Spectral Analysis of Severe Magnetic Fluctuations at Substorm Onset in the near-Earth Plasma Sheet Observed by THEMIS-D and -E

Severe magnetic fluctuations in the nightside near-Earth plasma sheet are considered as one possible instability to trigger substorm onset. Previous studies using GEOTAIL or AMPTE/CCE measurements have conducted the spectral analysis of these magnetic fluctuations [e.g., Ohtani et al., 1998; Volwerk et al., 2003; Shiokawa et al., 2005]. In our previous research by Xu et al. [2017], we made a statistical analysis of severe magnetic fluctuations at |X|=6-12 R_E using THEMIS-E measurements in 2013-2014, and showed their occurrence rates, spatial distribution and relations with plasma flow. The severe magnetic fluctuation events are identified as σ_B/B > 0.5, where σ_B and B are standard deviation and average value of magnetic field intensity during the time interval of local gyroperiod. The selected events using this criterion tend to occur near substorm onset. In this study, we further investigate the spectral property of the severe magnetic fluctuations obtained by the same criterion. Compared with previous spectral studies using GEOTAIL and AMPTE/CCE, we use the measurements by THEMIS with higher sampling rate (up to 128 Hz) and/or for different radial region at 6-12 R_E. We analyzed THEMIS-D and E measurements for 11 years in 2008-2018, and found 10848 severe magnetic fluctuation events. We did not find clear relationship between their occurrence rates and solar activity. We then further extracted 36 dipolarization events with multiple magnetic fluctuation events, because dipolarization is one of the signature at substorm onset. For these dipolarization events, we applied spectral analysis on magnetic field intensity data of both FGL (sampling rate: 4 Hz) and FGH (sampling rate: 128 Hz) modes. We do not see particular spectral peak at frequencies of 0.1-1 Hz. The spectral slopes tend to become steeper with increasing frequency from 0.1 Hz to 1 Hz. Above ~1-10 Hz, the noise from fluxgate magnetometer sensor seems to contaminate into the spectral slope. In the presentation, we are planning to show these spectral properties at different magnetic local times and radial distances during these dipolarization events.