Coupled transport and acceleration of suprathermal electrons by transit-time damping in corotating interaction regions

Abstract

Particles in space and astrophysical plasmas can be efficiently accelerated through interactions with compressive magnetohydrodynamic (MHD) turbulence. In this work, we investigate the coupled transport and acceleration of suprathermal electrons in corotating interaction regions (CIRs) of the solar wind based on the transit-time damping (TTD) mechanism. Using reconstructed Parker spiral magnetic fields from solar wind observations, we estimate the transverse magnetic field gradients that characterize compressive turbulence in CIRs. From resonant interactions with fast-mode MHD waves, we derive the momentum and pitch-angle diffusion coefficients and solve the Fokker–Planck equation to follow the evolution of the electron momentum distribution. Our results show that TTD-driven stochastic acceleration naturally produces suprathermal electron populations from an initial Maxwellian distribution. The resulting suprathermal fraction can reach values of order \(10^{-3} - 10^{-2}\), depending on the plasma conditions and heliocentric distance. As the transverse magnetic field gradient decreases with radial distance, the overall acceleration efficiency is reduced while the range of resonant interactions broadens. We further find that lower plasma beta enhances the acceleration efficiency due to stronger magnetic compressions. These results suggest that compressive turbulence in CIRs can play an important role in generating suprathermal electron populations in the heliosphere.