Computer Science
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...1e...6b&link_type=abstract
Cosmic Particle Acceleration, 26th meeting of the IAU, Joint Discussion 1, 16-17 August, 2006, Prague, Czech Republic, JD01, #6
Computer Science
Scientific paper
We use RHESSI hard X-ray observations to constrict electron acceleration in solar flares, generally considered as a primary recipient of the released energy. X-ray sources in the corona have been previously discovered and tentatively associated with bremsstrahlung emission of the acceleration site. RHESSI imaging spectroscopy can temporally resolve the non-thermal spectrum of the coronal source for the first time. We compare the time behaviour with the predictions of stochastic acceleration, as described by transit-time damping of MHD turbulence excited by reconnection. The results in 5 limb events indicate soft-hard-soft (shs) behaviour of the coronal source emission in the course of an X-ray peak (the more intense, the harder the spectrum). The shs behaviour thus constitutes a conspicuous property of the acceleration process. The temporal behaviour of the spectrum can be quantitatively described a pivot point near a photon energy of 20 keV at which the spectrum remains constant in time. We solve a diffusion equation for the interaction of waves and particles including trapping, escape and particle replenishment. The solution yields a spectrum that is approximately a power-law in the observed range of energies, having a spectral index in the observed range. However, the theoretically derived pivot point is generally at energy lower than observed. For this reason we include transport effects, such as produced by an electric potential, or scattering in the coronal source (trapping) to bring the pivot energy up to the observed value. Escaping particles propagate to the base of the loop in the dense chromosphere. These precipitating particles are identified as origin of the observed hard X-ray footpoints. Observations and simulations show that solar flare electron acceleration is a process that involves interactions with the connecting loop. The observations are consistent with stochastic acceleration in a relatively dense medium (up to 10^11 cm^-3) and high wave energy densities (up to 0.001 magnetic). The comparison also constrains the global flare geo-metry, requiring coupling between the coronal source and the footpoints.
Battaglia Marina
Benz Arnold O.
Grigis Paolo C.
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