MESSENGER’s low-altitude plasma observations in Mercury’s northern magnetospheric cusp

Abstract:

In its final year in orbit, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is conducting a low-altitude campaign that features several periods during which the periapsis altitude will be as low as 15–25 km. During many of the low-altitude orbits, the spacecraft passes through Mercury’s northern magnetospheric cusp, which has been shown by MESSENGER observations at higher altitude (200–500 km) to be a key region of coupling among the solar wind, the exosphere, and the surface regolith. Plasma and magnetic field observations at low altitudes should add substantially to an understanding of processes observed to date in this important region: (1) Protons have been observed flowing into the cusp and forming a persistent loss cone. Documentation of similar characteristics close to the surface would greatly strengthen predictions that these ions are precipitating onto Mercury’s surface. (2) Upwelling low-energy (100–300 eV) Na⁺-group ions (mass per charge 21-30) within the cusp appear to be locally energized from surface-sputtered ions or photo-ionized exospheric neutrals. Observing the energies of these ions over a larger range of altitudes may allow the functional dependence to be determined, which would reveal further clues concerning their acceleration mechanism. (3) High-energy (>1 keV) Na⁺-group ions have been shown to be the dominant planetary ion in the cusp, exceeding at times the abundance of even solar wind alpha particles (He²⁺). In contrast to their low-energy counterparts, these ions appear to have been swept into the cusp from the dayside magnetosphere on newly reconnected field lines. The low-altitude extent of these observations, especially when compared with that of protons, can be used to evaluate this hypothesis and others concerning the entry of these high-energy ions into the cusp. (4) Short-duration, deep depressions in the magnetic field magnitude, called cusp filaments, have been commonly observed within the cusp and contain plasma of magnetosheath energies. Variations of these signatures at low altitudes will be a measure of the access of plasma through the cusp to the surface and its response to forcing by the solar wind.