S I M U L T A N E O U S M E A S U R E M E N T S OF E U V A N D

S O F T X - R A Y S O L A R F L A R E E M I S S I O N *

D. M. HORAN and R. W. KREPLIN

E. O. Hulburt Center[or Space Research, Naval Research Laboratory, Washington, D.C. 20375, U.S.A.

Abstract. Broadband sensors aboard the Naval Research Laboratory's SOLRAD 11 satellites measured solar emission in the 0.5 to 3 ~, 1 to 8/~, 8 to 20 ~, 100 to 500/~, 500 to 800 ~, and 700 to 1030 bands between March 1976 and October 1979. Measurements of EUV and soft X-ray emission from a large number of solar flares were obtained. Although solar flare measurements in the soft X-ray bands are continuously made and used as a standard of a flare's geophysical significance, direct measurements of flare EUV emission are quite rare. We present measurements of the X-ray and EUV emission from several flares with special emphasis on the relative EUV response associated with flares in different categories determined by 1 to 8/~ soft X-ray flux. An example of a flare exhibiting an impulsive (nonthermal) phase is included.

Since 1960 satellite-borne broadband sensors have been successfully used to observe

full-disk solar soft X-ray emission, and have produced a nearly continuous record

of reasonably well calibrated data since 1968. Over the past decade our understand-

ing of the mechanisms producing the solar soft X-ray emission has increased to the

point where we confidently use solar flare emission in the 1 to 8 ~ band as a standard

for measurement of the geophysical significance of solar flares. However, attempts

to measure the full-disk solar E U V emission using broadband sensors have not met

with the same success. Most the E U V flare data currently available were obtained

by measuring the response of the Earth's ionosphere to sudden changes in solar

emission and then calculating the solar flux required to cause the observed iono-

spheric effects (Donnelly, 1968; Donnelly and Kane, 1978). However, measure-

ments of solar E U V radiation using sensors specifically designed for that purpose

have been made by the Naval Research Laboratory 's S O L R A D 11 satellites over

the period March 1976 through October 1979. The S O L R A D 11 satellites also

carried standard broadband X-ray sensors and so we are able to present examples of the E U V emission from solar flares of the three X-ray classes.

The broadband detectors used to measure the solar E U V emission are LiF

photosensitive surfaces shielded by beryllium, tin, or indium filters to limit the

bandwidth to a nominal 100 to 500 ~ , 500 to 800 ~ , and 700 to 1030/~, respec-

tively. Details of the determination of the conversion efficiency of each detector are

given by Horan and Kreplin (1980). Figure 1 shows the'conversion efficiency versus

wavelength for the S O L R A D 11 sensors superposed on the E U V spectrum pre- sented by Donnelly and Pope (1973). However, the pre-launch efficiency determina- tions shown are no longer accurate because the sensors shared the common fate of satellite-borne E U V sensors by undergoing post-launch changes. Fortunately the

* Proceedings of the 14th ESLAB Symposium on Physics o[Solar Variations, 16-19 Semptember 1980, Scheveningen, The Netherlands.

Solar Physics 74 (1981) 265-272. 0038-0938/81/0741-0265 $01.20. Copyright (~) 1981 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

266

10-1

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10

D. M. HORAN AND R. W. KREPLIN

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Fig. 1. Conversion efficiency of the SOLRAD 11 E U V sensors as a function of wavelength and the model EUV spectrum of Donnelly and Pope (1973). Nominal bandpasses for the detectors are considered

to be 100 to 500 ~ , 500 to 800 ~ , and 700 to 1030 A.

solar EUV emission observed is quite steady over time periods of hours when solar flares are not occurring. Therefore we could divide the measured flux ~b by the steady pre-flare" value ~b0 to obtain the relative changes in the E U V emission during flares. It is probable that the conversion efficiency versus wavelength curves are altered by the post-launch changes. It is nevertheless assumed that the bandpasses remained essentially 100 to 500/~, 500 to 800 ~ , and 700 to 1030 ~ , because the atmospheric attenuation observed as the satellites entered Earth's shadow were as expected for E U V band sensitivity.

