Other
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...1e...8k&link_type=abstract
Cosmic Particle Acceleration, 26th meeting of the IAU, Joint Discussion 1, 16-17 August, 2006, Prague, Czech Republic, JD01, #8
Other
Scientific paper
Solar flare electrons are accelerated into a quasi-thermal population of tens of keV. X-ray and γ-ray observations also imply long tails in both electron and ion populations, reaching tens of MeV and above. Simple estimates indicate that virtually all available electrons are affected, pointing to an initial bulk heating process rather than acceleration of a small or localized population. Flares appear to occur in conjunction with magnetic reconnection. However, since the reconnection site (diffusion region) only makes up an insignificant volume compared to flare loop dimensions, it is unlikely that reconnection itself is directly responsible for the observed massive heating. Recently, we have explored a model in which the first step takes place along the discontinuities that bound the reconnection inflow/outflow in a Petschek geometry. Traditionally, these are considered to be slow shocks close to the switch-off limit. The usual argument in solar MHD descriptions of slow shocks is that the plasma is ~isothermal, with most of the magnetic energy going into the reconnection outflow. However, from our earlier simulations of low-beta reconnection and from recent observations in the solar wind and at the Earth's magnetotail, we know this is not what happens. In an ion-kinetic plasma, upon entry into the discontinuity, ions are accelerated into beam-like, field-aligned distributions with ~Alfvén speed. The outflow thus harbours counter-streaming ion beams that eventually thermalize, with approximately half of the available energy going into thermal heating - in fast reconnection implying ion energization by a factor of ~0.5/β. In the corona this is very significant, with resulting energies easily at and above the observed thermal electron population. We show a set of new hybrid simulations (kinetic ions, fluid electrons) that is in agreement with the above scaling with β. We have evaluated the resulting ion distributions and explored the ensuing instabilities. We discuss their role in forming high-energy tails and in heating the electrons. Our work suggests a two-stage acceleration process in which the ions play the primary role both in initial heating, and in providing the free energy for electron heating and high-energy tail formation.
Krauss-Varban Dietmar
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