Influence of Mercury's Exosphere on the Structure of the Magnetosphere

Abstract

Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma‐surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet‐like configurations. We demonstrate how a sufficiently dense exosphere leads to self‐shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field‐aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the “guiding” magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations.

The BepiColombo Planetary Magnetometer MPO-MAG: What Can We Learn from the Hermean Magnetic Field?

“…Numerical simulations of the solar wind interaction with Mercury's magnetic field have reproduced much of the magnetospheric structure and dynamics observed by Mariner 10 and MESSEN-GER (e.g. Trávníček et al 2009;Müller et al 2012;Exner et al 2018;Dong et al 2019b;Exner et al 2020). These simulations give a global picture of the magnetosphere but particular results should be treated with caution, because numerical schemes as well as the treatment of boundary conditions may lead to physical behavior that may not be immediately visible.…”

“…The magnetic fields from these closure currents are also visible at low satellite altitude with an amplitude up to 50 nT and more. How much from the FAC is intercepted and closed via the magnetosphere is currently under debate (Exner et al 2020). Furthermore, Pederson-like closure currents within the planet's interior produce magnetic fields that must appear as internal multipoles in the field analysis.…”

The Mie representation for Mercury’s magnetic field

“…For the first application of the thin shell approximation simulated magnetic field data are analyzed. The magnetic field resulting from the plasma interaction of Mercury with the solar wind is simulated with the hybrid code AIKEF (Müller et al 2011), that has successfully been applied to several problems in Mercury's plasma interaction, (Exner et al 2018(Exner et al , 2020. The internal Gauss coefficients g 0 1 = −190 nT (dipole field), g 0 2 = −78 nT (quadrupole field) and g 0 3 = −20 nT (octupole field) (Anderson et al 2012;Thébault et al 2018;Wardinski et al 2019) are implemented in the simulation code.…”

The BepiColombo–Mio Magnetometer en Route to Mercury

“…MGF will detect and track the field-aligned currents from distant tail region (up to 6 R M ) down to the near-surface region. Although very weak, the exo-ionosphere may intercept some of the field-aligned currents as shown by Exner et al (2020).…”