Modes and characteristics of low‐frequency MHD waves in the near‐Earth magnetotail prior to dipolarization: Fitting method

Abstract

Spacecraft observations indicate that low‐frequency (0.006–0.025 Hz) fluctuations of the magnetic field appear a few min prior to a substorm‐associated dipolarization. These fluctuations are examined on the basis of the linear magnetohydrodynamic (MHD) fluid theory. We propose a “low‐frequency fitting method” for identifying the characteristics of the MHD waves, such as the mode, the wavevector, and the frequency. Using the magnetic field and the ion velocity data, the frequency in the plasma rest frame is obtained by removing the Doppler shift. The fitting method takes the inhomogeneity of the ambient magnetic field into account, so that the parameter which is equivalent to the sum of the field line curvature and the gradient scale length of the ambient magnetic field is obtained as an output. We applied the method to a selected dipolarization event in which Geotail remained in the vicinity of the magnetic equator and also in the same magnetic local time as the onset region of an auroral breakup. The low‐frequency fluctuations were detected from ∼4 min before the local dipolarization onset. What has been found are as follows: (1) the parallel magnetic field fluctuations had a strong correlation with the perpendicular ion velocity fluctuations, which indicates that the observed waves were explained as magnetosonic modes. The slow magnetosonic wave was identified ∼3 min before the dipolarization onset, whereas the fast magnetosonic wave was identified ∼1.5 min before the dipolarization onset. This fast wave was estimated to be propagating tailward with a phase velocity of 400 km/s and a period of 70 s in the plasma frame. (2) The perpendicular fluctuations of the magnetic field and the ion velocity were only weakly correlated, which indicates the presence of a mode that cannot be captured by the present method. Using the variance analysis, we show that this mode is likely to be a drift mode, which had almost zero frequency in the plasma frame and was propagating duskward together with the plasma bulk flow. A possible interpretation of the observed waves is briefly discussed with a relevance to previously proposed substorm initiation models. Particularly, the drift wave can be interpreted as a linear stage of the ballooning instability.