Spatial characteristics of magnetotail reconnection

We examine the properties of magnetic reconnection as it occurs in the Earth’s magnetosphere, first focusing on the spatial characteristics of the near-Earth magnetotail reconnection site, then analyzing the properties of cold plasma that may affect reconnection at the dayside magnetopause.

Two models are developed that empirically map the position and occurrence rate of the nightside ion diffusion region, which are based upon Geotail data (first model) and a combination of Geotail and Cluster data (second model). We use these empirical models to estimate that NASA’s MMS mission will encounter the ion-scale reconnection site 11±4 times during its upcoming magnetotail survey phase. We also find that the occurrence of magnetotail reconnection is localized and asymmetric, with reconnection occurring most frequently at the duskside magnetotail neutral sheet near YGSM∗ = 5 RE. To determine the physics that governs this asymmetry and localization, we analyze the time history of the solar wind, the instantaneous properties of the magnetotail lobes and current sheet, as well as the geomagnetic activity levels, all for a larger set of Geotail and Cluster reconnection site observations. We find evidence in our own results and in the preexisting literature that localized (small ∆Y) reconnection sites initially form near YGSM∗ = 5 RE due to an asymmetry in the current sheet thickness. If the solar wind driving remains strong, then localized reconnection sites may expand in the ±Y direction. The ∆Y extent of the reconnection site ap- pears to be positively correlated with the geomagnetic activity level, which is to be expected for a simplified “energy in equals energy out”-type picture of 3D reconnection.

We develop two new methods for determining the temperatures of plasmas that are largely below the energy detection range of electrostatic analyzer instruments. The first method involves the direct application of a theoretical fit to the visible, high-energy portion of the distribution function. The second method for determining temperatures involves a comparison of the energy-dependent and total plasma number densities. Both methods assume an infinitely thin sheath model for space- craft charging, a Maxwellian-type plasma, and bulk velocities that are strictly governed by E×B drift, which we model with a dipole magnetic field and a Volland-Stern electric potential field. The two methods are applied to RBSP observations of the plasmasphere proper. We find positive agreement with existing measurements of the temperatures, which were based upon data from low-altitude polar orbiting spacecraft. We also find evidence for in situ heating of the plasmasphere at the equator in the ring current overlap region. Finally, we apply these techniques to a single conjunction event, where MMS and RBSP provided simultaneous and nearly continuous coverage of the plasmasphere and plume from its equatorial base to the reconnecting magnetopause. We develop scaling laws for the temperature and density of the plasmasphere as a function of geocentric distance, showing that it is heated and density depleted by factors 20 and 200 (respectively) from L = 5 to the magnetospheric side of the reconnection boundary layer.