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T. Kim, J.T. Schmelz (University of Memphis)
Solar coronal imagers like TRACE, EIT on SOHO, and SXT on Yohkoh use a ratio of images taken through different passbands to determine the plasma temperature. This standard analysis uses an isothermal approximation and is used widely throughout the solar community. The accuracy and usefulness of this method depends in part on the nature of the observed plasma and, in particular, how truly isothermal it actually is. We have investigated this aspect of the temperature analysis by folding known plasma differential emission measure distributions through the various instrument responses provided in Solarsoft. We began with noiseless, strongly peaked Gaussian distributions, which represented the close-to-ideal case of an essentially isothermal plasma. We found that the standard analysis did an excellent job of reproducing the temperatures at the peak of the input distribution, even if this value was well off the peak of the instrument response function. This neat result begins to disappear, however, when we slowly broaden the Gaussian distributions or add a second peak to the differential emission measure distributions. For example, a broadened distribution produces temperature that is shifted by a small, yet noticeable amount from the center of the Gaussian. Introducing a second peak, of equal intensity as the first one, but at a different temperature, also recreates temperature different from the peak positions. Indeed, the temperatures that the instruments see lie somewhere in between where the Gaussian peaks actually occur. In the case where one peak is significantly more dominant than the other, the instruments seem to favor the temperature of the stronger peak. All in all, our results indicate that the standard analysis struggles to provide reliable temperature values for multithermal plasma. Solar physics research at the University of Memphis is supported by NASA grants NAG5-9783 and NAG5-12096.
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Bulletin of the American Astronomical Society, 36 #2
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