curve is displaced by 2 units.

TABLE II Calculations of "~-L (E § W), o~D and total ~- (quiet) in relation to scale height H.

Case of polar darkening. Disk units.

H 0.00 0.01 0.02 0.05 0. l 0

o~L (E + W) 2.00 2.4 2.6 2.9 3.1 ~D 3.14 3.4 3.6 4.1 4.9 ~,~ (quiet) 5.14 5.8 6,2 7.0 8.0

The po la r da rkened mode l has p roved sat isfactory for compar i son with ac tua l

images. Differences between observat ions and calculat ions can be s tudied in re la t ion

to o ther var iables such as mean wavelength and the phase of the sunspot cycle.

Significant t rends should be recognizable. I f i t is found tha t o ~ L ( E + W ) is close to


T A B L E III

Calcula t ions of l imb intensi ty [M (ct), l imb br ightening L (c0 and disk radius rD (ct) for var ious or ientat ions a (measured f rom the equator ) and for four values o f the scale height H.

Case of polar darkening. Disk uni ts .

H 0 ~ 30 ~ 40 ~ 50 ~ 60 ~ 70 ~ 80 ~ 90 ~

I~i (~) 0.01 12.5 10.8 9.6 8.0 6.2 4.3 2.2 0.0 0.02 8.9 7.7 6.8 5.8 4.4 3.0 1.6 0.0 0.05 5.7 4.9 4.4 3.7 2.8 2.0 1.0 0.0 0.10 4.1 3.6 3.1 2.6 2.0 1.4 0.7 0.0

L(c0 0.01 0.52 0.42 0.36 0.28 0.20 0.12 0.03 0.00 within 0.02 0.50 0.40 0.34 0.26 0.19 0.10 0.03 --0.01 disk 0.05 0.46 0.37 0.32 0.24 0.16 0.08 0.02 - -0 .02

0.10 0.42 0.34 0.28 0.21 0 . 1 3 0.06 0.00 - 0 . 0 4

L (a) beyond l imb

L (~) total

r . (~)

0.01 0.21 0.18 0.15 0.12 0.09 0.05 0.02 0.00 0.02 0.28 0.24 0.20 0.16 0.11 0.06 0.02 0.00 0.05 0.41 0.34 0.28 0.21 0.14 0.08 0.01 0.00 0.10 0.53 0.42 0.35 0.26 0.17 0.08 0.01 0.00

0.01 0.73 0.60 0.51 0.40 0.29 0.17 0.05 0.00 0.02 0.78 0.64 0.54 0.42 0.30 0.16 0.05 --0.01 0.05 0.87 0.71 0.60 0.45 0.30 0.16 0.03 - -0 .02 0.10 0.95 0.76 0.63 0.47 0.30 0.14 0.01 - -0 .04

0.01 1.04 1.04 1.04 1.03 1.03 1.03 1.02 1.00 0.02 1.07 1.07 1.07 1.06 1.06 1.05 1.04 1.00 0.05 1.16 1.15 1.14 1,14 1.12 1.10 1.07 1.00 0.10 1.28 1.25 1.25 1.24 1.21 1.17 1.11 1.00

the calculations in Table II, or shows some regular relation with mean wavelength,

then YL measurements can be used to derive ~(quiet) and thereby put ~ s meas-

urements onto disk units. An evaluation of H is necessary for all applications of the models and this can be

determined from r D. However, to obtain ro from the measured r,,, one must correct for the spacial resolution of the pinhole cameras. It is clear from the SL 306 (Leicester)

image (Figure 5) that the resolution is almost entirely controlled by the pinhole size.

Thus, for a particular H, one may convolve a distribution of the Figure 4 type with

the circular distribution of a pinhole to obtain rm. Such calculations have been made

to reduce rm to r o. They gave ro = 1.15 as a reasonably consistent mean value. This

T A B L E IV

Expected values. Disk units .

rL = 0.96 Y L ( E + W) = 2.9 rD = 1.15 S D = 4,1 H = 0.05 ~7 (quiet) = 7.0

82 C.W.ALLEN

leads to H = 0.05 and to the Table IV values to be expected from a polar darkened optically thin solar atmosphere. It may be necessary to use Table IV for estimates when no better information is available.

4. Photometric Measurements

All XUV images of the sun have been obtained in conditions of some difficulty. Several unknown sources of error may exist and the problem is how to allow for these without distorting the results.

