Statistical Analysis of Severe Magnetic Fluctuations in the near-Earth Magnetotail Observed by THEMIS-E

Abstract:

In this research, we made a statistical analysis of severe magnetic fluctuations in the nightside near-Earth plasma sheet at |X| = 6 - 12 R_E. The severe magnetic fluctuations are considered as necessary cause for current disruption in the substorm model raised by Lui [2001]. We use two-year magnetic field data of 2013 and 2014 with a sampling rate of 4 Hz, which are obtained by the FGM instrument aboard the TH-E (P4) probe of THEMIS. 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. We find that the values of plasma beta for most fluctuation events are more than 10, indicating that they happen in the plasma sheet. Half of the fluctuation events happen with obvious decrease of AL index (10 nT). The estimated occurrence rates of severe fluctuation events are extremely low, 0.00093% at |X| = 6 – 8 R_E, 0.00796% at |X| = 8 – 10 R_E, and 0.0219% at |X| = 10 – 12 R_E. Assuming that four substorms occur every day with a 5-min duration of current disruption, we suggest that the low occurrence rate of severe magnetic fluctuation may indicate that the current disruption regions are very localized (0.24 R_E³, 2.0 R_E³, 5.6 R_E³ at |X| = 6 - 8, 8 - 10, 10 - 12 R_E) in the tail, or the current disruption model is not suitable for most substorm cases. Meanwhile, around 62% of the fluctuation events are accompanied by earthward ion flow with speed more than 100 km/s. This may suggest that either the severe magnetic fluctuations are caused by earthward ion flow, or fluctuations cause earthward ion flow by reducing the tailward pressure gradient force due to current disruption. For the latter case, the flow should be observed slightly after the severe fluctuations, even though the superposed epoch analysis of flow speed with the epoch time of severe fluctuations does not show obvious flow increase after the fluctuations.