Ballooning mode waves prior to substorm-associated dipolarizations: Geotail observations

We present in situ observations consistent with the ballooning mode in the vicinity of the magnetic equator at X(GSM) = -10 to -13 R(E) prior to substorm-associated dipolarization onsets. The ballooning instability is expected to have a wavevector along the Y direction and to give variation to the curvature of the ambient magnetic field lines. The magnetic field fluctuations appearing in the B(x) component are transported by the ambient plasma drift in the Y direction. A discrete frequency band would be identified in time series data if the mode has a discrete wavelength. The ballooning mode of this property was identified at the magnetic equator a few min before dipolarization onsets only when the plasma beta was large (20 to 70). Using low-energy ion velocity data, we show that the mode has almost zero frequency in the plasma rest frame so that w(sc) similar to k(y) . v(y), where w(sc) is the frequency in the spacecraft frame, and k(y) and v(y) are the wavenumber and the ambient plasma flow in the Y direction, respectively. This enables us to estimate the wavelengths of the ballooning mode, which were found to be of the order of the ion Larmor radius.