D I R E C T M E A S U R E M E N T S OF THE GRADUAL E X T R E M E

U L T R A V I O L E T E M I S S I O N F R O M LARGE SOLAR F L A R E S

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

E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, U.S.A.

(Received 8 July; in revised form 15 October, 1982)

Abstract. Broadband sensors aboard the Naval Research Laboratory's SOLRAD 11 satellites measured solar emission in the 0.5 to 3 A, 1 to 8 A, 8 to 20 ~, 100 to 500/k, 500 to 800/~, and 700 to 1030 ~ bands. Data from sixteen large flares show that the EUV emission is dominated by gradual emission which parallels the soft X-ray emission in duration and magnitude. The data are consistent with the separation of EUV and X-ray flare emission into two distinct components. A persistent component is made up of gradual EUV and gradual soft X-ray emissions. A brief component consists of hard X-rays, impulsive soft X-rays, and impulsive EUV emission.

1. Introduction

The Naval Research Laboratory's SOLRAD 11 satellites measured solar emission in the hard X-ray, soft X-ray, and extreme ultraviolet (EUV) portions of the electromagnetic spectrum. In a previous paper we used the hard X-ray and EUV emission measurements to analyze the impulsive component of solar flare emission (Horan et aL, 1982). In this paper we will use the soft X-ray and EUV emission measurements to describe the gradual component of flare emission.

Wood and Noyes (1972) concluded that EUV emission from flares without an impulsive (nonthermal) X-ray component tended to be weaker than that from flares with an impulsive X-ray component. Our observations do not show any relationship between the presence of impulsive X-rays and the magnitude of the EUV emission. We will show that the overwhelmingly dominant EUV emission from large flares is not impulsive emission and that this dominant gradual EUV emission is closely linked in magnitude and duration to the flare's gradual (thermal) X-ray emission. Almost all of the earlier direct measurements of solar EUV emission used spectrometer instruments which measured emission at specific wavelengths rather than across a broad bandwidth (Hall and Hinteregger, 1969; Hall, 1971; Wood etal., 1972). Observations by these instruments concentrated on emission lines associated with ionization stages prevalent at temperatures characteristic of the solar chromosphere and transition region. Emission from these lines was observed to rise rapidly to a peak which preceded the flare's He maximum, and then to decay rapidly. The few observations reported for lines from ions prevalent at coronal temperatures showed that these lines behaved differently. The coronal lines reached peak intensity after the flare's He maximum and had a more gradual decay than the noncoronal lines (Hall and Hinteregger, 1969; Hall, 1971). Wood and Noyes' conclusion concerning the strength of a flare's EUV emission was based on the behavior of the impulsive emission from noncoronal lines and failed to recognize the importance of the gradual emission from coronal lines.

Solar Physics 85 (1983) 303-312. 0038-0938/83/0852-0303501.50. Copyright �9 1983 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

304 D . M . HORAN ET AL.

2. Instrumentation

The measurements of soft X-ray emission were made using ionization chambers sensitive to the 0.5 to 3 .~, 1 to 8 A, and 8 to 16 A bands. These gas-filled, metal- windowed sensors have been demonstrated to be extremely stable over many years in orbit on earlier SOLRAD satellites. The conversion from detector current to solar X-ray flux in the 0.5 to 3 A, 1 to 8 ,~, and 8 to 20 ,~ bands was made as described by Kreplin (1961) and Dere et aL (1974). The broadband detectors used to measure solar EUV emission are LiF photosensitive surfaces shielded by beryllium, tin, or indium filters to limit the bandpass to a nominal 100 to 500 A, 500 to 800 A, and 700 to 1030 ~_, respectively. Due to post-launch changes in detector efficiency, absolute EUV emission values cannot be presented at this time. However, since the EUV emission was observed to be essentially constant for several hours between flares, we were able to obtain highly accurate relative changes in EUV emission during flares. By assigning a value to the pre-flare flux, we obtained an estimate of the absolute EUV energy flux during a flare. We used the spectrum of Donnelly and Pope (1973) and assigned values of 1.90 ergs cm -2 s -~, 0.19 ergs cm -2 s -~, and 0.49 ergs cm -2 s -1 to the preflare flux

levels in the 100 to 500 ,~, 500 to 800 A, and 700 to 1030 A bands, respectively. The SOLRAD 11 EUV sensors are discussed in greater detail in Horan and Kreplin (1980, 1981). Each of the three EUV bands was sampled every 7.5 seconds. Sampling intervals for the X-ray bands ranged between 7.5 and 15 s for the 0.5 to 3 A and 1 to 8 A bands, and between 15 and 30 s for the 8 to 20 A band.

3. Observations

Sixteen large X-ray flares were selected from the SOLRAD 11 data base because of the high quality of the telemetered data throughout the course of each flare, and because each flare was a single burst or very distinct multiple bursts. The flares were all X-ray Class X, which means their peak 1 to 8 A emission was at least 0.1 ergs cm -2 s -1 (Baker, 1970). The X-ray class was determined from SOLRAD-11 measurements and may differ from the designation of flare class by the National Oceanic and Atmospheric Administration's Space Environment Services Center (SESC) in that they may list some of our smaller Class X flares as Class M. This difference in classification occurs because, in converting GOES satellite data to solar X-ray energy flux, SESC assumes a solar emission spectrum which is better suited to the peak temperatures associated with solar flares than is the 2 x 10 6 K gray-body spectrum used for the SOLRAD conversions. Therefore, our peak 1 to 8 ,~ flux values are larger than SESC's by about a factor of two. The effect of the spectral assumption in converting to energy flux values is described in greater detail in Dere et aL (1974).

Figures l a - d show the broadband X-ray and EUV signatures of four of the selected flares. The lower three traces show the X-ray energy flux in ergs cm- 2 s- 1. The upper traces use a linear scale to show the ratio of the total energy flux in the EUV band, ~p, divided by the pre-flare level, tpo. The X-ray traces also use a linear scale for the energy

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best seen in the longer wavelength bands.

306 D . M . HORAN ET AL.

flux, rather than the usual logarithmic scale, for more direct comparison with the EUV emission. Measurements of 500 to 800 A emission were not available for the flares shown in Figure 1.

Figure la shows a Class X2 flare which occurred on 30 May, 1978. It was a single large burst with a rapid rise to a maximum in all bands within three minutes of flare onset and a fairly steady decay which took over an hour to return to pre-flare levels. Figure lb shows a Class X1 flare which occurred on 6 December, 1977. This flare had a much shorter lifetime than the flare of 30 May, 1978, primarily because the decay to pre-flare levels required less than twenty minutes. The jaggedness in the EUV profiles just prior to the peak is evidence of some impulsive emission during the rise to maximum, but the gradual emission is overwhelmingly dominant throughout the event. A Class X1 flare with an extremely long lifetime occurred on 13 February, 1978 (Figure lc). This flare took over an hour to reach maximum X-ray and EUV emission. Decay to pre-flare levels ranged from three hours for the 0.5 to 3 A band to over five hours for the 100 to 500 band. This flare profile has a bumpy appearance because several normal duration flares are superposed on the primary event. Data from the 700 to 1030 .~ band was not available for this flare. Figure ld shows a Class X1 flare which occurred on 16 Septem- ber, 1978. This flare appears to be a composite of the flares shown in Figures la and b. It is interesting to note that the 0.5 to 3 A emission looks like a single burst, but a second peak associated with the longer lasting component of the composite signature is more apparent for the longer wavelength bands. The peak emission at approximately 13 : 30 UT is primarily gradual emission although the EUV profiles show evidence of an impulsive emission during the rising phase.

Each of the four flares shown demonstrates that the EUV emission closely parallels the soft X-ray emission in duration. In addition, they show that the gradual EUV emission is the overwhelmingly dominant EUV emission. The four flares shown are representative of the sixteen studied. Table I lists the 16 flares and gives their X-ray classification based on the SOLRAD 11 measurements. A classification of X4 means that the peak 1 to 8 A energy flux was between 0.4 and 0.5 ergs cm -2 s- 1. The ratio of the EUV energy flux to the pre-flare base level, ~b/q~o, is given at time of peak emission for the 100 to 500 A band. Rise and decay times are given for the 8 to 20 .~ and 100 to 500 A emission. The rise times indicate the time required to go from 0.1 of the maximum emission to the maximum emission for that band. The decay time indicates the time required to go from maximum emission to half of the maximum emission. Rise and decay times were rounded off to the nearest minute. The data in Table I indicate that the EUV emission generally persists longer than the soft X-ray emission. This conclusion is borne out by close examination of the data and is most obvious for the longer lasting flares. Also the data show that the peak 100 to 500 A emission generally occurs after the peak 8 to 20 A emission. The only clear instance where the 100 to 500 emission reached maximum before the 8 to 20 A emission occurred during the flare of 6 December, 1977, Figure lb, and it is possible that the observed peak is due to the superposition of impulsive and gradual EUV emission rather than gradual EUV emission alone.

GRADUAL EUV EMISSION FROM LARGE SOLAR FLARES 307

TABLE I

X-ray class, time of peak emission, and rise and decay times of flares studied

Date SOLRAD-11 8 to 20 ~ emission 100 to 500 ~ emission class

max r ise decay max r ise decay peak (UT) (Min) (Min) (UT) (Min) (Min) qS/~po

12 Apr. 1977" X4 09 : 55 6 9 09 : 56 8 - 1.55 5 Aug. 1977" X1 14:17 4 9 14:18 5 8 1.14 6 Oct. 1977" X4 04 : 37 8 7 04 : 38 8 9 1.28 6 Dec. 1977" X1 19 : 37 2 3 19 : 36 2 4 1.21

11 Dec. 1977" X2 22:07 9 19 22:07 8 29 1.20 30 Dec. 1977" X2 04:11 2 2 04:11 3 3 1.17 8 Jan. 1978 X7 07 : 17 6 14 07 : 19 9 16 1.40

13 Feb. 1978 X1 03 : 04 90 85 03 : 20 94 98 1.26 29 Apr. 1978 X15+ 19:32 32 27 19:34 31 27 1.61 9May 1978 X4 14:44 11 19 14:46 12 20 1.24

30 May 1978" X2 19 : 29 3 12 19 : 30 4 14 1.14 llJul. 1978 X12 22:32 5 9 22:33 6 12 1.37 21 Jul. 1978 X3 19 : 12 - 35 19 : 14 19 42 1.23 16 Sep. 1978" X1 13 : 29 3 3 13 : 30 3 3 1.07 15 Oct. 1978 X4 09:45 12 21 09:46 13 38 1.17 3 Dec. 1978" X1 20 : 33 2 4 20 : 34 2 3 1.08

* 700 to 1030 It measurements obtained also.

Table I does not include information for the 500 to 800 ,~ or 700 to 1030 A bands.

Measurements of the 500 to 800 A emission were obtained for only one flare, that of

12 April, 1977. Measurements of the 700 to 1030 ]~ emission were obtained for nine

flares and they are identified by an asterisk after the date in Table I. The available

measurements of the 500 to 800 ]~ and 700 to 1030 A emission show that these bands

are essentially identical to the 100 to 500 ,~ band in time of occurrence of peak emission

and duration of enhanced emission (see also Horan and Kreplin, 1981).

Figure 2 is a plot of the maximum fractional increase in the 100 to 500 ,~ emission,

(q~ - q~o)/~bo, versus the numerical multiplier N in the flare's X-ray classification, XN.

It is assumed that virtually all of the peak emission was gradual, rather than impulsive,

emission. This assumption is reasonable because most of the flares showed no evidence

of impulsive emission in their EUV signatures. Those flares which did show evidence

of impulsive emission, such as shown in Figures lb and d, were still dominated by

gradual emission. This plot shows that a flare's peak 100 to 500 A emission tends to

increase as the flare's peak 1 to 8 ]~ emission increases. The data for the 700 to 1030

band show a similar relationship to the 1 to 8 A emission. Figure 3 is a plot of the sum

of the peak energy fluxes in the 100 to 500 ,~ and 700 to 1030 A EUV bands versus the

sum of the peak energy fluxes in the 1 to 8 A and 8 to 20 A soft X-ray bands. The energy

flux for each EUV band was obtained by multiplying the fractional flux increase,

((a - C~o)/~)o, by the assumed pre-flare reference level, 1.90 ergs cm -2 s i for the 100 to

500 A band and 0.49 ergs cm -2 s - 1 for the 700 to 1030 A band. Figure 3 demonstrates

that the gradual EUV energy flux from large X-ray flares is closely linked in magnitude

308 D . M . H O R A N E T A L .

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flux between 0.3 and 0.4 ergs cm -2 s - ~.

to the flare's soft X-ray emission, which is almost entirely gradual emission. This relationship between the peak EUV and soft X-ray energy flux may also be true for smaller flares (Horan and Kreplin, 1981).

The EUV emission parallels the soft X-ray emission so well that we became concerned that our EUV sensors were responding more to soft X-ray emission than to EUV emission. The knowledge that our EUV sensors underwent post-launch changes in sensitivity heightened this concern. Therefore, we plotted the increase in current over pre-flare level for the 100 to 500 A sensor versus the simultaneous increase in current for the 8 to 20 A sensor for each of the sixteen flares. We made similar plots for the 700

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G R A D U A L E U V E M I S S I O N F R O M L A R G E S O L A R F L A R E S 309

to 1030 A versus the 8 to 20 A sensors for the nine flares for which we had 700 to 1030 data. In every case the plot showed that the sensors were responding to different emission�9 Figure 4 shows the plot of the 100 to 500 ?t versus 8 to 20 ]1 sensor response for the long lasting, extremely large flare of 29 April, 1978, which we believe most clearly demonstrates the different response of the two sensors�9 On both axes of Figure 4 a value

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sensor had a scale value of 1.0 and the pre-flare current had a scale value of 0.

of 0 indicates the current measured prior to flare onset and a value of 1 indicates the peak current measured by the sensor�9 The observed tendency for the EUV emission peak to occur after the 8 to 20 A peak and the slower decay of the EUV measurements confirm that the EUV sensors are not responding to 8 to 20 A emission, and also indicate that they are probably responding to emission whose wavelength is longer than 20 ]i. We also plotted the increase in current over the pre-flare level for the 100 to 500 ?i sensor versus the simultaneous increase in current for the 700 to 1030 A sensor for the nine flares with 700 to 1030 it data. Again the plots demonstrate that the two sensors are responding to different emission and the clearest example is presented in Figure 5. Since the data indicate that both EUV sensors are responding to emission with wavelength greater than 20 A, but not to the same emission, it is reasonable to assume that the sensors are measuring the emission they were designed to measure�9

4. Discussion

The present observations of EUV and soft X-ray flare emission can be related to observations of a number of flares during the 1973 Skylab missions, in particular the flare of 15 June (Widing 1975; Doschek etaL, 1977; Widing and Dere, 1977), the

310 D.M. HORAN ET AL.

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Fig. 5. P lo t o f the c u r r e n t g e n e r a t e d by the f lare in the 700 to 1030 A senso r ve r sus the c u r r e n t

s i m u l t a n e o u s l y g e n e r a t e d by the f lare in the 100 to 500 ,~ sensor . T h e d a t a we re n o r m a l i z e d as for

F igure 4.

9 August flare (Dere and Cook, 1979), and the 5 September flare (Cheng and Widing, 1975; Dere et al., 1977). These flares display a pattern in which the flare structure just prior to He maximum is characterized by hot (107 K) loops which connect ribbons seen in emission lines formed over the 104 K to 106 K temperature range. The emission from the ribbons increases and decays rapidly. Then, as the temperature in the hottest loops begins to decrease, the regions between the ribbons become filled with loop structures which emit prominently in lines formed between 3 x 105K and 3 x 10 6 K. It is reasonable to associate impulsive EUV and impulsive soft X-ray emission with the ribbons observed by Skylab to be in the 10 4 K to 10 6 K temperature range. Also, the gradual EUV and gradual soft X-ray emission could be associated with the loop structures which fill the regions between the ribbons as the flare progresses. The l 0 7 K loops would contribute strongly to the soft X-ray emission, and probably to the EUV emission as well. Unfortunately the Skylab observations did not include the very early phases of the flares.

The response of the X-ray detectors to emission from a thermal plasma described by a typical theoretical emission spectrum is fairly well understood. Essentially, the X-ray detectors are sensitive to emission from plasmas at temperatures above 2 x 10 6 K , with the shorter wavelength detectors more sensitive to the emission from hotter plasmas. The spectrum in the bandpass of the EUV detectors is formed by line and continuum emission from plasmas spanning a much larger temperature range, roughly 2 x 10 4 K to 2 x 107 K (Dere, 1979). In addition, the spectrum of Donnelly and Pope (1973) was not meant to represent the flare spectrum. Thus there is considerably more uncertainty in interpreting the response of the broadband EUV detectors in terms of plasma temperatures in the source region.

GRADUAL EUV EMISSION FROM LARGE SOLAR FLARES 311

The separation of solar flare emission into two components has been suggested for X-rays by de Jager (1965) and for EUV emission by Kelly and Rense (1972). Hard X-ray, soft X-ray, and EUV data from SOLRAD-11 support the concept of two components of flare emission. Gradual soft X-rays and gradual EUV emission form one component and impulsive EUV emission, gradual and impulsive hard X-rays, and impulsive soft X-rays form the second component. Various terms, e.g. thermal, non- thermal, quasi-thermal, gradual, impulsive, have been used to identify the components. The terms thermal and nonthermal are not completely satisfying as distinguishing labels because a credible model identifies the source of impulsive EUV emission as thermal plasma which has been heated by nonthermal electrons. On the other hand, after separating the hard X-ray emission into gradual and impulsive components, we concluded that the gradual hard X-ray emission was linked with the impulsive hard X-ray and impulsive EUV emission and was not related to the gradual EUV emission (Horan et al., 1982). Since the gradual soft X-ray and gradual EUV emissions last much longer than the hard X-ray and impulsive emissions, we will distinguish the two components by the terms persistent and brief.

Although we divided the flare emission into two components, there must be some link between the components because their characteristic emissions follow a definite sequence when both are clearly present. The brief emission starts after the persistent emission, but before the persistent emission reaches a maximum (Horan et al., 1982). Although it is certain that emission from both brief and persistent components is present in some flares, it is not known whether both components are present in all flares. Brief emission is identified with certainty by the presence of hard X-rays or enhanced noncoronal EUV lines. Therefore, with broadband soft X-ray and EUV measurements alone, we must use the overall profile of the flare's EUV signature as an uncertain indication of the presence of brief emission.

5. Summary and Conclusions

Our data show that the dominant EUV emission from large solar flares is gradual emission and that this gradual EUV emission parallels the gradual soft X-ray emission in duration and magnitude. Data presented by Kelly and Rense (1972) show that the impulsive emission may be the dominant EUV emission in some large flares. In an earlier paper (Horan et aI., 1982) we concluded that the gradual EUV emission is not related to the impulsive EUV emission. Direct, broadband measurements of EUV emission were also made by Kelly and Rense (1972) and they presented a model of EUV flare emission in which the impulsive phase of a flare lasts for only an extremely small fraction of the total lifetime of the flare. However, they did not relate EUV emission and soft X-ray emission. In general the broadband EUV emission reaches a maximum later than the soft X-ray emission, and decays more slowly.

Measurements of hard X-ray, soft X-ray, and broadband EUV emission by NRL's SOLRAD-11 satellites are consistent with the concept of two components of flare emission. Because of their parallel development and decay, the gradual soft X-ray and

312 D . M . HORAN ET AL.

gradual EUV emissions form one component which we have designated the persistent

component . The impulsive EUV, gradual and impulsive hard X-rays, and any impulsive

soft X-rays form a second component , which we have labeled the brief component .

When both components are present the persistent component is observed first, but the

brief component begins before the persistent component has reached a maximum.

Acknowledgements

We wish to express our appreciation of the fine work performed by the Naval Research

Laboratory 's Space Systems Division in designing and constructing the S O L R A D 11

satellites. We are indebted to the personnel o f the Blossom Point Satellite Tracking

Facility for their vigilance in tending the spacecraft, to N A S A for their assistance with

data acquisition, and to N R L ' s Space Environment Da ta Analysis Center for processing

and cataloging the huge amount of data telemetered. We are also indebted to R. G.

Taylor o f N R L ' s Combust ion and Fuels Branch for his care in preparing the EUV

sensors for flight. We acknowledge the fine efforts o f G. E. Leavitt, K. O. Hayes, and

R . G . Reynolds in designing and integrating the EUV and soft X-ray experiments.

Finally we acknowledge the considerable and comprehensive contributions of E. W.

Peterkin as S O L R A D Project Manager.

References

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