On High-frequency Kinetic Alfvén Waves in Space and Solar Plasma

Abstract Kinetic Alfvén waves (KAWs) are ubiquitous in space and solar plasmas and are believed to be crucial for energy transfer and particle energization. Existing studies on KAWs primarily focus on the low-frequency approximation, where the wave frequency is much smaller than the proton cyclotron frequency (i.e., ω ≪ ω cp ). However, the wave properties of high-frequency KAWs with ω ≳ ω cp remain unclear. In this work, based on the two-fluid theory, we derive a general dispersion relation for KAWs spanning low-frequency to high-frequency regimes, and examine this dispersion relation and the electromagnetic properties of both low-frequency and high-frequency KAWs. Our findings reveal that, compared to low-frequency KAWs, high-frequency KAWs exhibit several distinct features: higher wave frequency ( ω ≳ ω cp ), propagation angle over a broader oblique angle range, significantly larger ratio of parallel to perpendicular electric field, and the magnetic helicity and magnetic compressibility that are highly sensitive to the plasma beta. The enhanced parallel electric field highlights the pivotal role of high-frequency KAWs in field-aligned particle acceleration. This work extends KAW theory to the high-frequency domain, providing key insights into KAW properties and the particle acceleration process in space and solar plasmas.