Statistical analysis of severe magnetic fluctuations in the near-Earth plasma sheet observed by THEMIS-E

摘要

We statistically analyzed severe magnetic fluctuations in the nightsidenear-Earth plasma sheet at 6–12  (Earth radii; 1  = 6371 km), because they are important for non-magnetohydrodynamics(non-MHD) effects in the magnetotail and are considered to be necessary forcurrent disruption in the inside-out substorm model. We used magnetic fielddata from 2013 and 2014 obtained by the Time History of Events and MacroscaleInteractions during Substorms E (THEMIS-E) satellite (sampling rate: 4 Hz).A total of 1283 severe magnetic fluctuation events were identified thatsatisfied the criteria ∕  0. 5, where and are the standard deviation and the average value of magnetic fieldintensity 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 ,respectively, and most events last for no more than 15 s. From theseoccurrence rates, we estimated the possible scale sizes of current disruptionby severe magnetic fluctuations as 3.83  by assuming thatfour substorms with 5 min intervals of current disruption occur every day.The fluctuation events occurred most frequently at the (distance in the geocentric solar magnetospheric coordinate system) close tothe model neutral sheet within 0.2 . Most events occur inassociation with sudden decreases in the auroral electrojet lower (AL) index and magnetic fielddipolarization, indicating that they are related to substorms. Sixty-twopercent of magnetic fluctuation events were accompanied by ion flow withvelocity   100 km s, indicating that the violation of iongyromotion tends to occur during high-speed flow in the near-Earth plasmasheet. The superposed epoch analysis also indicated that the flow speedincreases before the severe magnetic fluctuations. We discuss how both theinside-out and outside-in substorm models can explain this increase in flowspeeds before magnetic fluctuation events.