Self-consistent kinetic model of nested electron- and ion-scale magnetic cavities in space plasmas

Direct observations of cross-scale wave-particle energy transfer in space plasmas

The collisionless plasmas in space and astrophysical environments are intrinsically multiscale in nature, behaving as conducting fluids at macroscales and kinetically at microscales comparable to ion and/or electron gyroradii. A fundamental question in understanding the plasma dynamics is how energy is transported and dissipated across scales. Here, we present spacecraft measurements in the terrestrial foreshock, a region upstream of the bow shock where the solar wind population coexists with the reflected ions. In this region, the fluid-scale, ultralow-frequency waves resonate with the reflected ions to modify the velocity distributions, which in turn cause the growth of the ion-scale, magnetosonic-whistler waves. The latter waves then resonate with the electrons, and the accelerated electrons contribute to the excitation of electron-scale, high-frequency whistler waves. These observations demonstrate that the chain of wave-particle resonances is an efficient mechanism for cross-scale energy transfer, which could redistribute the kinetic energy and accelerate the particles upstream of the shocks.

Electron scale coherent structure as micro accelerator in the Earth's magnetosheath

Turbulent energy dissipation is a fundamental process in plasma physics that has not been settled. It is generally believed that the turbulent energy is dissipated at electron scales leading to electron energization in magnetized plasmas. Here, we propose a micro accelerator which could transform electrons from isotropic distribution to trapped, and then to stream (Strahl) distribution. From the MMS observations of an electron-scale coherent structure in the dayside magnetosheath, we identify an electron flux enhancement region in this structure collocated with an increase of magnetic field strength, which is also closely associated with a non-zero parallel electric field. We propose a trapping model considering a field-aligned electric potential together with the mirror force. The results are consistent with the observed electron fluxes from ~50 eV to ~200 eV. It further demonstrates that bidirectional electron jets can be formed by the hourglass-like magnetic configuration of the structure.