The soft X-ray measurements were made using gas-filled, metal-windowed ion- ization chambers of a type which has been demonstrated to be extremely stable over many years in orbit. Solar X-ray flux in the 0.5 to 3 Zk, 1 to 8 ~ , and 8 to 20 /~ bands was obtained from the detector responses as described by Kreplin (1961) and Dere et al. (1974).

E U V A N D S O F T X - R A Y S O L A R F L A R E E M I S S I O N 267

Each of the three EUV bands, 100 to 500 ~ , 500 to 800 ]~, and 700 to 1030 .~, was sampled every 7.5 s. Sampling intervals for the X-ray bands ranged between 7.5 and 15 s for the 0.5 to 3 ]k and 1 to 8 ~ bands, and between 15 and 30 s for the

8 to 2 0 A . The classification of X-ray flares as Class C, M, and X was proposed by Baker

(1970) and uses the peak flare emission in the 1 to 8 A band as a standard. Class C

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12 A P R I L 1977

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Ha I M P O R T A N C E 1B

X - R A Y CLASS X

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1 . 0 0

l x 1 0 - 1 . . . . . . . . . . p . . . .

1 • - 2

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1 • . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.5-3 A 1 x 1 0 - 6

P I I t t I~ I I I I I I t I I I I I I 0930 0940 0950 1000 1010 1020 1030 1040 1050 1100

T I M E (UT)

Fig. 2. Solar soft X-ray emission ~ in three bands and the ratio of the E U V flux to its pre-flare level ~b/~ho in three bands during the Class X flare of 12 April 1977. The X-ray fluxes are plotted on a

logarithmic scale and the E U V ratios are plotted on a linear scale.

2.20

2.00

1 . 8 0 cO

1 . 6 0 U3

1.40

1.20

1 . 0 0

268 D . M . H O R A N A N D R. W. K R E P L I N

flares have a peak 1 to 8 ~ flux between 1 x 10 -3 and 1 x 10 -2 ergs cm -2 S - 1 and are

considered incapable of major geophysical effects. Class M flares have a peak 1 to 8 ~ flux between 1 x 10 -2 and 1 x 10 -1 ergs cm -2 s -1 and are considered capable of

major geophysical effects. The largest flares, Class X, have a peak 1 to 8 ]~ flux which exceeds 1 x 10 -1 ergs cm -2 s -1

Figure 2 shows the E U V and X-ray emission associated with a flare of X-ray Class

X, H a Impor tance 1B which occurred on 12 April 1977 (Solar-Geophysical Data, 1977a). The upper three traces on the plot use a linear scale to show the ratio of the total emission in an E U V band, rb, divided by the pre-flare flux level, 4~0. The lower three traces use a logarithmic scale to show the X-ray flux ~b in ergs cm -a s -1. The

peak E U V emission in the 500 to 800 and 700 to 1030 ~ bands was more than double the pre-flare flux level. The peak emission in the 100 to 500 ~ band was about 55% greater than its pre-flare level. An impulsive component was present

during the early phases of the flare and is most noticeable on the 100 to 500 .~ trace although it is present on all three E U V traces. The two longer wavelength E U V bands had peak emission at the same time, but the peak emission in the 100 to 500

band may have been a few seconds earlier. The peak emission in the three X-ray

bands clearly preceded the E U V peaks by 1 to 3 rain. The peak 1 to 8 ]k flux was

0.5 ergs cm -~ s -1. The duration of the flare's E U V emission appears to be compar- able to its X-ray emission. The initial and final portions of the two shorter wavelength E U V traces are missing because the spacecraft was using a te lemetry format at those

times which did not te lemeter data f rom those two sensors. Figure 3 shows the E U V and X-ray emission associated with a flare of X-ray Class

