Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsp44a..03c&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SP44A-03
Physics
7507 Chromosphere, 7509 Corona, 7514 Energetic Particles (2114), 7519 Flares, 7554 X-Rays, Gamma Rays, And Neutrinos
Scientific paper
Most flares show strong soft X-ray (SXR; <20 keV) emission from a hot looptop source and hard X-ray (HXR; >20 keV) emission from footpoint sources. The latter is generally interpreted as non-thermal bremsstrahlung from downward-accelerated coronal electrons impacting the denser chromosphere, where they deposit most of their kinetic energy as heat. Heated chromospheric material then rises and fills the flare loop - a process known as chromospheric evaporation (CE). Evidence for CE is given by crystal spectrometer observations of blueshifted spectral line emission from footpoints, suggesting upward motion of hot evaporated plasma. CE has often been thought to be the primary source of thermal looptop plasma. In large (GOES X-class) flares, non-thermal energy deposition can be as much as ~1e29~erg/s. For typical non-thermal electron energies, chromospheric densities, and footpoint areas, this input power should yield ~5-20~MK CE material at the footpoints, but with an emission measure calculated at ~1e50~cm- 3. Such bright thermal emission should be clearly visible in RHESSI imaging spectra of large flares, but no such emission has yet been observed. This suggests that while CE may indeed be the primary source of thermal loop plasma, heating by non-thermal electrons may be less efficient than once believed. A heating efficiency of only ~10-50% would result in a CE emission measure of only ~1e48~cm-3, which would be more consistent with observed footpoint fluxes. We present an analysis of selected RHESSI flares, ranging in GOES class from ~M1 to ~X17. We employ imaging spectroscopy to obtain spectra of spatially-separated footpoints and looptop sources to determine the strength of the thermal components in each. The time evolution of the thermal footpoint signatures is compared to that of the HXR emission as well as to the looptop thermal emission and temperature. We calculate the energy contained in non-thermal electrons, the thermal energies at the footpoints and the looptop, and discuss the implications for the efficiency of heating by non-thermal electrons and the contribution of chromospheric evaporation to the flare thermal plasma.
Caspi Amir
Krucker Sa"m
Lin Robert P.
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