Abstract
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 σB∕B > 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.
Highlights
Substorms, which are important geomagnetic and auroral disturbances with timescales of 2 to 3 h, were first identified by Akasofu (1964)
From the appearance of a new X-type near-Earth neutral line, the eventual formation of a substorm current wedge resulted from earthward bursty bulk flow (BBF) and auroral initial brightening at the flow braking point
We focused on the magnetic fluctuations that violate ion gyromotion, which may cause ion kinetic effects such as current disruption at the substorm onset
Summary
Substorms, which are important geomagnetic and auroral disturbances with timescales of 2 to 3 h, were first identified by Akasofu (1964). From the appearance of a new X-type near-Earth neutral line, the eventual formation of a substorm current wedge resulted from earthward bursty bulk flow (BBF) and auroral initial brightening at the flow braking point These ideas about the time sequences of substorm-related phenomena in the magnetotail eventually became the outside-in model. On the other hand, Lui and Najmi (1997) showed that during the current disruption, the spectrum of accompanied magnetic fluctuations becomes intermittently broad, covering timescales from below to above the ion gyroperiod In such a particular circumstance, the collapse of the MHD condition causes continuous challenges for the study of magnetotail dynamics. We conducted a statistical analysis of occurrence rate and substorm-related properties of severe magnetic fluctuations at periods shorter than the local ion gyroperiod in the near-Earth plasma sheet
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