Current reduction in a pseudo‐breakup event: THEMIS observations

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 × 10 5 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.

A physical explanation for the magnetic decrease ahead of dipolarization fronts

“…Since the current density in the current sheet is usually a few nA m −2 (Mitchell et al, 1990), the corresponding dB x /dz should be a few nT/1000 km. Runov et al (2004) and Yao et al (2014b) have also shown that dB x /dz is 2-4 nT/1000 km from multi-spacecraft observations. We thus assume that observations in the two groups are separated by 1 R E across the neutral sheet, we are then able estimate an average redistributed current density before the arrival of DF.…”

“…Slavin et al (2003) also noted that the ambient plasma was compressed ahead of BBFs. Yao et al (2014b) suggested this compression may trigger a pseudobreakup, rather than a fully developed substorm. Simulations have suggested that the magnetic perturbation ahead of the BBF is caused by the reflected ions from the DF .…”

Magnetospheric vortices and their global effect after a solar wind dynamic pressure decrease

“…We calculate the total FAC of the vortex to be 1.5–3 × 10 5 A, which is on the order of the total current for a pseudobreakup. In the pseudobreakup reported by Yao et al [], the total current is ~ 7 × 10 4 A. These values are less than the total FAC in most moderate substorms, i.e., 10 6 A [e.g., Mcpherron et al , ; Kamide and Baumjohann , ].…”

Auroral streamer and its role in driving wave-like pre-onset aurora

“…(1) As shown previously in the literature, near-earth instabilities and reconnection usually take place in thin current sheet conditions (Büchner and Kuska 1999;Drake et al 1994;Nakamura et al 2006;Birn 1993, 1999). (2) It is also often reported that both reconnection and plasma instability are associated with magnetic dipolarization Lui et al 2008b;Yao et al 2013bYao et al , 2015; (3) Reconnection and reconnection outflows may also directly trigger a pseudo-substorm (or very small substorm) (Pu et al 2010;Yao et al 2014).…”