Drift-kinetic Measurements of Plasma Gradients in MMS Data

Blake Wetherton, University of Wisconsin-Madison

In magnetic reconnection, magnetic stress energy is converted into particle thermal and kinetic energy through the topological rearrangement of magnetic field lines. It is responsible for a host of energetic events in the universe, including coronal mass ejections, geomagnetic storms and the generation of terrestrial aurorae. While many models exist to explain reconnection, the crucial physics lie in a thin current layer around one electron skin depth wide. NASA's Magnetospheric Multiscale (MMS) mission seeks to directly investigate magnetic reconnection in Earth's magnetosphere with a tetrahedral formation of four spacecraft. These will provide some information on gradients in plasma properties, but the spacing between the MMS spacecraft tends to be around 10 electron skin depths, making gradients at the crucial scale poorly resolved. We present a drift-kinetic method for obtaining gradients in the plasma distribution function (and through it density) with data from a single spacecraft that should be valid so long as the plasma is magnetized (as is the case in most magnetospheric plasma, with a caveat at the electron diffusion region). This model is derived from drift kinetics, tested against results from a VPIC fully kinetic simulation of magnetopause reconnection, and applied to MMS spacecraft data to ascertain the geometry of the reconnection region and demonstrate gradient-scale resolution superior to applying finite difference methods between the four spacecraft.

Abstract Author(s): B.A. Wetherton, J. Egedal, P. Montag, A. Le, W. Daughton, B. Lavraud