Abstract
Abstract. This study presents an investigation on the occurrence of fast flows in the magnetotail using the complete available data set of the THEMIS spacecraft for the years 2007 to 2015. The fast flow events (times of enhanced ion velocity) are detected through the use of a velocity criterion, therefore making the resulting database as large as almost 16 000 events. First, basic statistical findings concerning velocity distributions, occurrence rates, group structures are presented. Second, Superposed Epoch Analysis is utilized to account for average profiles of selected plasma quantities. The data reveal representative time series in near and far tail of the Earth with typical timescales of the order of 1–2 min, corresponding to scale sizes of 3 RE. Last, related magnetic field disturbances are analyzed. It is found that the minimum variance direction is essentially confined to a plane almost perpendicular to the main flow direction while, at the same time, the maximum variance direction is aligned with flow and background field directions. The presentation of the database and first statistical findings will prove useful both as input for magneto-hydrodynamical simulations and theoretical considerations of fast flows.
Highlights
Fast flow events or flow bursts as part of bursty bulk flows (BBFs) (Angelopoulos et al, 1992) play a potentially important role in substorm physics and magnetotail dynamics
Baumjohann et al (1988) analyzed high-speed ion flows in AMPTE data in the central plasma sheet and plasma sheet boundary layer and found that the average bulk speed is below 100 km s−1, agreeing with Huang and Frank (1986)
A total of almost 16 000 events are found, many of them embedded in a group of fast flows, so-called bursty bulk flows
Summary
Fast flow events or flow bursts as part of bursty bulk flows (BBFs) (Angelopoulos et al, 1992) play a potentially important role in substorm physics and magnetotail dynamics. According to Angelopoulos et al (1992) a flow burst is a single velocity burst embedded into large-scale structures, series of flow bursts, which are termed bursty bulk flows Their close relation to various magnetospheric phenomena such as storms and substorms, Pi2 pulsations, and auroral activity makes it worth the effort to study them. Sharma et al (2008) gave a review on various transient phenomena in the magnetotail, focusing on studies performed by Ohtani et al (2004) who examined the temporal structure of fast flows in Geotail observations and simulations where it was found that fast flows are often accompanied by dipolar field changes and increases in density. Schmid et al (2015) performed statistical studies on dipolarization fronts and plasma flows in THEMIS data and hypothesize on the development of such events.
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