Cross-scale Dynamics Driven by Plasma Jet Braking in Space

Plasma jets are ubiquitous in space. In geospace, jets can be generated by magnetic reconnection. These reconnection jets, typically at fluid scale, brake in the near-Earth region, dissipate their energies, and drive plasma dynamics at kinetic scales, generating field-aligned currents that are crucial to magnetospheric dynamics. Understanding of the cross-scale dynamics is fundamentally important, but observation of coupling among phenomena at various scales is highly challenging. Here we report, using unprecedentedly high-cadence data from NASA's Magnetospheric Multiscale Mission, the first observation of cross-scale dynamics driven by jet braking in geospace. We find that jet braking causes MHD-scale distortion of magnetic field lines and development of an ion-scale jet front that hosts strong Hall electric fields. Parallel electric fields arising from the ion-scale Hall potential generate intense electron-scale field-aligned currents, which drive strong Debye-scale turbulence. Debye-scale waves conversely limit intensity of the field-aligned currents, thereby coupling back to the large-scale dynamics. Our study can help in understanding how energy deposited in large-scale structures is transferred into small-scale structures in space.