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Session 70 - Solar Flares and Ejections.
Display session, Thursday, June 13
The low energy cutoff of the nonthermal hard X-ray spectrum in solar flares is commonly taken to be 20 to 30 keV. This number is needed for estimates of the energy released in nonthermal electrons, and is in fact not known due to the presence of thermal emission, which in extreme cases can extend to energies >30 keV. In order to estimate the amount of nonthermal emission present in a flare, we need to estimate the amount of high temperature (20 to >30 MK) thermal emission. Our previous work using the Yohkoh BCS and HXT assumed isothermal plasma emission, and gave unsatisfactory results. In this work we use a Differential Emission Measure (DEM) estimated from GOES and Yohkoh data, which gives a better estimate of the high temperature thermal component. Using this data, we then use the Yohkoh HXT to find any breaks or cutoffs in the hard X-ray spectrum. Note that the cutoff in the electron spectrum is always higher than the cutoff in the photon spectrum; nonthermal emission that extends downto, say 10 keV, does not imply that the accelerated electron spectrum extends down that far. We use nonthermal thick-target numerical models to estimate the difference between photon and electron cutoffs, and infer the electron cutoff from the photon spectrum by comparing the data with these models.
Program listing for Thursday