Review of Mercury’s dynamic magnetosphere: Post-MESSENGER era and comparative magnetospheres

Abstract

This review paper summarizes the research of Mercury’s magnetosphere in the Post-MESSENGER era and compares its dynamics to those in other planetary magnetospheres, especially to those in Earth’s magnetosphere. This review starts by introducing the planet Mercury, including its interplanetary environment, magnetosphere, exosphere, and conducting core. The frequent and intense magnetic reconnection on the dayside magnetopause, which is represented by the flux transfer event “shower”, is reviewed on how they depend on magnetosheath plasma β and magnetic shear angle across the magnetopause, following by how it contributes to the flux circulation and magnetosphere-surface-exosphere coupling. In the next, Mercury’s magnetosphere under extreme solar events, including the core induction and the reconnection erosion on the dayside magnetosphere, the responses of the nightside magnetosphere, are reviewed. Then, the dawn-dusk properties of the plasma sheet, including the features of the ions, the structure of the current sheet, and the dynamics of magnetic reconnection, are summarized. The last topic is devoted to the particle energization in Mercury’s magnetosphere, which includes the energization of the Kelvin-Helmholtz waves on the magnetopause boundaries, reconnection-generated magnetic structures, and the cross-tail electric field. In each chapter, the last section discusses the open questions related to each topic, which can be considered by the simulations and the future spacecraft mission. We end this paper by summarizing the future BepiColombo opportunities, which is a joint mission of ESA and JAXA and is en route to Mercury.

MESSENGER Observations of Planetary Ion Enhancements at Mercury's Northern Magnetospheric Cusp During Flux Transfer Event Showers

“…The occurrence of FTE showers is high and depends on both of the magnetosheath plasma $$\beta $$, which is the ratio of the thermal pressure to the magnetic pressure, and the magnetic shear angle across the magnetopause (Sun, Slavin, Smith, et al., 2020). The high occurrence of FTE showers at Mercury is due to Mercury's magnetosphere being embedded in the solar wind with a low Alfvénic Mach number ( M A $$\lesssim $$ 4) (Slavin & Holzer, 1981; Sun et al., 2022), that is, the ratio of solar wind speed to the Alfvén speed. The low M A solar wind often leads to a magnetosheath with a low plasma $$\beta $$ that forms a plasma depletion layer (PDL) ahead of the magnetopause (Gershman et al., 2013; Slavin et al., 2014).…”

“…Mercury's magnetic field can hold off the constantly streaming solar wind with a subsolar magnetopause distance of around one thousand kilometers above the planet's surface (Siscoe et al., 1975 ; Slavin et al., 2008 ). As the closest planet to the Sun, Mercury is subject to the strongest solar wind driving compared to other planets in the solar system (Slavin & Holzer, 1981 ; Sun et al., 2022 ). One outcome is that the magnetic reconnection erosion effect on the dayside magnetopause is significant at Mercury (Slavin & Holzer, 1979 ; Slavin et al., 2014 , 2019 ), and it often generates flux transfer events (FTEs) on the magnetopause (Russell & Elphic, 1978 ; Russell & Walker, 1985 ; Slavin et al., 2009 , 2012 ; Sun, Slavin, Smith, et al., 2020 ).…”

“…FTEs occur at lower rates and are separated by tens of minutes at other planets (Jasinski, Akhavan‐Tafti, et al., 2021 ; Russell, 1995 ), such as Earth, Jupiter, and Saturn since they are embedded in the solar wind with a higher M A , see Sun et al. ( 2022 ) for a review.…”

“…This can be revealed by the fact that not only do FTE showers occur in high occurrence rates (around 52% of the magnetopause crossings) but also the FTEs are observed in quick succession, that is, separated by only a few seconds. FTEs occur at lower rates and are separated by tens of minutes at other planets (Jasinski, Akhavan-Tafti, et al, 2021;Russell, 1995), such as Earth, Jupiter, and Saturn since they are embedded in the solar wind with a higher M A , see Sun et al (2022) for a review.…”

Solar-wind electron precipitation on weakly magnetized bodies: the planet Mercury

“…Such dawn-dusk asymmetry is a consequence of the dawnward drift of electrons injected from the neutral line in the tail towards the planet [6,8,20]. Such dawnward enhancements can also be due to a dawn-dusk asymmetry in the plasma sheet thickness [34,Chap. 4 therein].…”