Oral Presentation
Precipitations of energetic electrons due to ULF modulation
Presenter: Hiroyo Ohya (Graduate School of Engineering, Chiba University)
Ultra low frequency (ULF) modulation of energetic electron precipitations has been observed by satellites, GPS-total electron content (TEC), high frequency (HF) Doppler sounders, Super Dual Auroral Radar Network (SuperDARN) HF radars, riometers and X-ray [e.g., Brito et al., 2012; Motoba et al., 2013]. However, there are few reports for the ULF modulation of the D-region ionosphere using very low frequency (VLF)/low frequency (LF) standard radio signals. In this study, we investigate the D-region signatures of the modulation due to the ULF waves using a network of VLF/LF standard radio waves 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 ATH (Athabasca, Canada, L = 4.31). We show the first observations of oscillations in intensities and phases on the NDK-ATH and WWVB-ATH paths with periods of 3-4 minutes during a small substorm of 05:25-05:50 UT (22:25-22:50 LT) on 4 June, 2017 (the AE index = 140 nT). When the solar wind dynamic pressure increased, the VLF/LF intensity decreased simultaneously, which suggests the whole magnetosphere was bumped by the solar wind. 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 magnetic pulsations with period of 3 minutes moved westward with velocity of 66.4 km/s at L = 3-4, These results show that ULF excitation due to a substom around midnight is related to the modulation of enegetic electron precipitation. In this presentation, we will discuss the cause of these VLF/LF oscillations.
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