Observations of dipolarization in the near-Earth plasma sheet

Abstract:

Using the conjunction observations of the electromagnetic fields and plasma from three probes of THEMIS (THA, THD and THE) in the near-Earth magnetotail, we investigate the spatial-temporal scales and properties of electromagnetic fluctuations in dipolarizations in the near-Earth plasma sheet (NEPS) during ten substorms in January and February 2008. The observation properties of dipolarization are found dependent on the locations of the spacecrafts in the near-Earth magnetotail. In the current sheet with the large plasma beta (β>2), the dipolarization is accompanied by the intense broad-band electromagnetic fluctuations. The B_z component increases significantly and its magnitude is comparable to the total magnetic field. The magnetic elevation angle increase sharply and the maximum value is around 80 degree. In the inner plasma sheet with plasma beta around 1 (β~1), the dipolarization is accompanied by the low frequency electromagnetic fluctuations. The magnetic elevation angle has multi-step increases. In the outer plasma sheet with small plasma beta (0.1<β<1), the dipolarization is accompanied by the ultra low frequency electromagnetic fluctuations. The magnetic elevation angle also has multi-step increases with very small beginning angle. The observation results indicate that the dipolarization occurring at substorm onset is a complex physical process with a small spatial and temporal scale. On the other hand, the dipolarization occurring during later substorm expansion phase with a larger spatial and temporal scale corresponded to the substorm enhancement. The earthward and dawnward ion bulk flow both play key roles in the dipolarization process. The dipoarization in the current sheet is chosen by requiring that the B_x component is around 0 nT with significant fluctuations, the B_y component is around 0 nT, the B_z component increases sharply and is close to the total magnetic field. The dipoarization in the current sheet is a key feature of substorm onset in the NEPS. This investigation presents clear dipolarization properties during substorms.