This section describes the photometry of four films containing a total of ten XUV solar images and two image spectra (spectroheliograms). The main purpose has been to measure and compare, the values of o~ s, o~ L, and o~ D from the images.

The images have been photometered at the University of London Observatory with a Hilger Recording Microphotometer. In general East-West (E-W) runs were used followed by N-S displacements between the runs. However, some early scans were along variously orientated diameters. Normally the slit width (on the film) was 0.04 ram, the slit length 0.08 ram, and the displacements about 0.10 mm.

From the definitions in Section 2 it is seen that most of the intensity and flux measurements are to be made in excess of some background such as ID (in the case of limb-brightening measurements) or Ii~ (in the case of active sources). The flux can be derived from (Ira--Ib), where Im is the maximum intensity of the feature and I b the background intensity. In order to estimate the flux of sources the approximation used was

~ S = AEWANS(/m -- lb) ,

where AEW and ANs are the EW and NS whole widths of the image at the half maximum intensity of the feature defined by

Iii2 = �89 + Ib).

For the limb flux the quantity A~w (Im--Ib) was determined for each run and then integrated along the NS displacements to give ~L.

The accuracy available from this procedure is appropriate to the problem. Since the solar images are very small (diam. 1.74 to 2.36 mm) the effects of grain and finite scanning spot can be severe. Some of the films are troubled with edge effects and some images give evidence of stray light. Photographic density-intensity curves are not always available and may be uncertain. Since inconsistent results are more likely to be due to errors than to extraordinary variations I have made compromises where they have seemed reasonable and reduced inconsistencies.

All films were obtained from Skylark rocket flights and have been labelled by the flight numbers SL 302, 303, 306. The results are recorded in the order in which the films were examined. The sources S are identified by the McMath Plage numbers and the position on the image is indicated approximately using the notation N, S, E, W =Nor th , South, East, West, C = central, L = limb.


SL 303 of April 9, 1965, at 0100 (Culham): The image studied was the zero order of a grating spectroheliogram described

by BURTON (1967, 1969). It is illustrated in figs. 8 and 9 of Burton's paper (preceding

this). It can be seen that there was a source on the limb hut none on the disk.

The relative response of the different wavelength regions could be estimated by

comparison with the SL 306 (Culham) spectroheliogram and is approximately:

30 - 100 ~ ( 1 0 ~ ) , 170 - 220,~(75~), 304 ~ ( 1 0 ~ ) .

The microphotometer scans were along diameters separated by 10 ~ of orientation.

The density-intensity curve was derived from the comparison of three spectrohelio-

grams taken with varying pinhole sizes.

The photometric results of SL 303 (Culham) are in Table V. The ~-L(E+W)

value is rather low compared with the expected 2.9 (Table IV), and this may imply

an appreciable optical thickness in the limb emission.

TABLE V

Measurements from SL 303 (Culham). du = disk units

~Q on film -- 1.16 mm rm = 1.18 ~'| Pinhole radius = 0.09 mm rD (adopted) = 1.15 ~(~

o~s (7767 NEL) = 0.49 du Y~ o~s = 0.49 du J=~ (E) = 1.07 du ~ (E + W) = 2.07 du 5L (W) = 1.00 du ~-D = 4.1 du

Y~o ~ =6.7 du

Microphotometer runs across the three spectroheliograms on the film (not all

shown in Burton's fig. 9) gave support to two observations made by eye, (1) that the

brightening of the source and the limb (relative to the disk) was no greater at 50 A

than at 180 A, and (2) that the Hen 304 & line showed no detectable limb or source

brightening.

SL 306 of Oct. 20, 1965, at 0520 (Culham): The image studied was the zero order of a grating spectroheliogram (BURTON,

1967, 1969) and is illustrated in figs. 9 and 10 of Burton's paper (preceding this).

There were three distinct sources and all other emission could be regarded as representing the limb or disk.

The relative wavelength response has been estimated from the spectroheliogram and found to be:

30 - 100 A (20~), 170 - 220 A (55~), 304 A (20~).

The microphotometer scans were along diameters spaced 10 ~ apart. The density- intensity curve derived from four spectroheliograms revealed a surprisingly high photographic contrast compared with SL 303.

