Abstract
Abstract. This article describes observations of a bursty bulk flow (BBF) in the outer central plasma sheet. The observations are made with the Cluster satellites, located approximately 19 RE downtail, close to the midnight sector in the Southern Hemisphere. 40–60 s after Cluster first detected the BBF, there was a large bipolar perturbation in the magnetic field. A Grad-Shafranov reconstruction has revealed that this is created by a field-aligned current at the flank of the BBF. Further analysis of the plasma moments has shown that the BBF has the properties of a depleted flux tube. Depleted flux tubes are an important theoretical model for how plasma and magnetic flux can be transported Earthward in the magnetotail as part of the Dungey cycle. The field aligned current is directed Earthward and is located at the dawn side of the BBF. Thus, it is consistent with the magnetic shear at the flank of an Earthward moving BBF. The total current has been estimated to be about 0.1 MA.
Highlights
Fast flows in the magnetotail have been subject to detailed research
We focus on the time interval immediately after the field aligned currents (FACs) observation, and we interpret the plasma moments as consistent with the concept of bursty bulk flow (BBF) as depleted flux tubes
There are three key observations in the figure which support the interpretation of the BBF as a depleted flux tube with an initial density reduction (Birn et al, 2004): 1) A decrease in the density starting around 22:54:20 UT. 2) An increase in temperature around the same time. 3) A dipolarization of the magnetic field
Summary
Fast flows in the magnetotail have been subject to detailed research. They are observed all the way from the tenuous plasma sheet boundary layer (PSBL) to the dense inner central plasma sheet (CPS) (Baumjohann et al, 1990). Flows in the PSBL can usually be characterized as field aligned beams (FAB) (Nakamura et al, 1992). They have distinctively different observational characteristics than flows in the middle. Flows in the CPS are typically convective and consist of a single bulk flow (Raj et al, 2002) They are very often referred to as bursty bulk flows (BBF) (Angelopoulos et al, 1992). The term BBFs referred to high speed flows (>400 km/s) where plasma beta exceeds 0.5 (Angelopoulos et al, 1992).
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