Particle Acceleration in Reconnecting Current Sheets with a Nonzero Magnetic Field

Abstract

Motion of charged particles in a reconnecting current sheet (RCS) isconsidered, taking into account not only the electric field inside itbut also all three components of the magnetic field. A new solution forthe particle trajectory is found for the case of a large longitudinalmagnetic field. It allows one to find the `'critical'' value of thefield, beyond which the particle motion in the sheet becomes adiabatic.The longitudinal component in RCSs in the solar atmosphere is likely toexceed this value (typically 0.1 of the main reconnecting field forelectrons). The longitudinal field tends to counteract the effect ofthe transverse magnetic field that serves to rapidly eject theparticles out of the sheet. Hence, a longitudinal component on theorder of the reconnecting component is necessary to explain theelectron acceleration in RCSs up to 10-100 keV during the impulsivephase of solar flares. The electron acceleration length turns out to be5 orders of magnitude smaller than the RCS length, placing strongrequirements on the electric field necessary to accelerate theparticles. This indicates that it is necessary to modify the simplisticrunaway acceleration models, which ignore the magnetic fieldaltogether. Depending upon the magnetic field structure in the RCS, theenergy can reside mainly in electrons or protons. Thus, the model givesa unified description for different regimes of particle acceleration inflares.