MMS Observations of the Multi-Scale Wave Structures and Parallel
Electron Heating in the Vicinity of the Southern Exterior Cusp
Abstract
Understanding the physical mechanisms responsible for the cross-scale
energy transport and plasma heating from solar wind into the Earth’s
magnetosphere is of fundamental importance for magnetospheric physics
and for understanding these processes in other places in the universe
with comparable plasma parameter ranges. This paper presents
observations from Magnetosphere Multi-Scale (MMS) mission at the
dawn-side high-latitude dayside boundary layer on 25th of February, 2016
between 18:55-20:05 UT. During this interval MMS encountered both inner
and outer boundary layer with quasi-periodic low frequency fluctuations
in all plasma and field parameters. The frequency analysis and growth
rate calculations are consistent with the Kelvin-Helmholtz Instability
(KHI). The intervals within low frequency wave structures contained
several counter-streaming, low- (0-200 eV) and mid-energy (200 eV-2 keV)
electrons in the loss cone and trapped energetic (70-600 keV) electrons
in alternate intervals. Wave intervals also showed high energy
populations of O+ ions, likely of ionospheric or ring current origin.
The counter-streaming electron intervals were associated with a
large-magnitude field-aligned Poynting fluxes. Burst mode data at the
large Alfven velocity gradient revealed a strong correlation between
counter streaming electrons, enhanced parallel electron temperatures,
strong anti-field aligned wave Poynting fluxes, and wave activity from
sub-proton cyclotron frequencies extending to electron cyclotron
frequency. Waves were identified as Kinetic Alfven waves but their
contribution to parallel electron heating was not sufficient to explain
the > 100 eV electrons, and rapid non-adiabatic heating of
the boundary layer as determined by the characteristic heating
frequency, derived here for the first time.