Numerical simulation of plasma processes driven by transverse ion heating

摘要

Numerical simulation is performed to study the plasma processes driven by transverse ion heating in a diverging flux tube. It is found that the heating drives a host of plasma processes, in addition to the well-known phenomenon of ion conics. The additional processes include formation of a density cavity topped by a density enhancement, formation of a reverse and forward shock pair with a 'double-sawtooth' structure in the flow velocity. The downward electric field near the reverse shock generates a double streaming situation consisting of two up flowing ion populations with different average flow velocities. A double streaming also occurs above the forward shock, where the ions energized by the heating are overtaking the relatively slow ions in the ambient polar wind. The energized ions appear as 'elevated' ion conics with a low-energy cutoff depending on the distance from the heating region. The parallel electric fields generated by the transverse ion heating have the following noteworthy features: the electric field near the forward shock is essentially unipolar, and it points upward; and for the heating localized in both space and time, the field has the features of a weak double layer. The electric field in the reverse shock region is modulated by the ion-ion instability driven by the multistreaming ions. The oscillating fields in this region have the possibility of heating electrons. The results from the simulations are compared with results from a previous study based on a hydrodynamic model. Effects of spatial resolutions afforded by simulations on the evolution of the plasma are discussed, demonstrating how a crude resolution can miss out plasma instabilities, affecting the plasma flow.