Interpreting energy-latitude dispersions in Mercury’s northern magnetospheric cusp with MESSENGER

We examine energy-latitude dispersions in Mercury’s northern magnetospheric cusp.These systematic trends in plasma energy originate from diSerences in particle travel timefrom 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 trackingof spacecraft motion with respect to the planetary field. We incorporate this relativemotion into a physical model for these dispersions and apply it to a set of MESSENGERcusp crossings with coherent dispersions. Fitting these dispersions returns the distance ofthe open-closed field line boundary from the cusp and the length of the field line to thereconnection site. We demonstrate that these physical parameters can be used to probeboth 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, thatthey indicate that the cusp is typically located poleward of the open-closed boundary by~6±3° invariant latitude, and that Mercury’s magnetosphere often exhibits steadyconditions for multiple Dungey cycle durations at a time. Individual case studies furtherdemonstrate that the structure of Mercury’s dayside magnetosphere can be decoupledfrom magnetopause reconnection and that MESSENGER can cross the daysidemagnetopause into open northward planetary field lines. As part of this work, we advanceuncertainty quantification and fitting techniques that can be applied to other features inMercury’s magnetosphere and other planetary systems.