M, H a Impor tance 1N which occurred on 11 February 1977 (Solar-Geophysical Data, 1977b). The E U V emission in the 500 to 800 ~ and 700 to 1030 ~ bands

was about 20% greater than the pre-flare flux levels. The shorter wavelength E U V band had an enhancement near 10%. The two longer wavelength E U V peaks occurred simultaneously and clearly followed the X-ray peak. The peak of the 100

to 500 ~ band is difficult to identify because of the flattened profile, but it did follow the X-ray peaks. The peak 1 to 8 ]~ flux was 0.05 ergs cm -a s -1. The flare's E U V and X-ray emission appear to be comparable in duration. The five longer wavelength traces are nearly back to their pre-flare levels 90 min after the start of the flare. The 0.5 to 3 ]~ trace stops at 22:15 because the signal dropped to the amplifier 's noise

level. The E U V emission associated with a Class C X-ray flare is shown in Figure 4.

This flare occurred on 3 August 1976. Although H a observations of the Sun were being made at the t ime of this flare, no enhancement of H a was repor ted (Solar- Geophysical Data, 1977c). This small flare caused an enhancement in the E U V 500 to 800 and 700 to 1030 ~ bands between 1 and 2%, and an enhancement less than 1% in the 100 to 500 /~ band. The E U V flare profiles are too flat to identify the t ime of peak emission but the three X-ray bands clearly peaked before the 500 to 800 and 700 to 1030 ~ bands. The flare profile for the 100 to 500 ~ band appears artificially truncated but no anomaly can be found in the exper iment operation,

EUV AND SOFT X-RAY SOLAR FLARE EMISSION 269

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1.00

1 1 F E B R U A R Y 1 9 7 7

I i l I 1 i I I I I I i I I I ] L I i [

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1.20

1.00

1.20

1.00

1 x 10 -1 ] I

1 x l 0 -2

l x 1 0 3

l x 1 0 -4

1 • -5

1 x l 0 6

Fig. 3.

5o0-8o0 ~,

100-500 ,&.

8-20/~

1 - 8 ,~

H(~IMPORTANCE 1N

X-RAY CLASS M

I I I I I I I I l I I I I I I I t I I 2130 2140 2150 2200 2210 2220 2230 2240 2250 2300

TIME (UT)

Solar soft X-ray fluxes and EUV flux ratios during the Class M flare of 11 February 1977.

telemetry, or data processing. Possibly the small size of the enhancement is respon- sible for the flattened flare profile. As in the case of the two larger X-ray flares, the duration of the E U V and X-ray flare emission was comparable. The five longer

wavelength traces all return to their pre-flare levels approximately an hour after the

start of the flare. In each of the three flare examples the 500 to 800 /k and 700 to 1030 ~ bands

had nearly identical percentage enhancements over the pre-flare flux: 110% enhancement for the Class X flare, 20% for the Class M, and 1.5% for the Class C flare. In each of the flares the percentage enhancement for the 100 to 500 ~ band was approximately half that of the other two E U V bands: 55% for the Class X flare,

10% for the Class M, and slightly over 0.5% for the Class C flare. The spectrum of Donnelly and Pope (1973) can be used to estimate the pre-flare flux level in each of the E U V bands. For a modera te level of solar activity their spectrum shows flux of 1.90 ergs cm -2 s -1 for the 100 to 500 ~ band, 0.19 ergs cm -2 S -1 for the 500 to

800 ~ , and 0.49 ergs cm -2 s -~ for the 700 to 1030/~ band. Table I shows the flare enhancement in each of the E U V bands based on the pre-flare flux levels f rom Donnelly and Pope. Although the percentage enhancement in the 100 to 500

270 D . M . H O R A N A N D R . W . K R E P L I N

c,

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I 1 I I r r I I I

I I I

3 AUGUST 1976

I I I I I I r I I I I I 1 I I

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Fig. 4.

