SOFT X-RAY SPECTROSCOPIC OBSERVATIONS OF A SOLAR FLARE

(Extended Abstract 1)

M. BRUNER Org. 91-20, Bldg. 255

Lockheed Palo Alto Research Laboratory 3251 Hanover Street, Palo Alto CA 94304

Daring a sounding rocket flight on 3uly 12, 1982 we had the good fortune to observe the soft X-ray spectrum of a solar flare at the time of its soft X-ray maximum. The geometry of the observation was such that the slit of the spectrograph crossed the inferred location of the flare loop system somewhere between the footpoints. The field of view sampled by the spectrograph was small, ranging from 2 to about 15 arc seconds along the loop axis and about 25 arc seconds across the loop system. The spectrum covered the 10 to 95/~ spectral range, and included lines whose excitation temperatures range from (0.5 - 10) • 106 K. Included in the spectrum were a number of lines whose intensity ratios are sensitive to electron density. The spectrum and its identification have been discussed by Acton et al. (1985). An analysis of the density diagnostic lines by Brown et

al. (1986) showed that the plasma included within the field of view of the spectrograph could not be in pressure equilibrium unless there were temperature gradients across the magnetic field lines. The results were consistent with a constant density of 3.0 • 106 cm -3 at all temperatures between 0.5 and 3.5 • 106 K.

In this analysis, we used the temperature distribution of electron density to invert the emission measure distribution, yielding temperature distributions of emitting volume. To interpret this distribution in terms of a geometric model, we postulated the existence in the pre-flare state of a bi-polar magnetic field configuration. At flare onset, chromospheric heating occurs at the footpoints of the magnetic field lines on which the primary energy release takes place. We assumed that the heating affected a relatively small area of the chromosphere, and resulted in the establishment of a horizontal temperature gradient. The mechanism by which the heating takes place is not particularly important; the key to the idea is the introduction of a horizontal temperature gradient.

We now consider the action of the chromospheric evaporation process in the presence of a temperature gradient. Material will be injected into the corona along local field lines as in the usual models, but with the difference that both the rate of injection and the temperature of the injected material will vary with horizontal position. The result will be the development of a loop system in which there are both temperature and pressure gradients in directions normal to the loop axis. In the simplest case of a radially symmetric temperature gradient in the chromosphere, the loop system will develop as a series of concentric shells surrounding a hot central core. The magnetic field, in effect, acts like a fiber optic bundle for plasma, reproducing on any cross section, a temperature and pressure image of the conditions existing at the base of the loop.

1Paper in preparation

Solar Physics 113 (1987) 101-105, �9 1987 by D. ReidelPublishing Company.

102

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13 July 1982 FLARE MODEL

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Figure 1: Model of the flaring loop observed on 13 July 1982, showing the run of temperature and pressure in concentric shells around the loop axis.

A loop with this geometry would have spectral properties similar to those observed in our experiment. A wide range of temperatures could be observed, even though the field of view was restricted to the top of the loop. The apparent pressure imbalance is understood in terms of the magnetic field, which supplies a compensating gradient in the magnetic pressure. The observed increase in gas pressure with temperature is expected as a natural consequence of the formation mechanism. The postulated horizontal temperature gradient could be formed by any of a number of mechanisms, such as thermal conduction in the chromosphere, a spatial dependence of the rate of primary energy release, or radiative heating of the chromosphere by X-rays emitted by material already in the loop as evaporation progresses. The model is appealing in its simplicity, and it is difficult to imagine a situation in which this formation process will not occur since that would require temperature gradients to be infinitely steep.

Since we did not have X-ray image information for the July 13th data, we are

soFT X-RAY SPECTROSCOPIC OBSERVATIONS OF A SOLAR FLARE 103

unable to include the observed loop geometry in the analysis. To test the plausibility of the shell model, we have assumed that the horizontal temperature distribution in the chromosphere was radially symmetric about the loop footpoint and that the loop system consisted of a series of cylindrical shells surrounding a hot core. We developed the physical model for this geometry by integrating the differential volume distribution with respect to the shell radius, deriving the distributions of temperature and gas pressure with respect to shell radius. The results are shown in Figure 1. The outer shell radius of about 9 • 10 s cm (i.e. about 25 arc sec diameter) is reasonable for the center of a flare loop system, and is consistent with the field of view of the spectrograph.

