A magnetic reconnection mechanism for ion acceleration and abundance enhancements in impulsive flares

The acceleration of ions during magnetic reconnection in solar flares is explored with simulations and analytic analysis. Ions crossing into Alfvénic reconnection outflows can behave like pickup particles and gain an effective thermal velocity equal to the Alfvén speed. However, with a sufficiently strong ambient out-of-plane magnetic field, which is the relevant configuration for flares, the ions can become adiabatic and their heating is then dramatically reduced. The threshold for nonadiabatic behavior, where ions are strongly heated, becomes a condition on the ion mass-to-charge ratio, m _i/m_pZ_i > 10√β_0x/2/ π, where m_i and Zi are the ion mass and charge state, m _p is the proton mass, and β _0x = 8πnT/B ²_0x is the ratio of the plasma pressure to that of the reconnecting magnetic field B_0x. Thus, during flares high mass-to-charge particles gain energy more easily than protons and a simple model reveals that their abundances are enhanced, which is consistent with observations.