Enhanced radial diffusion of radiation belt electrons caused by field line

The radial diffusion of radiation belt electrons due to interacting with ultra-low frequency (ULF) waves has traditionally been studied by assuming either a constant wave frequency at specific L drift-resonating with the electron or broad-band waves, resonating across a wider L range. We investigate a special case of radial diffusion caused by narrow-band ULF waves, generated by field line resonance (FLR), whose frequency varies with L in a manner that continuously satisfies the drift resonance condition throughout an electron’s radial motion. The conditions for this continuous resonance are derived for both non-relativistic and relativistic electrons in a dipolar magnetic field, which are further validated by two-dimensional test-particle simulations. The results show that, under conditions with inverse power-law relationship of -0.3~-0.5 between the wave frequency and L, relativistic electrons experience significantly enhanced radial diffusion, with the diffusion coefficient exceeding that of constant-frequency conditions by more than an order of magnitude.