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

Abstract

We investigated the modulation of D-region signatures due to ultra-low-frequency (ULF) waves using very-lowfrequency/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 fi rst observational report showing clear VLF/LF oscillations in the ULF range. The ground-based H-component magneticfi eld 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-fi eld 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 diff usion by the whistler-mode chorus wave is one possible mechanism accounting for the energetic electron precipitation.

Spatio‐Temporal Characteristics of IPDP‐Type EMIC Waves on April 19, 2017: Implications for Loss of Relativistic Electrons in the Outer Belt

“…In this study, we observed VLF/LF radio waves that are transmitted from stations in the United States: NDK (25.2 kHz, 46.37°N, 261.47°E, L = 3.0) and NLK (24.8 kHz, 48.20°N, 238.08°E, L = 2.9). The waves were received at Athabasca (ATH), Canada (54.6°N, 246.7°E, L = 4.3) (Hirai et al., 2018; Miyashita et al., 2020; Tsuchiya et al., 2018). This is a component of the Observation of CondiTion of ionized Atmosphere by VLF Experiment (OCTAVE), which is a network of VLF and LF radio wave receivers developed by Tohoku University and supported by the Institute for Space‐Earth Environmental Research (ISEE), Nagoya University, and the National Institute of Polar Research (NIPR).…”