Particle Acceleration in Magnetic Reconnection*

How magnetic reconnection efficiently produces a huge number of mildly relativistic energetic particles is an outstanding problem in solar physics and heliophysics. In particular, three major problems in solar particle acceleration have to be addressed: 1) the development of power-law energy spectra for both electrons and ions; 2) the "big number problem" of electrons. Recent observations discovered that the time to accelerate electrons to a power-law energy distribution in solar flares can be shorter than 50 ms while nearly the total number of electrons in the current sheet is accelerated in 1000s. 3) Observations suggest that the acceleration process of ions is related to the electrons'. In this talk, I will present a novel acceleration mechanism in magnetic reconnection. I will show how the velocity shear stored naturally in force-free currents of solar flares can drive an electron Kelvin-Helmholtz instability (EKHI) during magnetic reconnection. The EKHI efficiently accelerates electrons to a power-law energy spectrum with an index comparable to the observations in a few tens of ion gyro-periods (~ 0.1 ms for solar corona plasma). With the proceeding of reconnection, the EKHI induced Alfvenic turbulence can accelerate ions to broken power-law energy spectra. The simulation and theoretical results are well supported by solar x-ray and recent in-site MMS observations.