Hybrid Simulations of Solar Wind Proton Precipitation to the Surface of Mercury

Abstract

We examine the effects of the interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure on the solar wind proton precipitation to the surface of Mercury using a hybrid-kinetic model. We use our model to explain observations of Mercury's neutral sodium exosphere and compare our results with MESSENGER observations. For the typical solar wind dynamic pressure at Mercury our model shows a high proton flux precipitates through the magnetospheric cusps to the high latitudes on both hemispheres on the dayside, centered near the noon meridian with ∼11 • latitudinal extent in the north and ∼21 • latitudinal extent in the south, which is consistent with MESSENGER observations. We show that this two-peak pattern is controlled by the radial component (B x ) of the IMF and not the B z . Our model suggests that the southward IMF and its associated magnetic reconnection do not play a major role in controlling plasma precipitation to the surface of Mercury through the cusps. We found that the total precipitation rate through both of the cusps remain constant and independent of the IMF orientation. We also show that the solar wind proton incidence rate to the entire surface of Mercury is higher when the IMF has a northward component and nearly half of the incidence flux impacts the low latitudes on the nightside. During extreme solar events (e.g., coronal mass ejections), our model suggests that over 70 nPa solar wind dynamic pressure is required for the entire surface of Mercury to be exposed to the solar wind plasma.

Pre-flight Calibration and Near-Earth Commissioning Results of the Mercury Plasma Particle Experiment (MPPE) Onboard MMO (Mio)

“…It is widely accepted that Kelvin-Helmholtz (K-H) instability operates at Earth's magnetopause and plays a significant role in transporting mass and energy from the solar wind to the magnetosphere. MESSENGER observations showed that K-H vortices develop predominantly on the duskside (Gershman et al 2015). K-H waves as sources of these vortices are also detected mainly at dusk (Liljeblad et al 2014).…”

“…Because the typical scale of Mercury's magnetospheric structures are on the order of the proton Larmor radius, the ideal magnetohydrodynamics (MHD) approximation could be inadequate for describing the global dynamics of the magnetosphere. This is exemplified by MESSENGER observations, which showed that a typical proton gyroradius in Mercury's plasma sheet is ∼ 380 km (DiBraccio et al 2015) and that the thickness of the plasma sheet is comparable to the proton gyroradius (Sun et al 2017). For zero or weak guide magnetic fields, ion scale current sheets with a thickness comparable to the ion inertia length or the ion Larmor radius are predicted to become highly unstable for the current driven instabilities, which lead to quick triggering of magnetic reconnection (Shinohara and Fujimoto 2005).…”

Cross-comparison of global simulation models applied to Mercury’s dayside magnetosphere

“…In addition, the significant offset between the magnetic dipole origin and the center of the planet results in a surface magnetic field strength in the northern hemisphere that is double the nominal value estimated for the southern hemisphere [7,8,9]. These particularities culminate in fascinating particle precipitation patterns and differential space weathering that is as variable as the upstream solar wind [10,11,12]. The absence of a significant ionosphere makes Mercury's conductive core an integral part of the electrodynamic current closure and complicates the evolution of the complex local plasma environment even more [13,14].…”

“…Numerical simulations of the solar wind interaction with the Hermean magnetosphere have thus far adopted multi-fluid/magnetohydrodynamic [15,16,17,18] and hybrid approaches (representing the ions as computational particles and the electron populations as a (massless) fluid) [19,20,21,22,23,24,25,26,12]. These models, designed to focus on the ion kinetics, have been successful in recreating the general structure of Mercury's local plasma environment.…”

Cross-comparison of global simulation models applied to Mercury's dayside magnetosphere

“…Numerical simulations of the solar wind interaction with the Hermean magnetosphere have thus far adopted multi-fluid/magnetohydrodynamic [15,16,17,18] and hybrid approaches (representing the ions as computational particles and the electron populations as a (massless) fluid) [19,20,21,22,23,24,25,26,12]. These models, designed to focus on the ion kinetics, have been successful in recreating the general structure of Mercury's local plasma environment.…”