fl184.mariska03 Posted: 7-Aug-94 Updated: 13-May-95, 02-Aug-95, 7-May-96, 27-Jan-97 Events specified: Flares of 8-Aug-92 22:27 (occulted), 5-Oct-92 9:28 (occulted), 4-Oct-92 ? (comparison), 27-Sep-93 12:10 (comparsion), 28-Jun-92 14:25 (comparison)
J. T. Mariska (NRL), R. D. Bentley (MSSL), and T. Sakao (NAOJ)
The evidence is now overwhelming that in many flares significant hard and soft X-ray emission is produced in compact regions near the tops of flaring loops. This immediately raises the issue of whether the emitting source at the top has different physical characteristics that those at the loop footpoints. We plan to examine a small number of limb flares to determine the physical characteristics of the looptop emission using the BCS. Our primary goal is to determine whether this emitting structure is different from the footpoint source. We plan to do this by examining the Yohkoh HXT, BCS, and SXT data for a few limb flares which have the loop footpoints occulted by the Sun. For comparison we will also study a few limb flares in which the footpoints are also visible. The HXT data will yield hard X-ray maps of the flare site, SXT data will provide the soft X-ray context and some information on the physical parameters of the flare, and BCS data will provide detailed temperature, emisssion measure, and turbulent broadening evolution in Ca XIX, Fe XXV, and possibly S XV.
Our tentative list of flares to be studied includes:
8 Aug 92 22:27 (occulted) 5 Oct 92 9:28 (occulted) 4 Oct 92 ? (comparison) 27 Sep 93 12:10 (comparsion)28 Jun 92 14:25 (comparison)
We are still looking for other possible events.
Update 27-Jan-97
A paper entitled Observations of Thermal Plasma in a Solar Limb Flare by Mariska and Doschek has been submitted to ApJ.
Abstract: All but a small portion of the top of the 1991 October 21 flare was occulted by the solar limb. Thus both the Yohkoh Bragg Crystal Spectrometer (BCS) and Soft X-Ray Telescope (SXT) observed only a small region of flaring plasma. Using observations from those instruments, we have determined the temporal behavior of the emission measure, temperature, and nonthermal broadening in the looptop source. We find that for a portion of the decay phase of this flare the temperatures determined using the Fe XXV, Ca XIX, and S XV BCS channels and SXT filter ratios all agree. Assuming that the plasma is isothermal, we determine the abundances of Fe XXII, Fe XXIII, and Fe XXIV relative to Fe XXV. Comparing the results with recent ionization equilibrium calculations, we find that there are discrepancies between the Fe XXII to Fe XXV ratio, the Fe XXIII to Fe XXV ratio, and the Fe XXIV to Fe XXV ratio and ionization equilibrium calculations, which we attribute to uncertainties in the ionization balance calculations.
A list of new limb flare candidates for futher analysis is currently be constructed. Once this is completed, Yohkoh data on selected flares will undergo a detailed analysis.
Update 7-May-96
Analysis continues on a limb flare observed on 1991 Oct 21. This flare shows evidence for most of the plasma being isothermal for a portion of the event. Thus it provides a possible test of how well temperatures determined with the BCS agree with those determined from the SXT. In addition, if the flare plasma is mostly isothermal, BCS observations can be used to investigate ion abundances for several ions of Fe. A preliminary report of the results of the study will be presented at the AAS meeting in Madison Wisconsin in June and a paper is in preparation.
In addition, I have secured two years of funding from NASA to continue investigating occulted and nonocculted limb flares in order to understand better the differences between the looptop and footpoint emitting sources. That work will begin with a fresh survey of all the limb flare data to select suitable flares.
Update 02-Aug-95
A paper based on the work done to date on this topic will shortly be submitted to The Astrophysical Journal.
Title: Hard and Soft X-Ray Observations of Solar Limb Flares
Authors: J. T. Mariska, T. Sakao, and R. D. Bentley
Abstract: Using observations from the Yohkoh Bragg Crystal Spectrometer, Hard X-Ray Telescope, and Soft X-Ray Telescope, we have examined eight limb flares. Four of the flares have the footpoints occulted by the solar limb. We find that the occulted flares generally have softer hard X-ray spectra and smaller peak values of the nonthermal broadening velocity than nonocculted flares. All other physical parameters show no differences between occulted flares and nonocculted flares.
We also plan to continue working on additional flares. I will update the topic when additional progress has been made.
Update 13-May-95
We have now analyzed BCS data for eight limb flares, some occulted and some nonocculted. HXT maps have been made for most of these events, both to verify that the footpoints are occulted and to examine the evolution of each flare in hard X-rays. An initial report on the results will be presented at IAU Colloquium No. 153 and at the Solar Physics Division meeting of the American Astronomical Society. A paper describing the results is in prepartion.