A HYBRID SIMULATION OF CONTACT DISCONTINUITY

Contact discontinuities in a collisionless plasma are studied by hybri d simulations, in which ions are treated as particles and electrons ar e considered as a fluid. It is demonstrated that contact discontinuity with a stable density ramp can exist in cases with a finite electron temperature. An electron pressure gradient is present across the conta ct discontinuity, leading to the presence of a parallel electric field and hence field-aligned potential increase (DELTAPHI(parallel-to)) in the transition region. By reflecting ions at the discontinuity, this parallel electric potential peak reduces the interpenetration between hot and cold ions and maintains a stable density ramp across the conta ct discontinuity. The ratio of the field-aligned electric potential en ergy to ion thermal energy, eDELTAPHI(parallel-to)/kT(i), is found to be an increasing function of T(e)/T(i), where T(e) and T(i) are respec tively the electron and ion temperature.