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

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.

Investigation of ∼ 20–40 mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphere

“…The result of opposite polarization in the dusk-and dawnside magnetosphere thus supports the idea that the majority of not only the duskside but also the dawnside general ULF waves are driven by the KHI. Furthermore, James et al (2016) observed the average polarization of their observed ULF waves to be left-handed on the dawnside and right-handed on the duskside (in agreement with the result in this study) and suggested this to be a consequence of the KHI. The total power spectral density is on average larger for the KHI-ULF waves than the general ULF waves.…”

“…In addition, a study by James et al (2016) covering MESSENGER magnetosphere crossings over the time period 2011-2015 also suggests that ULF waves (occurring mainly at the dayside), with similar characteristics as those KHI-ULF waves reported in Liljeblad et al (2016), could be KHI driven.…”

“…These were narrowband ∼ 0.5 Hz waves with a polarization changing from right-hand circular to linear as the spacecraft moved towards the planet. Since the arrival of Mercury Surface Space Environment Geochemistry, and Ranging (MESSENGER), Mercury magnetospheric ULF waves have been observed a number of times, ranging from ∼ mHz (Liljeblad et al, 2016;James et al, 2016) up to ∼ 1 Hz (Boardsen et al, 2009(Boardsen et al, , 2012.…”

Field Line Resonance in the Hermean Magnetosphere: Structure and Implications for Plasma Distribution

“…Figure a shows that ρ eq takes values within the range 95–115 amu/cm 3 , where the highest values correspond to the lowest power index, α = 0. James et al () estimated a much higher density of ∼ 240 amu/cm 3 for the same wave event, but the method used to make that estimate assumed that the plasma mass density was constant along the field line and did not include the portion of the field line which existed between the surface and the core of Mercury (i.e., the wave was reflected at the planet's surface). James et al () calculated this value by inverting a simple time‐of‐flight calculation, a method which is invalid when the wavelength is comparable to the size of the system as it provides a poor estimate of the fundamental wave frequency (Singer et al, ).…”

“…The majority of the waves from these studies were observed toward dusk, in the dayside of Mercury's magnetosphere. Another recent study by James et al (), which examined all wave activity observed within the Hermean magnetosphere with frequencies below the local sodium gyrofrequency, found a large population of toroidally polarized wave activity across the whole dayside and the flanks of the magnetosphere and suggested that some of the observed waves may be Earth‐like toroidal resonances.…”