Low-frequency electromagnetic turbulence observed near the substorm onset site

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near-Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search-coil magnetometer and the spherical double-probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi-linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively, The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.