Plasma Dynamics at Mercury: Characteristics of the Dayside, Nightside, and Foreshock

Abstract

Mercury is the only other rocky planet beside Earth in the solar system with an intrinsic dipole field. Because Mercury is also 60% closer to the Sun than the Earth on average, it is an excellent test environment for planetary dynamics observable under extreme conditions. Mercury’s lack of a collisional atmosphere means particles can precipitate directly onto the planetary surface, contributing to erosion of the surface and loss of mass to the space environment. We seek to better understand processes at work in Mercury’s magnetosphere with the potential to affect the local flow of plasma, its energy, and the distribution of plasma precipitation across the planetary surface. Tracing of singly-ionized sodium through fields generated by a magnetohydrodynamic model has enabled us to show that Na+ ions with initial energies of ~1 eV are energized by Dungey cycle return flow in the dayside dipolar region by two orders of magnitude or more, enabling their escape from this region. Some of these ions pass to the planetary cusp, in which they precipitate onto the surface; and others pass into the flow around the planet in the magnetosheath, in which they are energized up to tens of keV before impacting the surface. These findings shed light on previously unexplained observations of high-energy planetary ions on Mercury’s dayside, and have important implications for surface sputtering. The magnitude of impact that plasma precipitation makes on Mercury’s surface is dependent not only on the momentum of the particles, but also on the region over which the particles precipitate. In addition to the planetary cusp, by analogy to Earth the other major region in which significant precipitation would be expected is through the nightside plasma sheet horns, which at Earth map to the auroral oval. We show for the first time that Mercury’s northern plasma sheet horn was frequently directly observed by magnetic field and ion sensors aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, and that proton precipitation rates in the plasma sheet horn are comparable to those in Mercury’s dayside cusp. Further analysis of hundreds of additional horn observations is ongoing, and has already begun to challenge our expectations on the regions of the surface over which precipitation is likely to occur. We have identified a sample set of over 300 horn observations, and present a characterization of their location and average conditions, for the purposes of future comparison with signatures of magnetospheric dynamics. Finally, following on previous work, nearly 40 foreshock populations at Mercury are categorized based on their energization process at the shock surface. Comparison of the populations in the survey to observations at Earth demonstrates a surprisingly similar environment at Mercury for particle energization, despite the vastly different parameter regime there. For the first time, waves observed in Mercury’s foreshock are statistically correlated with plasma data: diffuse ion populations show the anticipated correlation with so-called “30-second” waves, with which they are also correlated at Earth. We also show that Field-Aligned Beams, not correlated with any wave signature at Earth, are accompanied at Mercury by a previously discovered but unexplained wave mode.