Statistical characteristics and significance of low-frequency instability associated with magnetic dipolarizations in the near-Earth plasma sheet

摘要

Magnetic dipolarization has been considered as a key element of substorm phenomena. In this work we investigate the spectral features of the magnetic dipolarization fluctuations in frequency-time space by using the continuous wavelet transform technique. We present details of the analysis for three specific examples and the statistical results for 82 magnetic dipolarizations that occurred at X > ～-11.5 R_E in the near-Earth plasma sheet. We focus on a low-frequency regime defined here as 0.005 to 0.03 Hz for convenience sake (typically well below local proton gyrofrequency). On the basis of the three specific events, we explicitly demonstrate that the magnetic dipolarization fluctuations can be dominated by intense waves at one or more (typically 2-3) discrete frequencies in the low-frequency regime. Statistically, we find that this is the case for 59 (about 72%) out of the 82 events. In addition, we find that such a wave starts to grow in amplitude, thus implying occurrence of instability, typically minutes prior to the dipolarization onset time. The estimated exponential growth time is less than 2 min for ～68% out of the 72% events. The statistically averaged frequency for the strongest wave is～0.01Hz, which we argue is in the regime of ballooning instability. All these features are most clearly seen in the compressional component of magnetic fluctuations. For two of the three example events, it is demonstrated that the magnetic fluctuations on the perpendicular plane are linearly polarized for a given frequency while a more comprehensive statistical study of polarization features is left for a future work. On the basis of the results obtained in this work we conclude that the association of low-frequency instability with substorm-associated dipolarizations can be quite significant from a statistical viewpoint.