TC-1 and Cluster Observation of Substorm Dipolarization on Sep 14, 2004

Cao, X., Pu, Z., Zhang, H., Fu, S., Xiao, C., Zong, Q., Liu, Z., Cao, J., 
Korth, A., Frazen, M., Carr, C., Reme, H., Glassmeier, K.


We study the substorm depolarization occurring from 1730 to 1930 UT on Sep 14,
2004 based on DSP TC-1, Cluster and Polar data combined with the aurora and
geosynchronous energetic particle measurements and ground-based magnetic field
observations. During this time period the IMF is continuously southward. At
about 1803 UT Cluster observed signatures of continuous lobe reconnection in the
tail (Cao et al., 2005; Zhang et al., 2005; Pu et al., 2005). The ground-based
magnetometer records indicate that at about 1823UT, Pi2 pulsations started on
the magnetic field D-component in the mid-latitudes. IMAGE FUV WIC observation
shows that an aurora breakup took place at almost the same time. Two minute
later, energetic electron injection was observed by LANL-02A in the pre-midnight
sector at geostationary orbit. At about 1827UT TC-1, which was located at
(-10,-2, 0) Re (GSE) in the near-Earth tail, saw a sharp decrease of magnetic Bx
component, accompanied by sudden increases in both ion temperature and thermal
pressure. About 23 minutes later at about1850UT, the same signatures as observed
by TC1 were observed by Cluster at (-16, 1, 3) Re (GSE) in the mid-tail.
Meanwhile, POLAR, which was located much higher and more close to the Earth at
(-7.5, 3.5, -4.0) Re (GSE), saw a sudden change in the elevation of the magnetic
field at about 1855UT. Aforementioned time sequence of substorm activities
indicates that the expansion onset took place at about 1823UT and TC-1 and
Cluster observed dipolarization (tail expansion) 4 and 27 minutes later,
respectively. The wavelet analysis shows that the wave profiles of tail
expansion structures seen by TC-1 and Cluster both were very similar. Further
more, the power analysis illustrates that they both gad a characteristic period
peaked at about1.3-1.5min, almost identified to that of observed mid-latitude
Pi2 pulsations. This means that tail expansion was propagated from source region
in the near-Earth tail to the mid-tail. Detailed calculations imply that the
source of the dipolarization was situated at X=-7.7 Re to -8.6 Re and the
propagation speed was about 30km/s. A substorm paradigm is proposed to explain
the observations in this substorm event: Earthward flow produced by lobe
reconnection in the mid-tail created favorable conditions for substorm
initiation in the near-Earth; after the expansion onset the dipolarization front
propagates down to the tail.

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Fall Meeting, American Geophysical Union, San Francisco, U.S.A., 
5-9 December 2005