A statistical survey of ultralow-frequency wave power and polarization in the Hermean magnetosphere.

Timothy K Yeoman,Suzanne M Imber,Emma J Bunce,Matthew K James,Haje Korth

doi:10.1002/2016ja023103

Abstract

We present a statistical survey of ultralow‐frequency wave activity within the Hermean magnetosphere using the entire MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data set. This study is focused upon wave activity with frequencies <0.5 Hz, typically below local ion gyrofrequencies, in order to determine if field line resonances similar to those observed in the terrestrial magnetosphere may be present. Wave activity is mapped to the magnetic equatorial plane of the magnetosphere and to magnetic latitude and local times on Mercury using the KT14 magnetic field model. Wave power mapped to the planetary surface indicates the average location of the polar cap boundary. Compressional wave power is dominant throughout most of the magnetosphere, while azimuthal wave power close to the dayside magnetopause provides evidence that interactions between the magnetosheath and the magnetopause such as the Kelvin‐Helmholtz instability may be driving wave activity. Further evidence of this is found in the average wave polarization: left‐handed polarized waves dominate the dawnside magnetosphere, while right‐handed polarized waves dominate the duskside. A possible field line resonance event is also presented, where a time‐of‐flight calculation is used to provide an estimated local plasma mass density of ∼240 amu cm−3.

Highlights

We present a statistical survey of ultralow-frequency wave activity within the Hermean magnetosphere using the entire MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data set
Observations show that wave power is common throughout the magnetosphere and that compressional waves provide more of this wave power than the azimuthal or poloidal waves
Azimuthal wave power is most common within the dayside magnetopause, providing evidence that interactions with the solar wind such as the Kelvin-Helmholtz instability may be driving ULF wave activity within the magnetosphere, possibly through field line resonance

Summary

Introduction

The transition to a linearly polarized wave suggested that this may have been a resonance—Russell [1989] suggested that this wave could have been a fourth harmonic of the fundamental field line resonance (FLR) frequency, fFLR, based on some assumptions of field line length and Alfvén velocity, vA. Later it was argued by Southwood [1997] that this wave could not have been a pure FLR like those observed in the terrestrial magnetosphere as there was a significant compressional component to the wave, whereas terrestrial FLRs are shear Alfvén waves which oscillate predominantly azimuthally. Instead, Southwood [1997] suggested that these may be similar to standing waves at Earth modified by the presence of hot plasma [e.g., Southwood, 1976]

Results

Discussion

Conclusion