Ion holes in the slow solar wind: Hybrid simulations

Abstract

The spatio-temporal evolution of large-amplitude Alfvénic wave packets is investigated using a one-dimensional hybrid model. For conditions prevailing in the slow solar wind plasma, namely ß ∼ 1 ( ß being the ratio of kinetic pressure to magnetic pressure) and electron temperature Te greater than proton temperature Ti, we observe two branches of ion density holes. One is propagating forward and the other backward in the inertial frame of reference. Kinetic effects as well as strong coupling between density and magnetic field fluctuations are responsible for generating these holes. For the first time, we show the possibility of an Alfvénic wave packet decay into a new and unanticipated nonlinear wave mode that does not exist in the current literature. As T(= Ti/Te) increases, the ion holes disappear because of Landau damping. In the slow solar wind the scale sizes of these ion density cavities are ≤ 2sec. We predict the occurence of such cavites in the slow solar wind; these would be observable when high time resolution plasma data become available.