SM009-02
ULF Modulation of Energetic Electron Precipitations Observed by VLF/LF Transmitter Signals

Monday, 7 December 2020: 17:33
Virtual
Hiroyo Ohya1, Takuya Miyashita1, Fuminori Tsuchiya2, Mitsunori Ozaki3, Kazuo Shiokawa4, Yoshizumi Miyoshi5, Nozomu Nishitani6, Tomoaki Hori6, Mariko Teramoto7, Martin G Connors8 and Simon G Shepherd9, (1)Chiba University, Chiba, Japan, (2)PPARC, Tohoku University, Sendai, Japan, (3)Kanazawa University, Kanazawa, Japan, (4)ISEE, Nagoya Univ, Aichi, Japan, (5)ISEE, Nagoya University, Nagoya, Japan, (6)Nagoya University, Nagoya, Japan, (7)Kyushu Institute of Technology, Tobata, Japan, (8)Athabasca University, Athabasca, AB, Canada, (9)Dartmouth College, Thayer School of Engineering, Hanover, NH, United States
Abstract:
ULF (ultra low frequency, 1 mHz – 1 Hz) magnetic pulsations modulate the precipitation of electrons in the magnetosphere [Coroniti and Kennell, 1970]. The energetic electron precipitations (EEPs) associated with Pi2 pulsations (40-150 s) were reported [Asnes et al., 2004]. Auroral precipitations and energetic particle flux near the equator in nightside magnetosphere occur due to Pi2 modulation [Saka et al., 1999]. However, there have been few reports for the ULF modulation of high energy electrons that modulate the D-region ionosphere. In this study, we investigate the D-region signatures of the modulation due to the ULF waves using a network of VLF/LF transmitter signals in North America. The transmitter signals from NLK (USA, 24.8 kHz, L = 2.88), NDK (USA, 25.2 kHz, L = 2.98) and WWVB (USA, 60.0 kHz, L = 2.26) were observed by a receiver at ATHA (Athabasca, Canada, L = 4.31). We show the first observations of oscillations in intensities and phases on the NDK-ATHA and WWVB-ATHA paths with periods of 3-4 minutes during a small substorm at 05:25-05:50 UT on 4 June, 2017 (AE index = 140 nT). Based on ground-based magnetic observations, there were pulsations with the same periods with the VLF/LF oscillations both at high- and low-latitudes. The ground-based H-component magnetic field variations and Doppler velocity observed by the SuperDARN (Super Dual Auroral Radar Network) HF (high frequency, 8-20 MHz here) radars showed the same periodic changes as seen in the VLF/LF oscillations, suggesting that EEPs over the both paths was modulated by the ULF waves. We conclude that the ULF wave was the Pi2 pulsations associated with the substorm, because propagation direction of the wave was westward (66.4 km/s) from the pre-midnight sector, and the magnetic variations at low latitudes were in-phase over wide longitudes. The VLF/LF oscillations preceded magnetic variations at ATHA by several tens of seconds, which may be caused by time difference between the EEPs and ULF wave propagation. In this presentation, we will discuss possible causes of these VLF/LF oscillations.
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