The Velocity-Space Signature of Transit-Time Damping
arxiv(2024)
摘要
Transit-time damping (TTD) is a process in which the magnetic mirror force –
induced by the parallel gradient of magnetic field strength – interacts with
resonant plasma particles, leading to the collisionless damping of
electromagnetic waves and the resulting energization of those particles through
the perpendicular component of the electric field, E_⊥. In this study, we
utilize the recently developed field-particle correlation technique to analyze
gyrokinetic simulation data. This method enables the identification of the
velocity-space structure of the TTD energy transfer rate between waves and
particles during the damping of plasma turbulence. Our analysis reveals a
unique bipolar pattern of energy transfer in velocity space characteristic of
TTD. By identifying this pattern, we provide clear evidence of TTD's
significant role in the damping of strong plasma turbulence. Additionally, we
compare the TTD signature with that of Landau damping (LD). Although they both
produce a bipolar pattern of phase-space energy density loss and gain about the
parallel resonant velocity of the waves, they are mediated by
different forces and exhibit different behaviors as v_⊥→ 0. We also
explore how the dominant damping mechanism varies with ion plasma beta
β_i, showing that TTD dominates over LD for β_i > 1. This work
deepens our understanding of the role of TTD in the damping of weakly
collisional plasma turbulence and paves the way to seek the signature of TTD
using in situ spacecraft observations of turbulence in space plasmas.
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