MESSEGNER plasma and magnetic field observations of Mercury’s space environment are reviewed. Mercury’s magnetosphere is created by the solar wind interaction with its highly dipolar, spin-axis aligned magnetic field. Structurally it resembles that of Earth in many respects, but the magnetic field intensities and plasma densities are all higher at Mercury due to the increased solar wind pressure in the inner solar system. Magnetospheric plasma at Mercury appears to primarily of solar wind origin, i.e. H+ and He++, but with 10% Na+ derived from the exosphere. Like the Moon, solar wind sputtering and other processes promote neutrals from the regolith into the exosphere where they may be ionized and incorporated into the magnetospheric plasma population. The low plasma (i.e., ratio of plasma thermal to magnetic pressure) magnetosheath at Mercury results in strong plasma depletion layers adjacent to the magnetopause. In this environment magnetopause reconnection does not exhibit the “half-wave rectifier” response to interplanetary magnetic direction (i.e. low latitude reconnection is only observed at large magnetic shear angles) found at Earth. The comparable magnetic field intensities on the two sides of the magnetopause support reconnection for all non-zero shear angles with plasma as the primary parameter controlling the rate. Flux transfer events (FTEs) are observed at most magnetopause crossings often in “showers” with FTEs being encountered every ~ 10 s. Unlike at Earth, where FTEs account for only order 10-2 of the magnetic flux driving the Dungey cycle, the contribution of FTEs at Mercury appears nearly comparable to that of steady magnetopause reconnection. Mercury’s magnetotail sometimes displays similar loading/unloading to that observed at Earth during isolated substorms but the Dungey cycle-time at Mercury is ~ 2 – 3 min as compared to ~ 1 hr at Earth. This difference is due, at least in part, to the lack of an electrically conducting ionosphere at Mercury. Mercury’s magnetosphere can also exhibit Earth-like steady magnetospheric convection with quasi-periodic plasmoid ejection and near-tail dipolarization. Mercury’s highly resistive crust inhibits strong, long duration coupling via field-aligned currents. However, its large, highly conducting iron core supports strong “inductive” coupling. The currents induced in the outermost layers of the core by increased solar wind pressure, such as during coronal mass ejections and high-speed streams, are observed to decrease the compressibility of Mercury’s dayside magnetosphere. The effects of this inductive magnetosphere – core coupling on other aspects of magnetospheric dynamics at Mercury remain to be determined.
Caption: a) Increases in solar wind pressure at Mercury induce currents on the surface of the iron core (blue-green) that reinforce the dayside dipolar magnetic flux (yellow) emanating from the deeper dynamo region. b) Reconnection at the magnetopause transfers magnetic flux from all sources into the tail where it drives the Dungey cycle. Competition between induction and reconnection determines the amount of solar wind reaching the surface (Slavin et al., 2014).