Characteristics of magnetic dipolarizations in the vicinity of the substorm onset region observed by THEMIS

With conjunction observations of electromagnetic fields and plasma from Time History of Events and Macroscale Interactions during Substorm (THEMIS) in the near-Earth magnetotail, we investigate the spatial and temporal properties of substorm dipolarizations in the near-Earth plasma sheet (NEPS) during a substorm at 03:23 UT on 12 February 2008. Substorm dipolarizations with different features are detected by three near-Earth THEMIS probes (THA (P5), THD (P3) and THE (P4)) in the magnetotail. In the current sheet with a large plasma beta value (β > 2, where β is the ratio of the plasma thermal pressure to the magnetic pressure), the dipolarization within the substorm onset region, (−10.4, 2.8, −2.6)R_{E_gsm}, has a large initial magnetic field elevation angle, θ > 60°, θ = arctan (B_z/(B_x²+B_y²)^1/2), and is accompanied by energetic ion (tens to hundred keV) dispersionless injection detected by THD (P3). This substorm onset dipolarization is characterized by B_x and B_y components around 0 nT with significant fluctuations. The B_z component increases sharply and its subsequent magnitude approaches the total magnetic field, B_t. The maximum value of the elevation angle approaches 85° during the later substorm expansion phase. In the NEPS with β ~ 1, the dipolarization outside the substorm onset region is characterized by a magnetic elevation angle with a small beginning value of θ < 45° and following multi-step enhancements during the substorm expansion phase. The maximum value of the elevation angle approaches to 70° during the later substorm expansion phase. Our observation results indicate that characteristics of dipolarization with a large beginning elevation angle within the substorm onset region provide a new indicator to identify substorm onset location.