Mercury’s Ring Current: MESSENGER Observations and Test-Particle Simulations

Abstract

Energetic protons can carry a longitudinal electric current via their gradient and curvature drift around a planet and form a current system known as the ring current. The ring current has been observed in the intrinsic magnetosphere of Earth, Jupiter, and Saturn. However, there is still lacking evidence of ring current in Mercury’s magnetosphere, which contains significantly weaker and oppressive “dipolar” magnetic field and the charged particles are thought able to efficiently escape the magnetosphere through magnetopause shadowing and/or directly hitting the surface. Here we present the first observational evidence of Mercury ring current with the measurement of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER). The ring current is bifurcated under moderate solar wind forcing, which is caused by the off-equatorial magnetic minima on the noon side and tends to vanish during weak solar wind forcing. This morphology is validated by a test-particle simulation with a Mercury’s dynamic magnetic field model. The total energy stored in the ring current exceeds 5x1010 J during active times, indicating that magnetic storms may also occur in Mercury’s magnetosphere.