Velocity-Space Signature of Transit-Time Damping in Weakly Collisional Plasmas

The field-particle correlation (FPC) technique has established itself as a practical method for decoding energy transfer between the fluctuating electromagnetic fields and charged particles in weakly collisional plasmas using single-point measurement data. By effectively isolating the oscillatory energy exchange, the FPC technique allows for the identification of distinct velocity-space signatures attributable to each physical process. Among various damping mechanisms of turbulence, resonant wave-particle interactions, including Landau damping (LD), transit-time damping (TTD), and cyclotron damping (CD), are of particular interest. While prior research has elucidated the velocity-space signature of LD and CD, this study focuses on the physics of TTD. LD and TTD occur when the particles' parallel velocity approaches the wave's phase speed. Unlike LD, which is governed by the electrostatic force, TTD is driven by the interaction between the magnetic moment and magnetic mirror force, rendering it more naturally depicted by a gyrokinetic distribution function in the low-frequency limit. Employing the Astrophysical gyrokinetics simulation code, AstroGK, this study explores velocity-space signatures of TTD from both linear and turbulent perspectives, alongside the comparison with LD. The subsequent analysis of the simulated results reveals the varying contributions of TTD based on different initial conditions.