Observational evidence of ring current in the magnetosphere of Mercury

J.-T Zhao,X.-Z Zhou,J A Slavin,G Le,Q.-G Zong,C Yue,Y Wei,W.-J Sun,Y Liu,R Rankin,H Zhang,J M Raines

doi:10.1038/s41467-022-28521-3

Abstract

The magnetic gradient and curvature drift of energetic ions can form a longitudinal electric current around a planet known as the ring current, that has been observed in the intrinsic magnetospheres of Earth, Jupiter, and Saturn. However, there is still a lack of observational evidence of ring current in Mercury’s magnetosphere, which has a significantly weaker dipole magnetic field. Under such conditions, charged particles are thought to be efficiently lost through magnetopause shadowing and/or directly impact the planetary surface. Here, we present the observational evidence of Mercury’s ring current by analysing particle measurements from MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft. The ring current is bifurcated because of the dayside off-equatorial magnetic minima. Test-particle simulation with Mercury’s dynamic magnetospheric magnetic field model (KT17 model) validates this morphology. The ring current energy exceeds 5times {10}^{10} J during active times, indicating that magnetic storms may also occur on Mercury.

Highlights

A general ring current refers to the longitudinal electric current that results from the drift motion of energetic particles in the investigations of planetary magnetospheres such as Jupiter’s and Saturn’s2,3
The energy spectrum and pitch angle distribution (PAD) measured by the Fast Image Plasma Spectrometer (FIPS) instrument are presented in the first two rows (Figs. 1a, b and 2a, b), and the magnetic field observations are shown in Panels 1c and 2c
The magnetic field strength during this time interval was $ 160 nT, with Bz being the dominant component near the magnetic equator, indicating that the spacecraft was near the equator

Summary

Introduction

There is still a lack of observational evidence of ring current in Mercury’s magnetosphere, which has a significantly weaker dipole magnetic field Under such conditions, charged particles are thought to be efficiently lost through magnetopause shadowing and/or directly impact the planetary surface. Previous substorm observations have shown that rapid, frequent, and intense ion injection and energization processes occur in Mercury’s magnetotail, which could energetic protons to the inner magnetosphere[19] These energetic particles were considered very hard to be stably trapped due to strong magnetopause shadowing and surface absorption[20,21]. In situ measurements in the magnetotail have revealed quasi-trapped protons (i.e., protons that can only drift for a finite amount of time that is shorter than a complete drift period) with appreciable flux and showed their strong diamagnetic effect during some events[22] Both global magnetohydrodynamic (MHD) and hybrid (kinetic ions, electron fluid) simulations have reproduced quasi-trapped particles[22–25]. In situ observations and simulations are still required to determine whether energetic protons originating from the magnetotail can complete a full drift orbit and form a ring current, how the protons are distributed and how strong the ring current is

Methods

Results

Conclusion