MESSENGER Observations of Flow Braking and Flux Pileup of Dipolarizations in Mercury's Magnetotail: Evidence for Current Wedge Formation

Abstract

Similar to Earth, Mercury's magnetotail experiences frequent dipolarization of the magnetic field. These rapid (~2 s) increases in the northward component of the tail field (ΔBz ~ 30 nT) at Mercury are associated with fast sunward flows (~200 km/s) that enhance local magnetic field convection. Differences between the two magnetospheres, namely Mercury's smaller spatiotemporal scales and lack of an ionosphere, influence the dynamics of dipolarizations in these magnetotails. At Earth, the braking of fast dipolarization flows near the inner magnetosphere accumulates magnetic flux and develops the substorm current wedge. At Mercury, flow braking and flux pileup remain open topics. In this work, we develop an automated algorithm to identify dipolarizations, which allows for statistical examination of flow braking and flux pileup in Mercury's magnetotail. We find that near the inner edge of the plasma sheet, steep magnetic pressure gradients cause substantial braking of fast dipolarization flows. The dipolarization frequency and sunward flow speed decrease significantly within a region ~500 km thick located at ~900 km altitude above Mercury's local midnight surface. Due to the close proximity of the braking region to the planet, we estimate that ~10–20% of dipolarizations may reach the nightside surface of the planet. The remaining dipolarizations exhibit prolonged statistical flux pileup within the braking region similar to large-scale dipolarization of Earth's inner magnetosphere. The existence of flow braking and flux pileup at Mercury indicates that a current wedge may form, although the limitations imposed by Mercury's magnetosphere require the braking of multiple, continuous dipolarizations for current wedge formation.