REFERENCES

Acton, L.W., Bruner, M.E., Brown, W.A., Fawcett, B.C., Schweizer, W., and Spear, R.J.: 1985, Astrophys. J. 291,865.

Brown, W.A., Bruner, M.E., Acton, L.W., and Mason, H.E.: 1986, Astrophys. J. 301,981.

104 M. BRUNER

DISCUSSION

MOORE: How would a constant-pressure line look on your plot?

BRUNER: It would have a slope of -1. The points are not consistent with

constant pressure.

HUDSON: This is a question also addressed to Keith about the shape of the flare differential emission-measure distribution. Is it bimodal or unimodal?

BRUNER: Well, we don't have the observations to see the second peak.

HUDSON: So there's a minimum around i07 K?

STRONG: Yes, you normally see a minimum around that point. Marylin, were your fits done with a full differential emission-measure iterative program?

BRUNER: No, this is a Pottasch analysis. We tried many versions of an

iterative curve and had great trouble with stability, which we've recently overcome. The next version will have the iterative fits, which

are very similar.

RAMATY: How sensitive are the temperature determinations on the

assumption that the gases are always Maxwellians. Gases many times have

non-thermal tails, which would excite lines very easily, and I'm wondering if that would change things drastically.

BRUNER: I think that this part of the curve is suspect just for that

reason.

STURROCK: At those densities the ionization times will be about five

seconds.

BRUNER: And certainly we're looking at gradual-phase plasma here,

minutes after the impulsivephase.

ACTON: It's fair to point out that this was not a very impressive

flare. It had very little impulsive component and rose quite gradually, it was a filament-eruption flare, and you could actually see the

filament south of our slit as it was going off. We saw the peak of a broad maximum and there's no reason - although there were quite a lot

of X-rays - to think that there was an impulsive component.

UCHIDA: When you see a ragged structure like that, does that mean that the motion is also layered?

BRUNER: No, that's a fault in the spectrograph! Also one of the penalties you pay for having such a long range of wavelength is that

SOFT X-RAY SPECTROSCOPIC OBSERVATIONS OF A SOLAR FLARE 105

the system is not stigmatic, so differences along the slit turn out to

represent not positions along the solar disk, but differences in orientation of the slit on the Sun.

HIEI: When we see the NRL "overlappograph" data, it may seem like one loop but the position will be a function of line temperature. I think that the Skylab data shows not a core-like concentric structure, but simply different systems.

BRUNER: Yes, certainly, and you could interpret these data in that way too. There is no way to distinguish them from these data. My reason for liking shells is that I think I see things that look like shells!

FISHER: If this shell thing is right, if you look in increasing temperature you should see the shells merge together until there's just one? The reason you see two at low temperature is because you're seeing the part that's edge-on?

BRUNER: Yes. But we don't have enough data to see this behavior yet. But my preference for shells is not completely unfounded. If you heat very strongly at some point in the chromosphere, you're bound to establish a horizontal temperature gradient. While you may have multiple loops in the field, I claim that you're going to have a lateral temperature gradient any place that you've got heating. It's got to happen. It may be very steep, or the gradient may be so small as

to be undetectable, but we have to demonstrate that. If you have established a temperature gradient, then the guiding action of the magnetic field will lead that into a shell structure at the top. It'S like a fiber-optic bundle for plasma.

CHENG: Please. I would like to remind you that the "overlappograph" that you are talking about has a name, the XUV Spectroheliograph [Zirin guffaw]. It separates the images. It maybe should be called a "separateograph" or something like that.

BRUNER: Yes, that was an unkind term invented by people jealous of your laboratory's success!