UCLA

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.