MESSENGER Observations and Global Simulations of Highly Compressed Magnetosphere Events at Mercury

Abstract

We identify and examine all MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) crossings of Mercury's dayside magnetopause with magnetospheric field intensities ≥300 nT. The eight such events, which occurred under highly compressed magnetosphere conditions, are analyzed in the identical manner utilized by Slavin et al. (2014, https://doi.org/10.1002/2014JA020319). The results suggest that the eight highly compressed magnetosphere events represent the highest solar wind dynamic pressures for which the MESSENGER's orbit still passed below the magnetopause and provided measurements of the dayside magnetosphere. Using the magnetohydrodynamic model by Jia et al. (2015, https://doi.org/10.1002/2015JA021143) that electromagnetically couples Mercury's interior with its magnetosphere, a series of global simulations are conducted to quantitatively characterize the response of Mercury's magnetosphere to solar wind forcing. Combining the MESSENGER observations with the simulations, we have obtained a consistent picture of how Mercury's dayside magnetospheric configuration is controlled, separately and in combination, by induction‐driven shielding and reconnection‐driven erosion. For solar wind pressures of ∼40–90 nPa, compared with the average ∼10–15 nPa at Mercury's orbit, the shielding effects of induction in Mercury's core in standing‐off the solar wind typically exceed the erosion of the dayside magnetosphere due to reconnection for these events, most of which occurred under low magnetic shear conditions. For high magnetic shear across the magnetopause our simulation predicts that reconnection would dominate. Mercury's effective magnetic moment as inferred from magnetopause standoff distance ranges from 170 to 250 nT−RM3 for these events. These findings are of crucial importance for understanding the space weathering at Mercury and its contribution to the generation of Mercury's exosphere.

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

“…This value is comparable to the ratios determined for the dayside magnetopause at Mercury in previous studies (Slavin et al, 2008DiBraccio et al, 2013;Liljeblad et al, 2015;Jia et al, 2019;Dong et al, 2019). This value is comparable to the ratios determined for the dayside magnetopause at Mercury in previous studies (Slavin et al, 2008DiBraccio et al, 2013;Liljeblad et al, 2015;Jia et al, 2019;Dong et al, 2019).…”

“…In contrast, the HCM events with the highest magnetopause reconnection rates are located closer to the Chapman-Ferraro curve and the crossings with the smallest dimensionless reconnection rates are located closest to the Glassmeier et al curve on the right-hand side that includes the effects of induction. These conclusions are strongly supported by the global MHD simulations of the solar wind interaction with Mercury's magnetic field and conducting iron core by Jia et al (2019). For this reason, the HCM events do not appear to expose the dayside to direct solar wind impact; that is, their R ss values are greater than 1.…”

“…In addition to the DDM events, the eight highly compressed magnetosphere events from Jia et al (2019) are displayed for comparison. In addition to the DDM events, the eight highly compressed magnetosphere events from Jia et al (2019) are displayed for comparison.…”

“…Jia et al (2019) surveyed MESSENGER's four-year data set and identified eight passes through Mercury's dayside magnetosphere for which the magnetic field just inside of the magnetopause exceeded 300 nT. Indeed, Jia et al found that the induction currents increased the planetary magnetic moment by up to~25% during these mostly CME-related solar wind events (Jia et al, 2019). For these pressures the magnetopause at the subsolar point would be expected to approach very close to the planetary surface if it were not for the effect of the inductions currents.…”

“…Average magnetopause position for each of these events is given in Table 1 Figure 12. For this reason, we, again, follow the methodology of Slavin et al (2014) and Jia et al (2019) and assume a Newtonian pressure balance across the magnetopause, the MVA magnetopause normals, and, based upon the FIPS data, that the plasma pressure in the outer dayside magnetosphere is very small compared to the magnetic pressure . The magnetic field intensities in the magnetosheath for these two events were~50 to 70% of the total magnetic field just inside of the magnetosphere.…”