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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L07103, doi:10.1029/2008GL033269, 2008

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

M. H. Saito

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


Y. Miyashita

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


M. Fujimoto

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


I. Shinohara

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


Y. Saito

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan


K. Liou

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


T. Mukai

Japan Aerospace Exploration Agency, Tokyo, Japan


Abstract

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 β 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 ω sc ∼ k y · v y , where ω 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.

Received 15 January 2008; accepted 3 March 2008; published 3 April 2008.

Keywords: ballooning instability; substorm; dipolarization.

Index Terms: 2752 Magnetospheric Physics: MHD waves and instabilities (2149, 6050, 7836); 2790 Magnetospheric Physics: Substorms; 2704 Magnetospheric Physics: Auroral phenomena (2407); 2744 Magnetospheric Physics: Magnetotail; 3280 Mathematical Geophysics: Wavelet transform (3255, 4455).

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Citation: Saito, M. H., Y. Miyashita, M. Fujimoto, I. Shinohara, Y. Saito, K. Liou, and T. Mukai (2008), Ballooning mode waves prior to substorm-associated dipolarizations: Geotail observations, Geophys. Res. Lett., 35, L07103, doi:10.1029/2008GL033269.