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

摘要

Magnetic fluctuations in the near-Earth magnetotail are an important signature of substorm onset. In a previous statistical study, we reported their occurrence rates, spatial distributions, and relationship with plasma flows. In the present study, we investigated their spectral properties using 11 years of measurements from the Time History of Events and Macroscale Interactions during Substorms mission for 2008-2018.We found 10,848 severe magnetic fluctuation events with σ_B∕B > 0.5, where σ_B and B are the standard deviation and average, respectively, of the magnetic field intensity for the local proton gyroperiod. The occurrence rates of severe magnetic fluctuations show no clear dependence on the F_(10.7) index in one solar cycle.We extracted 36 dipolarization events with severe magnetic fluctuations. In the power spectral density (PSD) of the magnetic fluctuations during dipolarizations, the steepness of the spectral slope increased with increasing frequency in almost all the events. The average PSDs are shown sorted by (a) distance to the neutral sheet and (b) ambient magnetic field intensity. In all groups, the slopes of the average PSDs increased abruptly from below ~10~(-1.3) Hz (0.05 Hz) to above ~10~(-1.3) Hz, which is close to the gyrofrequency of O~+ ions. It is the first time that a change of slope near the proton gyrofrequency (frequency range: 0.05-1 Hz) was found in cases of larger ambient magnetic field intensity, implying that the magnetic fluctuations were relatively strong near the proton gyrofrequency. These results suggest that the magnetic fluctuations contribute to the nonmagnetohydrodynamic effect in the ion motion.