Statistical investigation of Kelvin‐Helmholtz waves at the magnetopause of Mercury

Abstract

A large study of Kelvin-Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 encounters of not only nonlinear KH waves but also linear surface waves, including the first observations of KH waves at the dawnside magnetopause. Most of the waves are in the nonlinear phase (90%) occur at the duskside magnetopause (93%), under northward magnetosheath magnetic field conditions (89%) and during greater magnetosheath B z (23 nT) values than in general. The average period and amplitude is 30 ± 14 s and 14 ± 10 nT, respectively. Unlike duskside events, dawnside waves do not appear at the magnetopause flank (< 6 magnetic local time). This is in agreement with previous observations and modeling results and possibly explained by finite Larmor radius effects and/or a lack of a large-scale laminar flow at the dawnside magnetopause boundary.

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

“…The large majority of KHWs observed in Liljeblad et al (2014) were identified at the duskside magnetopause. As shown by Liljeblad et al (2015), MESSENGER covers the Hermean magnetosphere almost symmetrically during the year 2011.…”

“…Perhaps these dawnside KHWs do indeed frequently develop at the magnetopause but are constantly repressed by certain conditions of the surrounding environment (such as a broader velocity shear layer or a low-latitude boundary layer present most often at the dawnside as reported in Liljeblad et al (2015)) and will therefore not be as clearly visible as the duskside KHWs. Thus, they are more difficult to identify with the criteria used in Liljeblad et al (2014). …”

“…The study by Liljeblad et al (2016) investigated all 131 MESSENGER magnetospheric traversals just prior to or after the observation of Kelvin-Helmholtz waves (KHWs) at the magnetopause during 2011-2013, which in turn were identified in an earlier study by Liljeblad et al (2014). Distinct ULF wave signatures in the mHz range could be detected in 44 out of these 131 magnetospheric passages.…”

MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

“…These orbits also passed through Mercury's dusk magnetopause. Analogous data from orbits passing through Mercury's dawn magnetopause, also examined, were not used in this analysis because of the absence of abundant Na + or frequent evidence for K‐H vortices [ Raines et al ., ; Liljeblad et al ., ]. Instead, we focus on the differences between the dayside and nightside of the dusk terminator, where there is a strong spatial gradient in Na + content.…”

“…Many large‐scale structures of the magnetosphere have been successfully reproduced by global simulations using both magnetohydrodynamic (MHD) [ Kabin et al ., ; Benna et al ., ] and hybrid techniques [ Wang et al ., ; Trávníček et al ., ; Schriver et al ., ]. Some features, such as the preferential formation of Kelvin‐Helmholtz (K‐H) vortices along Mercury's duskside magnetopause (MP) [ Boardsen et al ., ; Sundberg et al ., ], with a few also observed on the dawnside MP [ Liljeblad et al ., ] (Figure ), have been linked to kinetic scale plasma physics [ Nakamura et al ., ; Paral and Rankin , ]. These large‐scale (comparable to a Mercury radius, R M , or 2440 km) K‐H vortices may be capable of transporting substantial quantities of mass and energy between the solar wind and Mercury's magnetosphere, as has been observed at other planetary systems [e.g., Lee et al ., ; Hasegawa et al ., ; Masters et al ., ; Wilson et al ., ; Delamere et al ., ].…”