Abstract
Abstract. The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00–06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4~(−10.97, 2.04, −3.03) RE and P3~(−11.32, 1.15, −3.10) RE were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5~(−9.45, 1.07, −2.85) RE were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the Bx component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.
Highlights
The magnetic field dipolarization is a very key element in the substorm onset trigger process and usually accompanied with the plasma instabilities and waves
The magnetic field dipolarization occurs at the same time as substorm onset time at the inner edge of the current sheet in the near-Earth tail (Liou et al, 2002; Rostoker, 2002; Miyashita et al, 2000)
From the observations of multiple probes of THEMIS during this substorm process, multiple magnetic dipolarizations occurred during the substorm expansion phase
Summary
The magnetic field dipolarization is a very key element in the substorm onset trigger process and usually accompanied with the plasma instabilities and waves. The magnetic field dipolarization occurs at the same time as substorm onset time at the inner edge of the current sheet in the near-Earth tail (Liou et al, 2002; Rostoker, 2002; Miyashita et al, 2000). Huang et al (1992) pointed out that the energetic particles were heated and accelerated in the dipolarization processes in the near-Earth plasma sheet. The probes' footprints (THA: diamond; THD: triangle; THE: pointed out that the continuously magnetic dipolarizations ner near-Earth plasma sheet was not usually accompanied by were ass8ociataesdtewriistkh) tahree padladsemdainsheeaectheaxupraonraslioinmsa.geO.n the high speed earthward ion bulk flow.
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