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 fce; 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. Plain Language Summary Cross‐scale energy transfer is a fundamental problem in plasma physics, in which wave‐particle interaction plays an important role. Previous studies provide a limited and incomplete picture of the coupling process between high‐frequency whistler mode waves (HFWs, electron‐scale) and low‐frequency fast‐magnetosonic waves (LFWs, ion‐scale), and therefore further investigation is still needed to clarify their coupling mechanism. Based on MMS satellite data, we present a promising coupling process between HFWs and LFWs in the Earth's foreshock region. Observation results indicate that suprathermal electrons are perpendicularly heated by the LFWs in the high‐magnetic‐field region via betatron acceleration. These electrons could generate the HFWs through cyclotron resonance, which is confirmed by both observations and theoretical calculations. Therefore, this event shows the energy transfer among LFWs, suprathermal electrons, and HFWs, illustrating that the energy is directly transported from the ion scale to the electron scale. Our finding provides a potential generation mechanism for HFWs, which may be highly important for understanding the electron dynamics in the Earth's foreshock region. Key Points Using Magnetospheric Multiscale satellite data, we present the evidence of the energy transfer from ion‐scale low‐frequency fast‐magnetosonic waves (LFWs) to electron‐scale high‐frequency whistler mode waves (HFWs) in the Earth's foreshock region Due to the magnetic compression of