Ion beta dependence on the development of Alfvénic fluctuations in reconnection jets

摘要

The generation of magneto-hydro-dynamic (MHD) to ion-scale fluctuations in collisionless magnetic reconnection is discussed using a two-dimensional electromagnetic hybrid code. It is shown that reconnection jets become turbulent specifically in low beta conditions, β_(i0) < 0.1–0.2 (where β_(i0) is the ion plasma beta in initial inflow regions). The fluctuations observed in reconnection jets consist of outgoing Alfvénic fluctuations. As probable candidates for the origin of Alfvénic fluctuations, this study focused on the dynamics in the plasma sheet boundary layer (PSBL) and a current sheet. We suggest that PSBL ion dynamics play an important part in excitation and suppression of waves. PSBL beam ions drive Alfvén waves in MHD to ion scale, kλ_i < 0.5 (λ_i is ion inertial length), independent of β_(i0). On the other hand, because the beam temperature is highly correlated with that of inflowing ions, the waves decay by cyclotron damping as the value of the inflow ion beta increases. Local linear analysis suggests that this damping signature changes in β_(i0) ～0.1–0.2 and suppresses the wave activity of Alfvén modes in high beta reconnection jets.