A two-dimensional numerical study of ion-acoustic turbulence and its potential impact on magnetic reconnection onset

During the current sheet formation process that precedes the fast stage of reconnection events, micro-instabilities potentially capable of providing a large effective resistivity can be triggered, which may determine the reconnection onset and properties of the reconnecting system. A prominent example is the ion-acoustic instability (IAI). Ion-acoustic waves are one of the most fundamental oscillation modes in plasmas, and are widely observed in the heliosphere; however, the nonlinear development of the IAI remains poorly understood, partly because of the restriction to one-dimensional simulations. To address this issue, we perform two-dimensional Vlasov-Poisson simulations of this problem for the first time and study the linear and nonlinear evolution of the IAI in a collisionless plasma. We circumvent the problem of the realizability of the initial conditions by driving the system towards the ion-acoustic-unstable state via an imposed, weak external electric field. We demonstrate that two-dimensional effects are significant in the effectiveness of ion-acoustic turbulence. We show that the famous Sagdeev formula of anomalous resistivity overestimates the effective collision frequency under the weak electric field, rather than underestimating it as claimed by many previous studies. As it has been reported in some previous numerical studies, no steady saturated nonlinear state is ever reached in our simulations. After saturation of the instability, the bulk of electrons stays marginally unstable to the IAI; however, a run-away tail that is freely accelerated by the external electric field forms, which may lead the system to become Buneman unstable at later time. We also briefly discuss how these findings may impact the current sheet formation process and the onset of magnetic reconnection.