ULF modulation of energetic electron precipitation observed by VLF/LF radio propagation

We investigated the modulation of D-region signatures due to ultra-low-frequency (ULF) waves using very-low-frequency/low-frequency (VLF/LF) radio propagation. Three transmitters from the United States (NLK, NDK, and WWVB) were received at Athabasca (ATHA), Canada. We observed oscillations in intensities and phases on the NDK-ATHA and WWVB-ATHA paths with a period of 3 to 4 minutes, associated with ULF waves during a substorm at 05:25–05:45 UT on 4 June, 2017. This is the first observational report showing clear VLF/LF oscillations in the ULF range. The ground-based H-component magnetic-field variations and Doppler velocity observed by the SuperDARN HF radars showed the same periodic changes as seen in the VLF/LF oscillations, suggesting that energetic electron precipitation (EEP) over the WWVB-ATHA and NDK-ATHA paths was modulated by the ULF waves. Based on ground-based magnetic-field data, we concluded that the ULF wave corresponded to the Pi2 pulsations associated with the substorm, because the propagation direction of the wave was westward (66.4 km/s) from the pre-midnight sector, the magnetic variations at low latitudes were inphase over wide longitudes, and the magnetic variations at ATHA slightly preceded those at low latitudes. A rising-tone chorus emission was observed in the frequency range of 5 kHz to 6 kHz at ATHA during the VLF/LF oscillations. Pitch-angle diffusion by the whistler-mode chorus wave is one possible mechanism accounting for the energetic electron precipitation.