Mechanisms for Generating Finite Cross-Tail Jets in the Plasma Sheet

Abstract:

In the magnetotail, brief periods of fast plasma flow (``bursty bulk flows''--BBFs) provide much of the sunward transport of mass, energy, and magnetic flux. Although usually interpreted as resulting from magnetic reconnection acting in the mid- or more-distant tail, the precise generation mechanism for these flows has never been clearly established. Observationally, a key feature of these flow fronts or jets is that their full cross-tail width in the plasma sheet is of the order of 1--3 $R_E$. The present work examines the relative ability of reconnection and ballooning/interchange (BICI) processes to produce such finite-width fronts. 3D particle-in-cell simulations are used to initiate reconnection in finite-width regions of the plasma sheet and to generate BICI heads from regions of the plasma sheet containing a tailward-decreasing entropy profile. For the reconnection jets, the front is observed to expand duskward (in the ion diamagnetic drift direction) to form a structure some 15 $d_i$ wider than the initial localization width. The BICI heads have a comparable extent. Both types of jets feature abrupt increases in the equatorial $B_z$ field and both tend to break up in $y$ due to a secondary interchange instability on a scale of 1--2 $d_i$. A distinguishing feature between the two types of jets is that the BICI fronts are preceded by an off-equatorial signal involving wave activity near the ion cyclotron frequency that involves intense (30--50 mV/m) electric fields and magnetic perturbations of the order of 10\% of the ambient main field. The possibility of producing repeated jets by these mechanisms will be discussed as well.