Ultra Low Frequency Modulation of Energetic Electron Precipitation in the D-Region Ionosphere in a Magnetically Quiet Time Using OCTAVE Very Low Frequency and Low Frequency (VLF/LF) Observations

There are several studies of energetic electron precipitation (EEP) modulated by ultra low frequency (ULF) during substorms, although there are few in geomagnetically quiet times.At present, it is not clear how often such quiet time EEP occurs and the mechanism involved.In this study, we investigate EEP modulated by ULF (,5 Hz) from Observation of CondiTion of ionized Atmosphere by VLF Experiment (OCTAVE) very low frequency (VLF; 3 kHz to 30 kHz) and low frequency (LF; 30 kHz to 300 kHz) observations in geomagnetically quiet times.The OCTAVE is a worldwide network of VLF/LF signals that we established.The VLF/LF transmitter signals from four transmitters (NLK, NDK, WWVB, and NAA) in the USA were received at Athabasca (ATH; 54.7 8 N, 113.3 8 E, L ¼ 4.3), Canada.There were oscillations in amplitude on the NDK-ATH and NLK-ATH paths with periods of 240 s and 270 s during magnetically quiet times from 11:20 Universal Time (UT) to 11:40 UT on October 9, 2017, respectively.The amplitudes of the VLF/LF variations were 3 dB and 1 dB on the NDK-ATH and NLK-ATH paths, respectively.Based on the wave-hop method, reflection height varied by ~4.3 km during the VLF/LF oscillations, indicating that the electron density in the D-region ionosphere increased by ~680 cm À3 (110%) at a 91 km height.The H component of ground-based magnetic field data at ATH and low latitudes showed the geomagnetic Pi2 pulsations with a similar period to that in the VLF/LF signals.The main cause of the VLF/LF oscillations could be ULF-modulated EEP due to the lowering of mirror points.