Buneman Instabilities Relevant to Magnetic Reconnection

When displacement currents are negligible a simple argument using realistic parameters in the magnetotail or magnetopause shows that parallel current sheets need to be extremely narrow (thicknesses on the order of the electron skin depth) in order to have high enough drift velocity to excite electrostatic instabilities such as Buneman instabilities, which can evolve into phase space holes. Electron phase-space holes together with electron beams have, in fact, been observed by Cluster in association with magnetotail reconnection [1]. Parallel current sheets thin enough to support Buneman instabilities may develop electromagnetically by processes such as tearing instabilities. Such current sheets are localized near the separatrix around the x-point in a magnetic island configuration [2]. These currents have orthogonal spatial variation in both the density, n, and (mean) drift velocity, v_d, which are factors in the current, J = qnv_d for each species. To help identify the source of observed holes near reconnection sites, we will employ theoretical methods and Vlasov simulations to better understand the effect of various current sheet configurations on Buneman thresholds and growth rates, as well as on the characteristics of hole formation and longevity. The spatial distribution of current can be controlled by variation in density (e.g., a "diamagnetic" current with constant v_d), sheared velocity profiles, or combinations of the two. [1] C. Cattell et al., J.~Geophys.~Res., 110, A01211 (2005). [2] J. F. Drake et al., Science, 299, 873 (2003); P. L. Pritchett, Phys.~Plasmas, 12, 062301 (2005). Research supported by DOE, NSF, NASA and University of Colorado.