On the propagation of low-frequency fluctuations in the plasma sheet: 2. Characterization of the MHD eigenmodes and physical implications
Résumé
In a first analysis of the low-frequency fluctuations observed by Cluster during a
crossing of the plasma sheet [Louarn et al., 2004], we identify oscillations that likely
correspond to MHD eigenmodes. They are intense (10 nT), have a rather short period
(20–25 s), have a wavelength of 5 Re, and are observed during an active period that
follows a substorm onset. We show here that they combine both sausage and kink
modes propagating in the x direction with a common spectral peak at 0.04 Hz, the
sausage oscillations presenting also a secondary peak at 0.13 Hz. Using a complete
theory of the linear MHD response of the plasma sheet, we reconstruct the sheet
oscillations starting from given external perturbations. This method leads to quantitative
predictions concerning the magnetic and pressure fluctuations. The existence of natural
resonant frequencies of the sheet can indeed be foreseen and the relative weight of
the kink-like and sausage-like modes can also be estimated. We show that the kink
mode (0.04 Hz) and the high-frequency sausage mode (0.14 Hz) are perfectly
compatible with the MHD model of Harris sheet oscillations, with wavelength of 6 Re
and 3 Re, respectively. The corresponding amplitude of the pressure and magnetic
fluctuations also appears to be remarkably consistent with the theoretical predictions.
Conversely, the low-frequency sausage mode (0.04 Hz) is hardly explained by the MHD
model. We propose that it could be a nonideal MHD or a kinetic mode, as the tearing
mode.
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