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H.D. Winter III, J.T. Schmelz (University of Memphis), J.L.R. Saba (Lockheed Martin)
Understanding features and phenomena on the Sun requires knowledge of the basic plasma parameters, such as composition, temperature, emission measure, electron density, filling factors, and their distributions. Establishing the distribution of emission measure (the amount of emitting material) with temperature is the first step needed to proceed with most of the interesting physics of active regions -- including heating processes, cooling timescales, and loop stability. The reliability of emission measure distributions derived from spectroscopic observations usually depends upon the validity of the assumptions about the absolute elemental abundances, ionization fractions of the emitting ions, and the electron density. Inaccuracies in the electron density assumptions can lead to emission measure distributions that do not correctly describe the observed plasma.
To model the temperature distribution of NOAA Active Region 7563, we have combined broad-band filter data from the Yohkoh Soft X-ray Telescope (SXT) with simultaneous spectral data of iron lines from the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) taken during its flight on 1993 August 17. We have used a forward-folding technique to determine an emission measure distribution of the active region loops using different assumptions for the electron density while holding other assumptions about the plasma constant. We have found that: (1) Assuming an electron density of 109 cm-3 yields a good degree of agreement between theoretical and observed results. (2) With an electron density of 1010 cm-3 it becomes impossible to derive an emission measure with good agreement between theoretical and observed results.
The results of our multithermal analysis imply that an average electron density of 109 cm-3 is a satisfactory assumption for the plasma of NOAA Active Region 7563 as viewed by the SXT and the SERTS instruments. Solar physics research at the University of Memphis is supported through NASA grant NAG5-7197.
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