HR: 1340h
AN: SM13C-1460    [Abstracts]
TI: Geotail observations of low-frequency MHD waves prior to dipolarization onsets in the near- Earth magnetotail
AU: * Saito, M
EM: msaito@stp.isas.jaxa.jp
AF: ISAS/JAXA, 3-1-1 Yoshinodai Sagamihara, Kanagawa, 2298510, Japan
AU: Miyashita, Y
EM: miyasita@stp.isas.jaxa.jp
AF: ISAS/JAXA, 3-1-1 Yoshinodai Sagamihara, Kanagawa, 2298510, Japan
AU: Fujimoto, M
EM: fujimoto@stp.isas.jaxa.jp
AF: ISAS/JAXA, 3-1-1 Yoshinodai Sagamihara, Kanagawa, 2298510, Japan
AU: Shinohara, I
EM: iku@stp.isas.jaxa.jp
AF: ISAS/JAXA, 3-1-1 Yoshinodai Sagamihara, Kanagawa, 2298510, Japan
AU: Saito, Y
EM: saito@stp.isas.jaxa.jp
AF: ISAS/JAXA, 3-1-1 Yoshinodai Sagamihara, Kanagawa, 2298510, Japan
AU: Mukai, T
EM: mukai@stp.isas.jaxa.jp
AF: JAXA, Chiyoda-ku, Tokyo, 1008260, Japan
AB: Spacecraft observations indicate that low-frequency (0.006-0.025 Hz) fluctuations of the magnetic field appear a few minutes prior to substorm-associated dipolarization onsets. On the basis of Geotail observations, we investigate such fluctuations in the vicinity of the magnetic equator in particular in the near-Earth magnetotail at X= -8 to -12 R_E. In order to characterize the observed fluctuations both physically and quantitatively, we propose a new method for estimating characteristics of wave parameters, such as the mode, the phase velocity in the plasma frame, the wave number, and the Doppler shift. In this method, the inhomogeneity of the ambient magnetic field is taken into account. Band-passed data of magnetic field and ion velocity fluctuations are analyzed on the basis of the linear MHD equations. It is found that the fluctuations in the low-frequency range consist of various modes of MHD waves. Particularly, the fast magnetosonic wave was identified and was propagating tailward a few minutes prior to a dipolarization onset, though there are also dipolarization events in which no fast wave was identified. A more prominent finding in the present study is the presence of a mode that has almost zero frequency in the plasma frame. Such a zero-frequency mode can convect with the ambient plasma flow and hence can be measured with a finite frequency by a spacecraft. This mode may correspond to a linear stage of the ballooning instability. These characteristics of the MHD waves may provide crucial constraints on the conventional substorm initiation models.
DE: 2752 MHD waves and instabilities (2149, 6050, 7836)
DE: 2764 Plasma sheet
DE: 2790 Substorms
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting