Adv.Spme Rev. Vol. 17, No.4/5,pp.(4/5)101_(4/5)104, 1996 1995 COSPAR

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COMPARISON OF SOLAR FLARE EMISSION MEASURES FROM BROADBAND SOFT X-RAY AND ULTRAVIOLET SPECTROGRAPH OBSERVATIONS

J. W. Cook, K. Waljeski, D. Moses and G. E. Brueckner

E. 0. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 203755352, U.S.A.

ABSTRACT

Joint observations of a solar flare were obtained by the AS&E Imaging X-ray Telescope and the NRL High Resolution Telescope and Spectrograph (HRTS). We compare emission measures from soft X-ray and HRTS data. A small isolated X-

ray loo g

close to the HRTS slit position has an emission measure ne 2A~ of

3.5~10~ cmD5, compared to an emissio2 measure of 2.7~10~~ crne5 obtained from the intensity of flaring Fe XXI 1354 A plasma along the HRTS slit.

INTRODUCTION

Joint observations of a solar flare were obtained by the AS&E Imaging X-ray Telescope and the NRL High Resolution Telescope and Spectrograph (HRTS) on 21 November 1990 in coordinated sounding rocket launches. The collaborative observations provide coverage over a wide temperature range from the temperature minimum through coronal and flare temperatures, although over a short time period limited by the sounding rocket flights. The original goal of this collaborative effort was to obtain high spatial resolution observations of a large, complex active region. The occurence of flaring activity during the time of launch allowed a small X-ray class C flare to be observed. In this paper we concentrate on only one aspect of the flare observations, a quantitative comparison of the emission measure of the high temperature flare plasma. Reliable diagnostic information provides the foundation for all quantitative analysis of the flare phenomenon.

INSTRUMENTS AND OBSERVATIONS

The soft X-ray payload consisted of a high spatial resolution X-ray telescope with a fused silica grazing incidence mirror of the Wolter Schwarzschild type I design. X-ray observations were obtained with a polypropylene filter and with a thick and a thin beryllium filter. The beryllium filters sample emissions from hotter plasmas than the polypropylene filter views. The final spatial resolution is film limited, and appears to be approximately 3 arc sec. Further information on this payload can be found in Moses et al. 111.

The HRTS instrument consisted of a 30 cm Cassegrainotelescope, a broadbgnd spectroheliograph tuned to a wavelength region (10 A FWHM) around 1600 A, a stigmatig slit spegtrograph which covered a wavelength range from just below La 1215 A to 1700 A along a 920 arc set length slit, and an Ha imaging system. The spatial resolution obtained on this flight was near 1 arc sec.

The X-ray payload was launched on 21 November 1990 at 18:15 UT from White Sands, while the HRTS instrument was launched at 18:40 UT. In addit ion, groundbased observations were obtained at a number of sites, including magnetograms from NSO/Kitt Peak and videomagnetograms and Ha images from Big Bear Solar Observatory. The target area was the trailer sunspot complex in a large active region, NOAA AR6368, at N18W19 on the solar disk. Figure 1 shows a composite of images from the flaring area. Magnetograph images showed a “tongue” of leader polarity approximately 1 arc min x 5 arc set in size extending back into the opposite polarity trailer spot penumbra. This active

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(4/5)102 J. W. Cook et al.

region was the site of repeated class C X-ray flaring from the night before $he launch through the flights of both sounding rockets. X-ray and HRTS 1600 A and Ha images show that this region consisted of an arcade of highly sheared, low lying chromospheric loops spanning the magnetic neutral line associated with the intrusive "tongue" feature. This area was the site of flaring activity at all temperatures. Larger, overlying loops connected this region to the leader polarity region. Additional small scale loop structures were present in Ha and 1600 A images near this site, but were not present in the X-ray image. All observations indicate flaring in small loop structures, seen over a range of temperatures, which are closely associated in position.

DIAGNOSTICS OF FLARE TEMPERATURE PLASMAS

We compare the soft X-ray observations with the HRTS Fe XXI observations for information on the high temperature plasmas in the flaring active region. The X-ray observations can be used to find temperatures and emission measures from intensities in images through broadband filters having different spectral distributions. We show in Figure 2 the diagnostic curves for the ratio of intensities through several pairs of the soft X-ray filters as a function of the plasma temperature, and the emission measure per unit intensity as a function of temperature for these same individual filters. The sensitivity of the filter intensity ratios with increasing temperature flattens above 8~10~ K, and only a lower limit can really be placed on the temperature above this. The emission measure is determined from the intensity through an individual filter, assuming that the emission comes from an isothermal plasma at the temperature determined from the filter ratio. Even though we may find only a lower bound temperature for plasmas hotter than 8~10~ K, the emission measure is less sensitive to the exact temperature value at higher temperatures (see Figure 2). Temperature and emission measure maps over a 120 x 120 arc set field covering the flaring active region are shown in Figure 3.

