[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-P13] ULF modulation of VLF/LF transmitter signals in the D-region ionosphere

*田中 健太郎¹、大矢 浩代¹、土屋 史紀²、塩川 和夫³、三好 由純³、寺本 万里子⁴、Martin Connors⁵、中田 裕之¹ (1.千葉大学大学院工学研究院、2.東北大学大学院理学研究科惑星プラズマ・大気研究センター、3.名古屋大学宇宙地球環境研究所、4.九州工業大学、5.Athabasca University)

キーワード：ULF波、VLF/LF波、高エネルギー降下電子

Energetic electron precipitation (EEP, >100 keV) from radiation belts to the D-region ionosphere during substorms has been studied since the 1960’s using very low frequency (VLF, 3-30 kHz)/low-frequency (LF, 30-300 kHz) radio transmitter signals and riometers (Thorne and Larsen, 1976). The VLF/LF technique is most sensitive to the ionization of the D-region ionosphere, which is caused by EEP with energies typically >100 keV (Rodger et al., 2012). Several studies have reported ionospheric modulations due to ultra low frequency (ULF) Pc5 and Pi2 modulations (Asnes et al., 2004). Modulation of D-region due to EEP by ULF waves during a substorm was reported (Miyashita et al., 2020). However, there was only one report for the EEP associated with ULF modulation using VLF/LF transmitter signals. In this study, we investigate two EEP events associated with the ULF modulation that occurred on 05:00-05:30 UT on May 5, 2011 and 11:15-11:40 UT on October 9, 2017, using the VLF/LF transmitter signals observed in North America. The four transmitters were NDK (25.2 kHz, 46.37N, 261.47E, L=3.0); WWVB (60.0 kHz, 40.67N, 254.95E, L=2.3); NLK (24.8 kHz, 48.20N, 238.08E, L=2.9 ); and NAA(24.0 kHz, 44.65N, 292.72E, L=3.0). One receiver at Athabasca (ATHA), Canada (54.6N, 246.7E, L=4.3) was used in this study. During 05:00-05:30 UT on May 5, 2011(a small substorm), oscillations in amplitude and phase of VLF/LF waves with a period of 250-350 s were observed on the NDK-ATHA and WWVB-ATHA paths. The H component of ground-based magnetic field data around ATHA varied with a period of 300-400 s, which was similar to that of VLF/LF waves. However, any discrete ULF waves could not be identified in the geomagnetic field on the ground. The VLF/LF oscillations could be caused by changes in D-region current due to variations in electron density. During 11:15-11:40 UT on October 9, 2017(magnetically quiet time) oscillations in amplitude of VLF/LF waves with a period of 200-300 s were observed on the NLK-ATHA and NDK-ATHA paths. Furthermore, the H-component of ground-based magnetic field variations around ATHA and the low latitudes showed periodic variations of 200-300 s. When the solar wind speed was less than 300 km/s, long period (more than 200 s) Pi2 pulsations were generated by the cavity mode of the magnetosphere (Kwon et al., 2013). In this event the solar wind speed was small, and the period of the ground magnetic field was 200-300 s. Therefore, we considered that the VLF/LF oscillations would be caused by EEP due to Pi2 ULF modulation associated with cavity resonance. In this session, we will discuss the cause of the VLF oscillations in detail.