  Boundary layers in space and astrophysical plasmas are the location of
complex dynamics where different mechanisms coexist and compete eventually
leading to plasma mixing. In this work, we present fully kinetic
Particle-In-Cell simulations of different boundary layers characterized by the
following main ingredients: a velocity shear, a density gradient and a magnetic
gradient localized at the same position. In particular, the presence of a
density gradient drives the development of the lower hybrid drift instability
(LHDI), which competes with the Kelvin-Helmholtz instability (KHI) in the
development of the boundary layer. Depending on the density gradient, the LHDI
can even dominate the dynamics of the layer. Because these two instabilities
grow on different spatial and temporal scales, when the LHDI develops faster
than the KHI an inverse cascade is generated, at least in 2D. This inverse
cascade, starting at the LHDI kinetic scales, generates structures at scale
lengths at which the KHI would typically develop. When that is the case, those
structures can suppress the KHI itself because they significantly affect the
underlying velocity shear gradient. We conclude that depending on the density
gradient, the velocity jump and the width of the boundary layer, the LHDI in
its nonlinear phase can become the primary instability for plasma mixing. These
numerical simulations show that the LHDI is likely to be a dominant process at
the magnetopause of Mercury. These results are expected to be of direct impact
to the interpretation of the forthcoming BepiColombo observations.
