Electron acceleration by lower hybrid waves in magnetic reconnection regions

Abstract

A new model for electron acceleration in magnetic reconnection regions is developed analytically, tested using quasilinear simulations, and the qualitative predictions compared with observations of reconnection in Earth’s magnetotail and the solar corona. The model involves lower hybrid (LH) waves, produced by a drift instability (LHDI) in reconnection regions, stochastically accelerating electrons parallel to the magnetic field by the Cherenkov resonance (LH drive or LHD). Analytic theory shows that LH waves produced by LHDI have the correct wave numbers to cause parallel electron acceleration from thermal to highly superthermal and even relativistic energies, for sufficiently low plasma β and long evolution times. Several previous Vlasov and particle-in-cell simulations show growth of LH waves by LHDI. Assuming that LHDI produces LH waves with the wave numbers and energy densities (≈50% that of the thermal ion plasma) found in the Vlasov simulations, quasilinear simulations with the correct mass ratio show creation of a superthermal electron tail resonant with the LH waves and parallel to B. The tail reaches about 20 electron thermal speeds in ≈600 lower hybrid times but extends to higher speeds with increasing time. When applied to Wind spacecraft data from Earth’s magnetotail, the model is qualitatively consistent with the occurrence of LH waves, primarily parallel electron acceleration, no ion acceleration, and overall time scale for the acceleration event. Similarly, it is not inconsistent with the primarily parallel electron acceleration, perpendicular ion heating, and time scales inferred from x-ray and radio observations of solar flares. Moreover, the conversion efficiencies from thermal and magnetic energies to semirelativistic electrons are estimated to be over 100 times lower for the magnetotail reconnection data than for the solar corona, not unexpected from the model since LH waves should be favored by the lower plasma beta conditions found in the corona.