The Influence of IMF Clock Angle on Dayside Flux Transfer Events at Mercury

Roger P Leyser,Suzanne M Imber,James A Slavin,Stephen E Milan

doi:10.1002/2017gl074858

Roger P Leyser, Suzanne M Imber + Show 2 more

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https://doi.org/10.1002/2017gl074858

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Abstract

AbstractAnalysis of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) data has shown for the first time that the orientation of the interplanetary magnetic field (IMF) in the magnetosheath of Mercury plays a crucial role in the formation of flux transfer events (FTEs) at the dayside magnetopause. During the first 4 Hermean years of MESSENGER's orbit around Mercury, we have identified 805 FTEs using magnetometer data. Under conditions of near‐southward IMF, at least one FTE was detected on nearly 70% of passes through the magnetopause but the observation rate during northward IMF was less than 20%. FTEs were also observed preferentially in the prenoon sector.

Highlights

Mercury was first discovered to have an intrinsic global magnetic field by Mariner 10 (Ness et al, 1974, 1975), and details of the nature of its magnetosphere were refined through measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft when it became the first satellite to orbit (Mercury Anderson et al, 2011, 2012)
Data has shown for the first time that the orientation of the interplanetary magnetic field (IMF) in the magnetosheath of Mercury plays a crucial role in the formation of flux transfer events (FTEs) at the dayside magnetopause
Under conditions of near-southward IMF, at least one FTE was detected on nearly 70% of passes through the magnetopause but the observation rate during northward IMF was less than 20%

Summary

Introduction

Mercury was first discovered to have an intrinsic global magnetic field by Mariner 10 (Ness et al, 1974, 1975), and details of the nature of its magnetosphere were refined through measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft when it became the first satellite to orbit (Mercury Anderson et al, 2011, 2012). Studies by Slavin et al (2012) and Imber et al (2014) have demonstrated that FTEs at Mercury occur more frequently than those seen at Earth and are considerably larger with respect to the size of the magnetosphere This is attributed to the reconnection-driven formation of FTEs being greatly enhanced due to the stronger interaction between the IMF and the Hermean magnetic field. DiBraccio et al (2013) found that the dimensionless reconnection rate displayed very little dependence on the magnetic shear angle between the two regimes, contrary to similar investigations at Earth (e.g., Sonnerup, 1974) They attributed this to a low Alfvén Mach number, MA, and low plasma β (the ratio of thermal pressure to magnetic pressure) in the Hermean magnetosheath. Our analysis suggests that the formation of FTEs at Mercury exhibits a strong dependence on the orientation of the IMF, with a considerably enhanced production rate for magnetopause crossings during which the magnetic shear angle was large

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