Particle acceleration in reconnecting current sheets with a nonzero magnetic field

Motion of charged particles in a reconnecting current sheet (RCS) is considered, taking into account not only the electric field inside it but also all three components of the magnetic field. A new solution for the particle trajectory is found for the case of a large longitudinal magnetic field. It allows one to find the ''critical'' value of the field, beyond which the particle motion in the sheet becomes adiabatic. The longitudinal component in RCSs in the solar atmosphere is likely to exceed this value (typically 0.1 of the main reconnecting field for electrons). The longitudinal field tends to counteract the effect of the transverse magnetic field that serves to rapidly eject the particles out of the sheet. Hence, a longitudinal component on the order of the reconnecting component is necessary to explain the electron acceleration in RCSs up to 10-100 keV during the impulsive phase of solar flares. The electron acceleration length turns out to be 5 orders of magnitude smaller than the RCS length, placing strong requirements on the electric field necessary to accelerate the particles. This indicates that it is necessary to modify the simplistic runaway acceleration models, which ignore the magnetic field altogether. Depending upon the magnetic field structure in the RCS, the energy can reside mainly in electrons or protons. Thus, the model gives a unified description for different regimes of particle acceleration in flares.