Cross-Scale Energy Transfer From Ion-Scale to Electron-Scale Waves in the Earth's Foreshock Region

Cross-scale energy transfer is a fundamental problem in plasma physics but is poorly understood. Based on Magnetospheric Multiscale satellite (MMS) data, we present the evidence of the energy transfer between ion-scale and electron-scale waves in the Earth's foreshock region. Low-frequency fast-magnetosonic waves (LFWs, ∼0.2 Hz; ion-gyration scales) are observed in the solar wind upstream of the Earth's bow shock. Due to the magnetic compression of LFWs, suprathermal electrons (∼10–100s eV) are adiabatically heated in the perpendicular direction, which leads to the high anisotropy in the high-magnetic-field region. Then high-frequency whistler mode waves (HFWs, 0.1–0.5 f_ce; electron-gyration scales) are excited by those anisotropic electrons through cyclotron resonance. Therefore, this study reveals how energy is transported from LFWs to HFWs, suggesting that wave-particle interactions have played a key role in cross-scale energy transfer in collisionless plasmas.