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Annales Geophysicae An open-access journal of the European Geosciences Union
Ann. Geophys., 35, 1131-1142, 2017
https://doi.org/10.5194/angeo-35-1131-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Regular paper
09 Oct 2017
Statistical analysis of severe magnetic fluctuations in the near-Earth plasma sheet observed by THEMIS-E
Heqiucen Xu1, Kazuo Shiokawa1, and Dennis Frühauff2 1Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan
2Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany
Abstract. We statistically analyzed severe magnetic fluctuations in the nightside near-Earth plasma sheet at 6–12 RE (Earth radii; 1 RE = 6371 km), because they are important for non-magnetohydrodynamics (non-MHD) effects in the magnetotail and are considered to be necessary for current disruption in the inside-out substorm model. We used magnetic field data from 2013 and 2014 obtained by the Time History of Events and Macroscale Interactions during Substorms E (THEMIS-E) satellite (sampling rate: 4 Hz). A total of 1283 severe magnetic fluctuation events were identified that satisfied the criteria σBB > 0. 5, where σB and B are the standard deviation and the average value of magnetic field intensity during the time interval of the local proton gyroperiod, respectively. We found that the occurrence rates of severe fluctuation events are 0.00118, 0.00899, and 0.0238 % at 6–8, 8–10, and 10–12 RE, respectively, and most events last for no more than 15 s. From these occurrence rates, we estimated the possible scale sizes of current disruption by severe magnetic fluctuations as 3.83 RE3 by assuming that four substorms with 5 min intervals of current disruption occur every day. The fluctuation events occurred most frequently at the ZGSM (Z distance in the geocentric solar magnetospheric coordinate system) close to the model neutral sheet within 0.2 RE. Most events occur in association with sudden decreases in the auroral electrojet lower (AL) index and magnetic field dipolarization, indicating that they are related to substorms. Sixty-two percent of magnetic fluctuation events were accompanied by ion flow with velocity V > 100 km s−1, indicating that the violation of ion gyromotion tends to occur during high-speed flow in the near-Earth plasma sheet. The superposed epoch analysis also indicated that the flow speed increases before the severe magnetic fluctuations. We discuss how both the inside-out and outside-in substorm models can explain this increase in flow speeds before magnetic fluctuation events.

Citation: Xu, H., Shiokawa, K., and Frühauff, D.: Statistical analysis of severe magnetic fluctuations in the near-Earth plasma sheet observed by THEMIS-E, Ann. Geophys., 35, 1131-1142, https://doi.org/10.5194/angeo-35-1131-2017, 2017.
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In this study, we statistically analyzed severe magnetic fluctuations in the nightside near-Earth plasma sheet. For the first time, we showed the occurrence rates of these fluctuations. The superposed epoch analysis also indicated that the flow speed increases before the severe magnetic fluctuations. We also discussed how both the inside-out and outside-in substorm models can be used to explain these observed results.
In this study, we statistically analyzed severe magnetic fluctuations in the nightside...
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