• BepiColombo
• boundary
• fluid
• instability
• kinetic
• lower-hybrid
• magnetopause
• Mercury
• plasma
• space

Boundaries separating two plasmas with different properties are ubiquitous in space (e.g.
the solar corona, planetary magnetospheres, astrophysical shocks and jets) as well as in
laboratory plasmas (e.g. tokamak and thrusters). Such boundaries are often the location
of plasma mixing and particle energization processes, thus affecting the whole plasma
system from the local/small scales to the global/large scales. This work focuses on a
specific kinetic instability that develops in such boundaries characterised by strong density
gradients, namely the lower-hybrid-drift instability (hereafter LHDI), and its interplay
with other fluid-scale instabilities 1 , targeting applications in space physics. The study
has been carried out by means of analytical computation and numerical simulations. In
particular, it aims at answering the following questions. (i) In which conditions is the
LHDI the dominant mechanism in terms of particle acceleration and mixing processes?
(ii) Do we expect to see the effect of such instability in future space missions on different
solar objects? If so, with which intensity and efficiency? In this document, I shall present
the analytical kinetic theory ( starting from the Vlasov equation) developed to study the
linear growth of the LHDI, as well as innovative 3D full-kinetic simulations used to study
the nonlinear stage of the LHDI and its interplay with other instabilities.
This study finds that the LHDI acts as a major player in shaping any plasma boundary
thinner than the ion skin depth. Nonetheless, we found that the LHDI electron acceleration
is negligible for typical space parameters considered in this study. Indeed, the saturation
of the LHDI, associated to the trapping of the ions in the wave potential, occurs at electric
energy levels small compared to the typical electron thermal energy, which naturally leads
to a slow and inefficient electron acceleration process. On longer time-scales, the LHDI,
via a coupling with the drift-kink instability, forms large scale finger-like structures, which
lead to strong cross-field plasma transport and a disruption of the initial configuration. In
particular, the Kelvin-Helmholtz instability (KHI) is dominant only when strong velocity
shear are realized, and it creates a diffuse layer, while for strong density gradients the
LHDI dominates the layer dynamics, creating plasma intrusions. Large scale intrusions
analog to the ones born in the LHDI nonlinear stage are created by the Rayleigh-Taylor
instability (RTI), this coupled mechanism is studied for the first time in a curved layer,
and we show its potential to create large scale finger-like structures extending far away
from the layer.
The results concerning electron acceleration are very general and valid for any plasma
contexts able to trigger the LHDI. Here our study intends to address (i) ongoing space
mission BepiColombo targeting Mercury’s highly kinetic magnetosphere and (ii) cometary
plasma physics, e.g. past mission Rosetta, especially for the part of interplay LHDI-RTI.