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~ ~ 1-8 A

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H. IMPORTANCE S U B F L A R E

X-RAY CLASS C

l I I { I I I I I r r 1305 1315 1325 1335 1345 1355

TIME (UT)

Solar soft X-ray fluxes and EUV flux ratios during the Class C flare of 3 August 1976. Note that EUV ratios use a different scale than that used in Figures 2 and 3.

band was less than in the two longer wavelength bands, the 100 to 500 A band had the greatest energy flux increase during the flare because the 100 to 500 A pre-flare level is greater than that of the other two EUV bands together. The total EUV flux enhancement for each flare was comparable in order of magnitude to the total 0.5

TABLE I

Peak flare emission in ergs cm -2 s -1

Wavelength Flare X-ray class

X M C

700-1030 A 0.5 0.10 0.007 500-800 A 0.2 0.04 0.003 100-500 A 1.1 0.19 0.010

Total EUV 1.8 0.3 0.02 0.5-20 A 1.1 0.1 0.01

E U V A N D S O F T X - R A Y S O L A R F L A R E E M I S S I O N 271

to 20 ~ X-ray energy flux increase. The total E U V flare flux increased by about an

order of magnitude as the flare size progressed f rom Class C to Class M and f rom Class M to Class X. However , it must be emphasized that the flares presented are

examples and they may not be typical of their X-ray class. It is common to divide X-ray emission into impulsive (nonthermal) and gradual

(thermal) components. The Class X flare of 12 April 1977 had an impulsive component which is most easily seen on the 100 to 500 ~ trace of Figure 2.

Fig. 5.

12 APRIL 1977 I I I I 1 I I I I I I

2 00 IMPORTANCE 1B 700-1030 . ~ ~ �9 CLASS X ~ ~

1.80 / ~ 2.00

o ,o r s

0

.20 g_

~- 1.00-

L I I I I I I I I I I 1

0947 0949 0951 0953 0955 0957 TIME (UT)

EUV flux ratios during rising phase of Class X flare of 12 April 1977 plotted on time scale which enhances the impulsive (nonthermal) component.

Figure 5 shows the same flare replot ted on a scale which emphasizes the impulsive component . The dashed lines are an estimate of the contributions f rom the gradual component . The impulsive component appears almost insignificant in comparison

with the gradual component . However , based on Donnelly and Pope 's spectrum the total E U V energy enhancement in this impulsive component of the Class X flare is 0.2 ergs cm -2 s -1 which is comparable to the 0.3 ergs cm -2 s -1 peak enhancement associated with the Class M flare of 11 February 1977.

References

Baker, D.: 1970, 'Flare Classification Based upon X-Ray Intensity', AIAA Paper 70-1370, Huntsville, Alabama.

Dere, K. P., Horan, D. M., and Kreplin, R. W.: 1974, Y. Atmospheric Terrest. Phys. 36, 989.

272 D. M. H O R . A N A N D R. W. K R E P L I N

Donnelly, R. F.: 1968, SolarPhys. 5, 123. Donnelly, R. F. and Kane, S. R.: 1978, Astrophys. J. 222, 1043. Donnelly, R. F. and Pope, J. H.: 1973, 'The 1-3000 A Solar Flux for a Moderate Level of Solar Activity

for Use in Modeling the Ionosphere and Upper Atmosphere', Tech. Rep. ERL 276-SEL 25, NOAA, Boulder, Colorado.

Horan, D. M. and Kreplin, R. W.: 1980, J. Geophys. Res. 85, 4257. Kreplin, R. W.: 1961, Ann. Geophys. 17, 151. Solar-Geophysical Data: 1977a, No. 398, Part II, U.S. Department of Commerce, Boulder, Colorado. Solar-Geophysical Data: 1977b, No. 396, Part II, U.S. Department of Commerce, Boulder, Colorado. Solar-Geophysical Data: 1977c, No. 390, Part II, U.S. Department of Commerce, Boulder, Colorado.