Electron Vortex Generation in Earth's Collisionless Bow Shock: MMS Observations

In astrophysics and space, supercritical shock is generated when an object interacts with an incoming supersonic plasma stream. Its downstream plasmas are highly turbulent, containing abundant vortices on all scales from magnetohydrodynamic to electron gyroscales. Understanding the production of these vortices is at the forefront, especially on the electron scale. Using ultrafast measurements of NASA's Magnetospheric Multiscale spacecraft, we report on the fortunate multi-spacecraft observation of the formation of an electron vortex directly generated inside the Earth's quasi-parallel bow shock transition and propagated to the downstream turbulent magnetosheath. The vortex is generated inside the shock transition by anisotropic similar to 100-600 eV electrons trapped in an ion-scale magnetic hole which could show a tornado-like magnetic morphology. Our results demonstrate that the electron vortex can develop not only as a product of the forward cascade but also from the shock transition into its downstream turbulence, which adds to the short-scale turbulence and dissipation. Turbulence is a ubiquitous process in fluids where nonlinear interactions take place to generate dynamically evolving vortex structures across a broad range of scales. It plays a leading role in the cascade of mass and energy transport to increasingly small scales until it is dissipated as heat. In ionized plasmas, turbulence is more complex when considering electromagnetic fields, different kinds of charged particles, waves, and dissipation mechanisms. For studying plasma turbulence, a readily available natural laboratory is the near-Earth environment. The solar wind plasma encounters the Earth's magnetosphere, generating a supercritical bow shock. The plasma downstream of the bow shock is highly turbulent and contains abundant plasma vortices from the macroscale to the electron gyroscale. Therefore, understanding the bow shock's role in vortex generation is crucial and essential to understanding the development of turbulence in collisionless magnetized plasmas. This paper presents a precise origin of electron-scale vortex for unpredictable and random plasma turbulence. The vortex can directly start inside the collisionless shock as a small-scale vortex before being transported downstream turbulence, instead of the popular belief that developing from the downstream turbulence and larger-scale vortices. Electron vortex can directly start inside the supercritical shock as a small-scale vortex before being transported downstream turbulence Evidence for its generation inside the shock transition is provided by identifying the energetic shock electron population as its source The vortex could show a tornado-like magnetic morphology, coupling with a magnetic hole and propagating to the downstream magnetosheath