Reconnection Exhaust Jets as the Progenitor of Magnetotail Transients

Abstract:

In the Earth's magnetotail, brief periods of fast plasma flow (``bursty bulk flows''---BBFs) provide much of the sunward transport of mass, energy, and magnetic flux. As a result of coordinated studies using multi-satellite observations, it has been demonstrated that these fast flows are confined to flow channels in the plasma sheet whose full width is of the order of 1--3 $R_E$ in the dawn-dusk direction and 1.5--2 $R_E$ in the north-south direction. The present study uses 3D particle-in-cell simulations to determine the properties of the exhaust jets produced by reconnection initiated in finite-width regions of the plasma sheet. Relatively narrow fronts ($< 10 d_i$) expand in the ion-drift direction to reach widths of 15--20 $d_i$. Broader initial fronts (25--50 $d_i$) tend to form a 10--15 $d_i$ width higher-speed structure on the dawn side of the front. Thus the minimum width of an exhaust jet appears to be on the order of 1 $R_E$. All of these fronts exhibit a tendency to filament into structures of order 1--2 $d_i$ in width, apparently due to the action of the ballooning/interchange instability. At the dawnward side of a front, the incoming ions are slowed sharply, and reflected ions appear downstream. Behind the front is a turbulent region with a strong localized duskward ion drift structure. Substantial increases in both ion and electron fluxes are observed at energies of 15--25$E_{th}$. Isolated small-scale ($d_e \ll L \ll d_i$) regions of net dissipation of strength 1--2 nW/m$^3$ appear at the front. The implications of these results will be discussed.