Direct Acceleration at Sites of Magnetic Reconnection
Yuri E. Litvinenko
University of New Hampshire, Durham, New Hampshire, USA
Electric fields induced by the changing magnetic field at sites of magnetic
reconnection can efficiently accelerate charged particles in the solar corona.
This review begins with estimates for the electric field magnitude in flare
models and presents some of the theoretical results for the electron and
proton acceleration in reconnecting current sheets in solar flares.
Particular emphasis is placed on models for collisionless
acceleration in a large-scale reconnecting current sheet with a nonzero
magnetic field and a highly super-Dreicer electric field of order a few V/cm.
Particle orbits in model current sheets are discussed using an approximate
analytical approach that allows one to identify the effects of both the
electric and magnetic field components on the particle motion.
Formulas for the particle energy gains and acceleration times are presented.
The question of self-consistency between the current sheet particle motion
and fields is also addressed. Given a super-Dreicer electric field in
the sheet, it is the magnetic field structure in the sheet that determines
both the electron to proton ratio for the accelerated particles and their
typical energies and spectra. The analytical results form the basis for the
electric field acceleration models in solar flares.
In particular, physical conditions can be identified that lead to either
flares in which electrons primarily generate hard X-rays in the energy range
of tens of keV or flares with unusually large electron fluxes at gamma-ray
energies extending up to a few tens of MeV.