Interpretation of multiple peaks observed in the quiet-time energy spectra of inner belt electrons

Multiple peaks observed in the energy spectra of electrons at the lower edge of the inner radiation belt ( L ⩽ 1.3) in the South Atlantic Anomaly region during magnetically quiet periods are interpreted in terms of cyclotron resonance involving whistler-mode waves radiated from multiple ground-based VLF transmitters. Our results show that out of six peaks observed at L = 1.22 and L = 1.31, three peaks corresponding to higher energies are formed as a result of pitch angle diffusion into the loss cone driven by first-order cyclotron resonance interaction involving the waves radiated from transmitters NSS (21.4 kHz), OMEGA (13.6 kHz) and OMEGA (10.2 kHz). We suggest that the remaining peaks at lower energies in the spectra are formed as a result of cyclotron resonance involving third harmonics of the waves radiated from some of the above and other transmitters. Other possibilities suggested for the formation of multiple peaks are also discussed.