E*;

10000

c i?

100

10

I 1

E+‘g6 6 62 64 66 66 7 72

Log Temperature

Fig. 2. Temperature and emission measure diagnostic curves from soft X-ray polypropylene (PP) and beryllium (Be) filter intensities.

The wavelength coverage of the HETS spectrograph includes the high temperature flare line of Fe XXI at 1354.08 A, characteristically emitted by an 11 x lo6 K plasma (Figure 4). The emission measure can be more directly determined from line than from broadband observations, and in addition yields diagnostic information from the line profile such as Doppler velocities and nonthermal broadening. We calculated the emission measure ne2AL of the plasma at T = 11 x lo6 K from the Fe XXI 1354 i intensity, 3.92 x 10-s

using the coronal iron abundance of Meyer 121, the ionization balance for iron of Arnaud and

Rothenflug /3/ giving a maximum ionization fraction of 0.23 at T = 11~10~ K. and the value; A,1 = 6190 s-l, and (nu/nion)/ne 2 7.1 x lo-l4 cm3 for ne = (l-

, of Mason et al. 141. For an Fe XXI intensity of 991 ergs cmm2 , the emission measure is 2.7 x 10zg cmD5.

Comparison of Sotar FIare Emission Meannes

Fig. 1. Flaring area in AR6368 (scale 4 arc min in height). Ritt Pe magne !$og ,ram; X-ray polypropylene filter image; X-ray Be filter image; 1600 Ai mage; HRTS Hn imagei HRTS slit spectrum showing the wavelengt conta iini ng the Fe XXI 1354 A flare temperature emission line.

ak HB

ht !TS .egi on

Fig. 3. Temperature and emission measure maps of the soft X-ray flaring plasr na. Temperatures from the PPlthin Be diagnostic, and emission measures from the thin Be image assuming temperatures from the previous map.

(4/5)104 I. W. Cook et al.

The X-ray data covers the complete active region, while the HRTS spectrograph slit views a 0.5 arc set cut through this area, intersecting only one complex of flaring activity. It is encouraging that the values found for the emission measure at similar temperatures and locations from the X-ray broadband diagnostics and the Fe XXI line calculation are reasonably close. However, we point out again that the two measurements are 25 min apart in time.

Fe XXI 1354.1 FROM HRTS Vll

‘5wr-j- Jaw _

2500 _

h

..; . .,... 0 *@!d,LblJ I346 ,350 1 552 1354 1356

WAKLENGTH (A)

I,!

Fig. 4. Profile of the Fe XXI 1354 f line in the flaring area. The dashed line shows the fit for the parameters given. The Fe XXI line and the blended C I line are fit by two gaussian profiles.

DISCUSSION

HRTS observations show ghat the hottest flare plasma is concentrated in small regions of Fe XXI 1354 A emission which are cospatial or close to narrow Ha ribbons seen in the HRTS Ha images. The Ha ribbons are composed of a series of small, bright loops which bridge a narrow neutral line seen in the groundbased magnetograms. Fe XXI emission occurs along the HRTS slit at locations where the slit intersects one of the Ha loops. A$ the same slit locations, transition region spectra of C IV 1548 and 1550 A (lo5 K) show strong nonthermal line broadening and complex Doppler shifts. The narrow ribbons are also cospatiai with the intense flaring X-ray emission. The line width of the Fe XXI 1354 A line shows nonthermal broadening of 45 km s-l, but no Doppler shift of the peak. The footpoints of larger coronal X-ray loops are cospatial with the small HQ loops. This is consistent with a picture of low lying emerging flux reconnecting with pre-existing coronal loops, producing the high temperature flare plasma in a current sheet above the smal emerging chromospheric loops, similar to the flare model of Heyvaerts and Priest 151.

REFERENCES

1. D. Moses, J.W. Cook, J.-D.F. Bartoe, G.E. Brueckner, K.P. Dere, D.F. Webb, J.M. Davis, J.W. Harvey, F. Recely, S.F. Martin, and H. Zirin, Ap. J., 430, 913 (1994).

2. J.-P. Meyer, ApJS, x, 173 (1985).

3. M. Arnaud and R. Rothenflug, AA Supplement, 60, 425 (1985).

4. H.E. Mason, G.A. Doschek, U. Feldman, and A.K. Bhatia, AA, 73, 74 (1979).

5. J. Heyvaerts and E.R. Priest, Solar Phys., 47, 223 (1976).