Dipolarization Observed by TC1 and Cluster During Substorm in Sep. 14, 2004

Abstract

We study the substorm dipolarization occurring from 1730 to 1930 UT on Sep 14, 2004 based on DSP/TC1, 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 was continuously southward. Substorm onset started at about 1822 UT. Three minute later at 1825 UT TC1, 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 the ion temperature and thermal pressure. Two minutes further later, energetic electron injection was observed by LANL-02A in the pre-midnight sector at geostationary orbit. About 23 minutes later at 1850 UT, 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 1822UT and TC1 and Cluster observed dipolarization (tail expansion) 3 and 28 minutes later, respectively. 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 about X = -(8.5 - 8.7) RE and the tailward progression speed was about (70 82) km·s-1. A substorm paradigm is proposed to explain the observations in this substorm event: Earthward flow produced by magnetic reconnection in the mid-tail creates favorable conditions for substorm initiation in the near-Earth; after the expansion onset the dipolarization front propagates down to the tail.