Electron scale magnetic holes generation driven by Whistler-to-Bernstein mode conversion in fully kinetic plasma turbulence

Magnetic holes (MHs) are coherent structures typically observed in turbulent plasmas, characterized by a sharp decrease in the magnetic field magnitude. MHs exist in different sizes, from magnetohydrodynamic to kinetic scales. Magnetospheric Multiscale (MMS) observations have revealed that electron scale MHs are very common in the turbulent Earth’s magnetosheath, potentially playing an important role in the energy cascade and dissipation. Nevertheless, the origin of MHs is still unclear and debated. In this work, we use fully kinetic simulations, initialized with typical Earth's magnetosheath parameters, to investigate the role of plasma turbulence in generating electron scale MHs. We identify a new turbulent-driven mechanism capable of generating MHs at scales of the order of a few electron inertial lengths. This mechanism involves the following steps: first, large-scale turbulent velocity shears produce localized regions with strong perpendicular electron temperature anisotropy; these regions quickly become unstable, producing oblique  whistler waves; then, as whistler fluctuations propagate over the inhomogeneous turbulent background, they develop a quasi-electrostatic component, evolving into Bernstein-like modes; the electric field of Bernstein-like modes produces filamentary electron currents that turn the wave into a train of current vortices; these vortices finally merge into a larger vortex that reduces the local magnetic field magnitude, ultimately evolving into a coherent electron scale MH. This work provides numerical evidence of a turbulence-driven mechanism for the generation of electron-scale MHs. Our results have potential implications for understanding the formation and occurrence of electron scale MHs in the Earth’s magnetosheath and other turbulent environments.