Gyro-resonant scattering of radiation belt electrons during the solar minimum by fast magnetosonic waves

摘要

In the current study, we perform statistical analysis of the magnetosonic (MS) waves (also often referred to as extremely low frequency (ELF) equatorial noise) in the range between the ion cyclotron frequency and the lower hybrid resonance frequency within 10° of the magnetic equator. Observations were made between 2 and 9 R_E using THEMIS Filter Bank (FBK) data. ELF waves with spectral power exceeding 10~(-6) nT~2/Hz are registered in ～3% of all samples in the inner magnetosphere. The survey has shown that, during the solar minimum, the average amplitude of equatorial ELF waves is less than 0.025 nT. Interpreting ELF events as MS waves, we have evaluated the corresponding wave-induced resonant scattering coefficients of radiation belt energetic electrons. We also study the effect of heavy ions on the scattering rates. The analysis reveals that the scattering by magnetosonic waves for various plasma compositions during geomagnetically quiet times is by up to two orders of magnitude slower than was previously reported and cannot significantly contribute to the long-term dynamics of the radiation belts. Computed electron scattering rates by magnetosonic waves extends to higher α_(eq) when the fraction of H~+ in the plasma decreases, while the range of pitch angles for which resonance occurs remains relatively insensitive to the plasma composition. While inclusion of multi-ion species into the wave dispersion relation produces noticeable changes in bounce-averaged scattering rates, the average rates are still significantly below typical scattering rates of chorus or hiss waves. Key PointsAverage MS waves amplitudes are small and the occurrence rate ～ 5%MS wave scattering more than 100 less efficient than by other wavesComposition can affect the scattering rates, but rates remain relatively low.