Substorm current wedge formation at Mercury informed from MESSENGER observations

Abstract:

Mercury’s magnetosphere exhibits brief, yet intense, substorm activity characteristically similar to that at Earth. Observations from the MESSENGER mission have indicated that the magnetosphere experiences magnetic lobe loading/unloading, plasma sheet dipolarization, energetic particle injection, and auroral-like precipitation. Compared to Earth’s substorm activity, Mercury’s is shorter and relatively more dynamic due to Mercury’s smaller magnetospheric spatiotemporal scales, stronger upstream solar wind forcing, and lack of ionosphere. While both planetary magnetospheres share similar substorm dynamics, the formation of a substorm current wedge at Mercury has remained an open topic. At Earth, the substorm current wedge is a large-scale field-aligned current system that couples the near-tail region to the ionosphere and is formed from the braking and magnetic flux pileup of dipolarizations. We find that Mercury’s dipolarizations experience similar braking and pileup, suggestive of current wedge development at Mercury. However, since Mercury lacks an ionosphere such a current system is expected to close over the planet’s conducting core by passing radially through the planet’s resistive regolith. We present a new observationally-informed conceptual model of substorm current wedge formation at Mercury that addresses limitations imposed by both the magnetosphere’s small spatiotemporal scales and the addition of a resistive layer between the magnetosphere and its highly conducting inner boundary.