Current reduction in a pseudo‐breakup event: THEMIS observations. Issue 10 (13th October 2014) - Yao, Z. H.
Abstract Pseudo‐breakup events are thought to be generated by the same physical processes as substorms. This paper reports on the cross‐tail current reduction in an isolated pseudo‐breakup observed by three of the THEMIS probes (THEMIS A (THA), THEMIS D (THD), and THEMIS E (THE)) on 22 March 2010. During this pseudo‐breakup, several localized auroral intensifications were seen by ground‐based observatories. Using the unique spatial configuration of the three THEMIS probes, we have estimated the inertial and diamagnetic currents in the near‐Earth plasma sheet associated with flow braking and diversion. We found the diamagnetic current to be the major contributor to the current reduction in this pseudo‐breakup event. During flow braking, the plasma pressure was reinforced, and a weak electrojet and an auroral intensification appeared. After flow braking/diversion, the electrojet was enhanced, and a new auroral intensification was seen. The peak current intensity of the electrojet estimated from ground‐based magnetometers, ~0.7 × 105 A, was about 1 order of magnitude lower than that in a typical substorm. We suggest that this pseudo‐breakup event involved two dynamical processes: a current‐reduction associated with plasma compression ahead of the earthward flow and a current‐disruption related to the flow braking/diversion. Both processes are closely connected to the fundamental interaction between fast flows, the near‐Earth ambient plasma, and the magnetic field.