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Ballooning mode waves prior to substorm‐associated dipolarizations: Geotail observations
Author(s) -
Saito M. H.,
Miyashita Y.,
Fujimoto M.,
Shinohara I.,
Saito Y.,
Liou K.,
Mukai T.
Publication year - 2008
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2008gl033269
Subject(s) - substorm , physics , ballooning , gyroradius , magnetic field , wavelength , plasma , wavenumber , magnetosphere , field line , instability , computational physics , geophysics , atomic physics , tokamak , optics , mechanics , quantum mechanics
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.