A Fermi mechanism for particle acceleration during magnetic reconnection
James F. Drake
Univ. of Maryland and Space Sciences Lab, Berkeley
January 22, 2007, 4PM
Magnetic fields play a fundamental role in the dynamics of a variety
of plasma systems, ranging from laboratory fusion experiments to the
solar corona and astrophysical accretion discs. Observations indicate
that the dissipation or release of magnetic energy occurs in explosive
events, examples being solar and stellar flares, substorms in the
Earth's magnetosphere, and disruptions in laboratory fusion
experiments. The energy release occurs through a process called
magnetic reconnection, in which regions of oppositely directed
magnetic field self-annihilate, converting magnetic free energy into
energetic beams, high velocity flows and thermal energy. Suprisingly
large amounts of the released energy appears in the form of energetic
electrons -- up to 50% or more during solar flares. The mechanism for
energetic electron production during magnetic reconnection has
remained a mystery for more than three decades. I will discuss the
basic physics of reconnection before presenting a new model in which
electrons are efficiently accelerated through Fermi reflection in a
bath of contracting magnetic islands. A link between the total energy
content of energetic electrons and the magnetic energy released is
established. Further, solutions for energy spectra suggest that
magnetic reconnection can compete with high mach number shocks as an
accelerator of cosmic rays. The talk will review key observational
data and emphasize basic physical principles to introduce the topic to
the non-specialist.
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