MESSENGER Observations of the Interplanetary Magnetic Field's Influence on Magnetotail Twisting at Mercury

Observations of various planetary magnetotails have indicated that the tail lobes may be displaced from their expected location due to a twist imposed by the solar wind-planetary interaction. Mercury is an ideal candidate to understand the influence of the Interplanetary Magnetic Field (IMF) on magnetotail twisting, given the strong fields at the planets location in the inner heliosphere. We analyze all measurements provided by the Mercury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) Magnetometer to investigate the influence of the IMF dawn-dusk By component on the Hermean magnetotail structure. We find that the cross-tail current sheet presents a ~ 2 twist, rotating in opposite directions in response to changes in IMF By direction from positive to negative. The dependence of this twist on the IMF By polarity is in agreement with previous observations associated with Earths and Marss magnetotails. To understand these results, we consider the initiation of the Dungey cycle where magnetic reconnection occurs along the dayside magnetopause between Mercurys intrinsic magnetic field and the shocked IMF in the magnetosheath. This process is responsible for generating open magnetic field lines, which are then convected to the magnetotail. When a strong IMF By component is present, a torque may be exerted across the magnetotail of Mercury, similar to what has been observed at Earth. Finally, we also consider the impact of induction effects due to Mercurys unique interior structure with a large iron core on the twist observed in Mercurys magnetotail.