Introduction

The magnetic field dipolarization is a very key element in the substorm onset trigger process and is usually accompanied with plasma instabilities and waves. The dipolarization is associated with the current wedge formation, the cross-tail current reduction, dispersionless energetic particle injection at the night side near geosynchronous orbit, and the auroral breakup. The magnetic field dipolarization occurs at the same time as substorm onset at the inner edge of the current sheet in the near-Earth tail. The magnetic dipolarization onset at the midnight geosynchronous altitude was related to some plasma instabilities prior to the auroral breakup. Simultaneous observations from multiple probes of THEMIS gave us a good opportunity to distinguish the temporal and spatial variations of the dipolarizations.

We used observations from multiple probes of THEMIS at different locations in the near-Earth plasma sheet to investigate the magnetic dipolarization feature in the vicinity of substorm onset and during substorm expansion phase, and to show some evidence for the relationship between the magnetic dipolarization and bursty bulk flow.

Observations

Multiple magnetic dipolarizations observed by THEMIS P5, P4 and P3 in the near-Earth plasma sheet during a substorm occurred at about 06:14 UT on February 15, 2008. In this interval of 06:00 - 06:40 UT, THD (P3) was farthest from the Earth, X~11.30 R_E and THA (P5) was nearest to the Earth, X~9.45 R_E. THE (P4) was between THA and THD, X~ -11.00 R_E. Figure 1(a) shows multiple dipolarizations observed by P5 probe during this substorm. Four dipolarizations from P5 were not accompanied by ion bursty bulk flow. Figure 1(b) and 1(c) present multiple dipolarizations observed by P4 and P3 probes during this substorm, respectively. The first two dipolarizations were very weak. They were not accompanied by ion bursty bulk flow. The later dipolarizations observed by P4 and P3 probes during substorm expansion phase had correlation with earthward ion bulk flow.

Figure 1. THEMIS multiple probes observed the dipolarizations during the substorm occurring at 06:14 UT on 15 February 2008 in the near-Earth plasma sheet. Figure 1(a), 1(b) and 1(c) are from THA, THE and THD, respectively. The red vertical dotted line marked the substorm onset time, i.e., the first magnetic dipolarization.

Click each image to enlarge.

Conclusions

There are multiple magnetic dipolarizations occurring during a substorm in the near-Earth plasma sheet. These dipolarizations have different mechanisms and properties. The dipolarization at the earthward side of the near-Earth plasma sheet has no relationship with high speed earthward ion bulk flow. It may be driven by some kinds of plasma instabilities. But the dipolarization far way from the Earth in the near-Earth plasma sheet usually has a relationship with high speed earthward ion bulk flow. Dipolarization at substorm onset is a local and small scale phenomenon, while dipolarizations during substorm expansion phase have larger spatial scale and are correlative with the azimuthal expansion of plasma sheet or high speed earthward ion bulk flow. The dipolarization process is accompanied by many kinds of other phenomena, not only by earthward ion bulk flow during substorms.

Source

Duan, S. P., Liu, Z. X., Liang, J., Zhang, Y. C., and Chen, T. (2011), Multiple magnetic dipolarizations observed by THEMIS during a substorm, Ann. Geophys., 29, 331-339, doi:10.5194/angeo-29-331-2011.

Biographical Note

Suping Duan is an associate professor with State Key Laboratory of Space Weather at Center for Space Science and Applied Research (CSSAR) in Chinese Academy of Sciences (CAS). Her research interests are space plasma instabilities and waves and the magnetospheric substorm.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong@igpp.ucla.edu