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Session 70 - Solar Flares and Ejections.
Display session, Thursday, June 13
Tripp Commons,

[70.04] Observations of Thermal Plasma in a Solar Limb Flare

J. T. Mariska, G. A. Doschek (NRL)

While the earliest phases of a solar flare are characterized mainly by nonthermal emissions, such as hard X-rays, the remainder of the event manifests itself primarily as thermal emissions at wavelengths ranging from X-rays through optical. This radiation comes from flare plasma covering a wide range of temperatures. Yohkoh\/ Soft X-Ray Telescope (SXT) observations show that in soft X-rays this emission often comes from complex emitting structures, with temperatures that vary from feature to feature. These temperatures, however, are based on filter ratios and are thus difficult to interpret for a multithermal plasma. Yohkoh\/ Bragg Crystal Spectrometer (BCS) observations also provide a temperature diagnostic for the soft X-ray emitting flare plasma. These temperatures often differ from those obtained using filter ratios, again demonstrating that flare plasma is multithermal. We report on observations of a limb flare observed on 1991 October 21 in which the temperatures measured using emission lines of Ca\,XIX and Fe\,XXV agreed, suggesting that this flare contained substantial amounts of isothermal plasma. We use SXT and BCS data from this flare to verify SXT filter ratio temperatures and to determine relative ion fractions of Fe\,XXII, Fe\,XXIII, Fe\,XIV, and Fe\,XXV for comparison with ionization balance calculations.

This work was supported by the NASA Office of Space Science and the Naval Research Laboratory.

Program listing for Thursday