Interpreting Energy-Latitude Dispersions in Mercury's Northern Magnetospheric Cusp With MESSENGER

Abstract

We examined energy-latitude dispersions in Mercury's northern magnetospheric cusp. These systematic trends in plasma energy originate from differences in particle travel time from the dayside magnetopause reconnection site to the low altitude, high latitude cusp. Compared to other planetary magnetospheres, the small size of Mercury's magnetosphere, large relative size of the cusps, and MESSENGER's orbital geometry require careful tracking of spacecraft motion with respect to the planetary field. We incorporate this relative motion into a physical model for these dispersions and apply it to a set of MESSENGER cusp crossings with coherent dispersions. Fitting these dispersions returns the distance of the open-closed field line boundary from the cusp and the length of the field line to the reconnection site. We demonstrate that these physical parameters can be used to probe both Mercury's solar wind coupling as well as the structure of the dayside magnetosphere. From the set of crossings, we find that dispersions are common to Mercury's cusp, that they indicate that the cusp is typically located poleward of the open-closed boundary by ∼6 $\pm$ 3° invariant latitude, and that Mercury's magnetosphere often exhibits steady conditions for multiple Dungey cycle durations at a time. Individual case studies further demonstrate that the structure of Mercury's dayside magnetosphere can be decoupled from magnetopause reconnection and that MESSENGER can cross the dayside magnetopause into open northward planetary field lines. As part of this work, we advance uncertainty quantification and fitting techniques that can be applied to other features in Mercury's magnetosphere and other planetary systems.