84 C.W. ALLEN

The photometric results are in Table VI, where the values enclosed in parentheses are an adjusted compromise. The direct measurements, without parentheses, give

values of o~ s and f fL much lower than the Leicester X-ray (about 40 A) images from the same flight, and also values of O~L lower than the Culham images in other flights. On the other hand the SL 306 Culham spectroheliograms show no detectable

TABLE VI Measurements from SL 306 (Culham). du = disk units

~| onfilm = 1.16mm rm = 1.20 ~| Pinhole radius = 0.08 mm rD (adopted) = 1.15 N|

ffs (8018 NWL) = 0.09 (0.16) du ~ ~Ts = 0.23 (0.40) du o~s (8032 NC) = 0.07 (0.12) du ~,~L (E d- W) = 1.14 (2.0) du o~s (8029 SE) = 0.07 (0.12) du ~D = 4.1 (4.1) du o~L (E) = 0.49 (0.9) du ~L (W) = 0.65 (1.1) du Y~ Y = 5.5 (6.5) du

evidence, either by eye or microphotometry, that f f s or o~ L are greater at 50 A than at 180 A. There is therefore some justification for restandardizing the SL 306 (Culham) results on ~ ' t rather than ~'D" In the corrected results fiE(E-l-W) has been adjusted to 2.0 in rough agreement with other Culham data, and the o~ s values increased in the same proportion. These corrected values (in parentheses) should be used for systematic comparisons.

Very rough estimates of the fluxes for the Heir 304 A line may be made by comparing the corresponding features in the spectroheliograms. On the corrected scale the estimates are:

o~s(8018 ) -- (0.04); ~s(8032) -~ (0.08), f f s (8029) _~ (0.03),

"~L (E + W) ~-- (0.9), (for He lI 304 _~).

SL 306 of Oct. 20, 1965, at 0520 (Leicester): The Leicester film (RusSELL and POUNDS, 1966) recorded seven X-ray solar

images, as illustrated in Figure 5. The individual images were exposed simultaneously but they differed in pinhole size, focal length, filter, etc., the details of which are given in the original paper. It is particularly necessary to discriminate between the image groups A, C, D, which had thin aluminium filters, B, E, G which had a filter combination of polypropalene and aluminium (described as filter B in the paper), and F which had a thick aluminium filter.

From the filter transmission (PouNDS and RUSSELL, 1966) combined with the solar spectral distribution the derived ranges of response are:

A, C, D 10-45 • (95 ~o) B, E, G 8-40 A (42 ~) , 44-60 A (53 ~ )

F 6-25 A (95 ~ )

SYSTEMATIC PHOTOMETRY OE XUV SOLAR IMAGES 85

Fig. 5. Solar XUV images recorded on Oct. 20, 1965, from flight SL 306 (Leicester). Data from the five images A, B, E, F, G have been given in Table VII.

The microphotometer scans were E - W with N - S displacements. The density-

intensi ty curve was obtained from informat ion provided by K. A. Pounds and was in

reasonable agreement with the relative densities of the images. The photographic

contrast was about the same as the SL 303 (Culham) film.

Only the images A, B, E, F, G were suitable for photometry. The results set out

in Table VII are considered the best that can be derived f rom the images but some

adjustments have been made to the direct measurements. It has been noticed by

Russell and Pounds that the images A and B differed remarkably in image contrast

whereas the only experimental difference was in the filter. The contrast difference is

much more than could be expected from the change of wavelength response distribu-

t ions and it applies to both the sources and the limb. In the search for an explanat ion

TABLE VII

Measurements from SL 306 (Leicester). du = disk units

Mean Units A B E G F BEG

~| on film mm Pinhole radius mm rm ~ | rD (adopted) .~| ~-~s (8018 NWL) du o~s (8032 NC) du �9 ~-s (8029 SE) du E g s du

L (E) du ~ (W) du

o~z (E § W) du JD (adopted) du 22 g du

0.87 0.87 0.99 0.99 - 0.99 0.06 0.06 0.03 0.13 - 0.13 1.20 1.20 1.16 1.20 - 1.18 1.15 1.15 1.15 1.15 1.15 1.15 0.21 0.28 0.24 0.33 0.28 0.47 0.22 0.26 0.22 0.32 0.27 0.45 0.15 0.17 0.15 0.19 0.17 0.21 0.6 0.7 0.6 0.8 0.72 1.1 1.0 1.2 0.9 1.1 1.04 1.0 1.5 1.7 1.6 1.6 1.58 1.5 2.5 2.9 2.5 2.7 2.62 2.5 4.1 4.1 4.1 4.1 4.1 4.1 7.2 7.7 7.2 7.6 7.44 7.7

86 C.W.ALLEN

it is noticed that D has the same filter as A. Also D is superimposed by stray

light that would be below threshold if reduced in proportion the to A/D intensities.

