Dr. Jing-Huan Li

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 of two events observed in the foreshock, a region upstream of the terrestrial bow shock. The fluid-scale ultra-low-frequency waves and ion-scale magnetosonic-whistler waves simultaneously resonate with the ions. The ion acceleration from ultra-low-frequency waves leads to velocity distributions unstable to the growth of magnetosonic-whistler waves, which in turn resonate with the electrons. Eventually, the energized electrons contribute to the excitation of electron-scale high-frequency whistler waves. These observations demonstrate that the chain of wave-particle resonance is an efficient mechanism for cross-scale energy transfer, which could redistribute the kinetic energy and accelerate the particles upstream of the shocks.