MESSENGER observations and global simulations of highly compressed magnetosphere events at Mercury

Mercury's comparatively weak intrinsic field, lack of an appreciable atmosphere and its close proximity to the Sun lead to a magnetosphere that undergoes more direct space-weathering interactions than other planets. The shielding effect due to the induction currents in the planetary core and erosion of the dayside magnetosphere due to magnetopause reconnection, compete against each other for dominance in controlling the large-scale structure of Mercury's magnetosphere. Here weidentify and examine all MESSENGER crossings of Mercury's dayside magnetopause with magnetospheric field intensities >= 300 nT. A total of 8 such events have been identified, all of which occurred under highly compressed magnetosphere (HCM) conditions. Our analysis suggests that the 8 HCM 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) 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 exceeds 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 stand-off distance ranges from 170 to 250 nT-R_M³ for these events. These findings, presented in Jia et al. (2019), are of crucial importance for understanding the space weathering at Mercury and its contribution to the generation of Mercury's exosphere.