Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury

Abstract

The NASA MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft entered orbital phase around Mercury on 18 March 2011. A surprising consistent feature in the data returned is large-scale vortices that form exclusively on the dusk side of the magnetosphere. Here we present global kinetic hybrid simulations that explain these observations. It is shown that vortices are excited by a Kelvin-Helmholtz instability near the subsolar point, which grows convectively along the dusk-side magnetopause. Virtual time series along a track approximating a flyby of the MESSENGER show correspondence with the satellite data; the data contain sawtooth oscillations in plasma density, flow and magnetic field, and exhibit the observed dawn-dusk asymmetry. It is shown that asymmetry between dawn and dusk at Mercury is controlled by the finite gyroradius of ions and by convection electric fields. Mercury's magnetosphere offers a natural laboratory for studying plasma regimes not present in other planetary magnetospheres or the laboratory.

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

“…Even though we do observe dawnside nonlinear KH waves and linear waves under certain magnetic field conditions, there is a total absence of dawnside KH waves at the flank. This observation may be connected to a result from the simulations of the solar wind interaction with Mercury by Paral and Rankin [], showing a lack of a steady magnetopause boundary at the dawn flank. The authors interpret this as a possible consequence from a Rayleigh‐Taylor instability that breaks up the magnetopause boundary, which makes it highly difficult for dawnside KH waves to grow convectively along the flank.…”

“…Moreover; KH waves have been observed at both the dawnside and duskside terrestrial magnetopause [e.g., Chen and Kivelson , ; Hasegawa et al , ], whereas at Mercury only duskside waves have so far been reported [ Slavin et al , ; Boardsen et al , ; Sundberg et al , , ]. Many authors have suggested that the asymmetry between the duskside and dawnside magnetopause is mainly due to finite Larmor radius (FLR) effects [e.g., Nagano , ; Glassmeier and Espley , ; Nakamura et al , ; Sundberg et al , ; Paral and Rankin , ]. The influence of the ion gyromotion on KH wave growth has been studied both by a FLR modified MHD approach [e.g., Nagano , ; Glassmeier and Espley , ; Sundberg et al , ] and by fully kinetic or global kinetic hybrid simulations [e.g., Nakamura et al , ; Paral and Rankin , ].…”

MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

“…Numerical simulations predict large differences in the structure and growth of K‐H vortices at the kinetic scale between positive (dusk) and negative (dawn) field‐aligned vorticity cases [ Nakamura et al ., ; Paral and Rankin , ]. The simulations of Paral and Rankin [], for example, show a dawn‐dusk asymmetry in the formation of K‐H waves in a system with Mercury‐like geometry. However, they did not include the presence of planetary ions, and their simulations have K‐H wave periods of ~10 s, a factor of ~2 smaller than most of those observed in Mercury's magnetosphere.…”

“…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 ., ].…”

Particle Orbits at the Magnetopause: Kelvin‐Helmholtz Induced Trapping

“…In global magnetospheric models, it remains numerically challenging to capture local shear flow related details, although KHI development has been demonstrated for a northward interplanetary magnetic field condition at Earth in Guo et al (2010), in global kinetic hybrid simulations for Mercury (Paral & Rankin, 2013), where the vortices are mainly advected to the dusk side, and recently also in the case of the Jovian magnetosphere, where dawn-dusk assymmetries play a crucial role (Zhang et al, 2018). As we are interested in how the details of the KHI development determine local particle orbits, we will use the fully 3-D, local box DMLR configurations representative of the magnetopause variation during KHI and study particle motion in specific phases of the MHD evolution.…”