Thermal Imaging of Multi-Temperature Flare Plasma with RHESSI Visibilities

Solar flares contain thermal plasma at multiple temperatures. Observations with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that the most intense (GOES X-class) flares commonly achieve maximum temperatures of ~30-50 MK. Recent analysis reveals that in such flares, the "super-hot" (>30 MK) plasma exists simultaneously with, but separately from, the ~10-20 MK plasma observed in nearly all flares; the X-ray spectra are well-fit by two distinct isothermal components. RHESSI images at multiple energies can show differing morphologies and/or centroid positions, as each thermal component contributes a different but varying amount to the flux in each energy band, making it difficult to separately characterize the spatial properties of the multiple emitting plasmas. We present a novel technique to visualize the individual thermal plasmas by combining RHESSI X-ray visibilities using the precise but spatially-unresolved spectra and model fits, isolating the contributions from each thermal component to effectively image as a function of temperature rather than energy. This technique is computationally inexpensive and generalizable to an arbitrary number of thermal components, e.g. a binned differential emission measure model of the temperature distribution. We apply this technique to the 2002 July 23 X4.8 event to show that the super-hot and cool plasmas are spatially distinct and to examine their evolution throughout the flare. We then discuss the implications for plasma heating in large solar flares.

FIGURE CAPTION: Image contours of the two individual thermal components in the 2002 Jul 23 X4.8 event, derived using our technique; the non-thermal contours are shown for reference. (Figure not shown in ADS abstract.)