Consequently A has now been corrected for the same proportion of stray light thus

bringing the results nearer to B. E and F being very faint images are also susceptible

to faint stray light and a correction has been made in each case that makes their ~-L

more consistent with denser images.

One apparent result from Table VII is that the source fluxes increase in the order

A:B, E, G:F . However, the high values for the shortest wavelength image F may be

much influenced by the adjustment described above. When the Leicester and Culham

SL 306 results are compared it confirms a trend that o~ s values are greater at shorter

wavelengths.

SL 302 of Dec. 17, 1964, at 0335 (Culham): The image was from a simple pinhole camera (BURTON, 1967, 1969). It is il-

lustrated in fig. 7 of Burton's paper (preceding this). There were six clear sources,

three on the limb and three on the disk.

It may be presumed that the wavelength response is similar to SL 303 (Culham).

The microphotometer scans were E-W with N-S displacements. No density-

intensity curve was available hence the curve for SL 303, which was on the same

emulsion (Path6 SC5), has been used.

The photometric results of SL 302 (Culham) are in Table VIII. As might be

TABLE VIII

Measurements from SL 302 (Culham). du = disk units

~| onfilm = 1.18mm rm = 1.22N| Pinhole radius = 0.08 mm rD (adopted) = 1.15 N| .~-s(7606 NWL) = 0.18 du ~s(7613 SWC) = 0.17 du o~s (7618 NE) = 0.09 du o~s (7617 SE) = 0.07 du ~s(7622NEL) = 0.11 du 5s (7619 SEL) = 0.10 du ~ o~s = 0.72 du o~L(E) = 0.74 du o~5(E + W) = 1.45 du YL(W) = 0.71 du YD = 4.1 du

Y~- =6.3 du

expected the source fluxes from this EUV data are less (relative to the quiet sun)

than the extracted sources fluxes from the Leicester X-ray data (RusSELL, 1965; POUNDS and RUSSELL, 1966).

5. Conclusion

The measurements given in the present paper are not extensive enough for a systematic correlation with solar activity, nor are they accurate enough to establish any physical relations with certainty. The following conclusions are tentative.


S Sources: The source emissions ~-s are systematically greater (relative to the disk

or the limb) at shorter wavelengths. There is reason to consider the changes to be

associated with a difference of excitation conditions between sources and normal atmosphere.

L Limb: The limb emission ~-i. also appears to be greater (relative to the disk) at shorter wavelengths. However, this cannot be ascribed to excitation differences and, if real, must be associated with optical depth of the emerging XUV radiation. The

short wavelength values of ~ L are close to optically thin predictions which suggests that all X-ray photons escape. A knowledge of the variation of ~ L with wavelength (or other excitation parameters) is needed if ~-L is to be used to standardize the source fluxes ~'s-

D Disk: The disk intensity is irregular and, no doubt, associated with past solar activity. In spite of this it is reasonable to adopt a mean disk intensity ID for applying the models of Section 3 and comparing the limb brightening. The measured diameters of the disk r,, have led fairly consistently to rD= 1.15 and H=0.05 .

Acknowledgements

This work may be considered as a contribution to British Rocket-borne solar astron- omy. It was stimulated by the successful images obtained by the Culham Laboratories and Leicester University, and was made possible by the loan of such images. Generous help from Dr. R. Wilson and Dr. W. M. Burton (Culham) and from Dr. K. A. Pounds (Leicester) is acknowledged. The microphotometry has been carried out with help from Miss Sh. M. A. Youssef, who is now engaged in correlating published XUV data with solar activity.

References

ABRAMOWITZ, M. and STEGUN, I. A. : 1965, Handbook of Mathematical Functions, Dover Publ., New York, p. 417.

BURTON, W. M.: 1967, Culham Laboratory Memorandum, CLM-M66. BURXON, W. M.: 1969, SolarPhys. 8, 53. GRADSHTEYN, I. S. and RYZHIK, I. M.: 1965, Tables of Integrals, Series and Products, Academic

Press, New York, p. 316, Formula 3.366 (2). POUNDS, K. A. and RUSSELL, P. C. : 1966 in Space Research, Vol. VI (ed. by Smith-Rose), Spartan

Books, New York, p. 38. RUSSELL, P. C.: 1965, Nature 206, 281. RUSSELL, P. C. and POUNOS, K. A. : 1966, Nature 209, 490.

systematic photometry of xuv solar images

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