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

Abstract

Abstract. We statistically analyzed severe magnetic fluctuations in the nightside near-Earth plasma sheet at 6-12 R E (Earth radii; 1 R E = 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 σ B /B > 0.5, where σ B 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 R E , 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 R E 3 by assuming that four substorms with 5 min intervals of current disruption occur every day. The fluctuation events occurred most frequently at the Z GSM (Z distance in the geocentric solar magnetospheric coordinate system) close to the model neutral sheet within 0.2 R E . 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.

Severe Magnetic Fluctuations in the Near‐Earth Magnetotail: Spectral Analysis and Dependence on Solar Activity

“…We analyzed the 4-Hz sampled magnetic field intensity data obtained by the THEMIS D and THEMIS E probes for 2008-2018 using the method applied by Xu et al (2017) in previous statistical analyses. A severe magnetic fluctuation event was defined as B ∕B > 0.5, where B andB are the standard deviation and average, respectively, of the magnetic field intensity for the local proton gyroperiod.…”

“…We focused on the variations of magnetic field intensity that may violate the gyromotion of ions. The details of this criterion can be found in Xu et al (2017). In total, we found 10,848 severe magnetic fluctuation events.…”

“…Dipolarization can be defined as an increase in the elevation angle (= arctan(B z ∕B xy )) of the magnetic field in geocentric solar magnetospheric (GSM) coordinates, where B xy is the magnetic component in the GSM-XY plane. The criterion used by Xu et al (2017) tends to select magnetic fluctuations at or near substorm onset. Therefore, to focus on those fluctuations related to substorm onset, from the already selected severe magnetic Journal of Geophysical Research: Space Physics 10.1029/2020JA027834 Figure 3.…”

“…coordinates. This condition was used by Xu et al (2017) for the initial selection of severe magnetic fluctuation events. 2.…”

“…Despite the above studies, the spectral properties of magnetic fluctuations in the near-Earth magnetotail and their dependence on solar activity are still not well understood. Xu et al (2017) conducted a statistical analysis of these magnetic fluctuations and reported their occurrence rates, spatial distributions, and relationship with earthward plasma flows using 2 years of data from the THEMIS E satellite. Using 11 years of data from the THEMIS D and THEMIS E satellites, the present study further investigates (a) the occurrence rates of severe magnetic fluctuations in one solar cycle, (b) the spectral properties of severe magnetic…”