Ultra slow electron holes in collisionless plasmas: Stability at high ion temperature

Abstract

Numerical simulations recover ultraslow solitary electron holes (SEH) of electron-acoustic genre propagating stably well below the ion acoustic speed Cs, where no pure electron perturbation is known to exist yet, as they are disallowed by the ion response. Recovered at high ion temperature (Ti > Te), the reason for this stability (unaccelerated propagation, unseen before in existing literature) of SEH is traced to the loss of neutralizing cold ion response. In the opposite case of a background of sufficiently cold ions, Te>3.5Ti, SEHs are accompanied by an ion compression that yields phase velocities above Cs (ion acoustic genre) and accelerates them, forcing a jump over a forbidden velocity gap, and settle on the high velocity tail of the electron distribution fe. In the observed ultraslow structures having Ti > Te, however, the warm ions begin to supplement the electron response and show Boltzmannlike behavior, transforming the ion compression to decompression (rarefaction) at the hole location. SEHs, hence, belong to the continuous spectrum of slow electron acousticlike modes being triggered by the electron trapping nonlinearity. The results also suggest a scope of generalization of the basic EH theory.