MESSENGER observations of flux transfer event showers and their influences on sodium ions in Mercury’s dayside magnetosphere

Abstract:

Flux transfer events (FTEs) are magnetic flux ropes formed by magnetic reconnection on the magnetopause between the planetary magnetic field and the interplanetary magnetic field (IMF) involving two or more X-lines. In FTEs, open magnetic flux is concentrated in flux ropes (FTE-type flux ropes), but open flux created by reconnection at a single X-line also drapes about the flux ropes and adds to the anti-sunward stress that carries them to higher latitudes and in the nightside magnetosphere. Both the magnetic field lines in FTE-type flux ropes and some the trailing post-FTE magnetic flux are open and have one end connected to the solar wind and the other to the planet. In Mercury's magnetosphere, FTEs occur frequently and when the rate exceeds ten flux ropes per minute, it is termed an FTE shower. Our study has surveyed the dayside magnetopause crossings over all five years of the MESSENGER Mission. FTE showers were quite common and accompanied around half of MESSENGER’s dayside magnetopause crossings. The occurrences of FTE showers and the physical properties of FTE-type flux ropes are shown to depend on the magnetic shear angle (θ) across the magnetopause current sheet and the magnetosheath plasma β (ratio of the plasma thermal pressure to the magnetic pressure) in the inner magnetosheath adjacent to the magnetopause. By performing the Lundquist force-free modeling on these FTE-type flux ropes, it is further shown that FTE-type flux ropes carry most (i.e., 60% to 85%) of the magnetic flux needed to supply Mercury’s Dungey cycle, which is substantially different from other planetary magnetospheres. Furthermore, FTE showers are shown to be closely associated with enhancements in the sodium ion (Na⁺) density in Mercury’s dayside magnetosphere, especially in the high latitude magnetopause boundary layer around the northern cusp. The Na⁺ density and flux during FTE shower intervals are more than twice than those during the non-FTE shower intervals. These observations suggest that the higher precipitation rates reaching the surface underneath the cusps during FTE showers sputters large numbers of planetary ions upward along the local magnetic field lines into the high latitude magnetosphere and the plasma mantle in the nightside magnetotail.