Generation of turbulence in Kelvin-Helmholtz vortices at the Earth’s
magnetopause: Magnetospheric Multiscale observations
Abstract
The Kelvin-Helmholtz instability (KHI) at Earth’s magnetopause and
associated turbulence are suggested to play a role in the transport of
mass and momentum from the solar wind into Earth’s magnetosphere. We
investigate electromagnetic turbulence observed in KH vortices
encountered at the dusk flank magnetopause by the Magnetospheric
Multiscale (MMS) spacecraft under northward interplanetary magnetic
field (IMF) conditions in order to reveal its generation process, mode
properties, and role. A comparison with another MMS event at the dayside
magnetopause with reconnection but no KHI signatures under a similar IMF
condition indicates that while high-latitude magnetopause reconnection
excites a modest level of turbulence in the dayside low-latitude
boundary layer, the KHI further amplifies the turbulence, leading to
magnetic energy spectra with a power-law index –5/3 at
magnetohydrodynamic scales even in its early nonlinear phase. The mode
of the electromagnetic turbulence is analyzed with a single-spacecraft
method based on Ampère’s law, developed by Bellan (2016), for estimating
wave vectors as a function of spacecraft-frame frequency. The results
suggest that the turbulence does not consist of propagating normal-mode
waves, but is due to interlaced magnetic flux tubes advected by plasma
flows in the vortices. The turbulence at sub-ion scales in the early
nonlinear phase of the KHI may not be the cause of the plasma transport
across the magnetopause, but rather a consequence of three-dimensional
vortex induced reconnection, the process that can cause an efficient
transport by producing tangled reconnected field lines.