Bursty bulk flows in the inner central plasma sheet

Abstract

High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990)) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from • 9 to 19 R•r in the Earth's magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on 2 years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements which we call bursty bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 •9 ergs/R•r 2. 17 R•r and were argued to have an (inferred) scale size of 15 R•r in the Yas• direction and fast shock properties. 1DeparUnent Statistical studies have also been used to characterize plasma sheet flows. Prior to 1986 only a few of these studies addressed plasma sheet flows irrespective of substorm phase [Caan et al., 1979; Hayakawa et al., 1982; Slavin et al. 1985, 1987]. For example, Hayakawa et al. [1982] using IMP 6 data from distances of-15 R•r < XOSM < -33 R•r showed that high-speed flows (V,, > 300 km/s) can occur in the plasma sheet within 1 Re from the expected neutral sheet position defined by the Russell and Brody [1967] model, so one might infer that these flows occurred in the CPS. However, the above statistical studies either did not distinguish between the CPS and its boundary or they concentrated in the distant-tail regions [Slavin et al., 1985, 1987]. The first statistical assessment of the significance of the near-Earth CPS for magnetotail transport was made by Huang and Frank [1986]. They constructed a data base using 128-512 s resolution plasma data from the ISEE 1 satellite. They applied the criterion that high-speed flows (Vi > 150 km/s) occurring 1.5 R•r or more away from the GSM equatorial plane are in the PSBL. They found that the average speed in the CPS was low (around 50 km/s) regardless of geomagnetic activity (based on the AE index). They showed (see also Figure 2 of Huang and Frank [ 1987]) that even if high-speed flows existed in their data set, these were not representative of the average properties of the CPS. They therefore argued that even if high-speed flows of short time or spatial scales may occur in the CPS, they are statistically insignificant compared to the vast majority of the (low flow velocity) data. The above study did not attempt to assess the relative contributions of the CPS and the plasma sheet boundary to magnetotail transport. A new statistical selection criterion to distinguish between the central plasma sheet and its boundary was proposed by Baumjohann et al. [1988] (subsequently referred to as BJeta188): ...

Six auroral generators: A review

“…Nishimura et al [2010] observed the appearance of such streamers a few minutes before the onset of a substorm. Sergeev et al [1999] associated the streamers, which develop a few minutes before impulsive injections into the inner magnetosphere, with the bursty bulk flows (BBFs) of Angelopoulos et al [1992]. The optical arc appears to be part of an ionospheric flow channel.…”

Energy conversion regions as observed by Cluster in the plasma sheet

“…Typically, BBFs correspond to bursty high-speed flow events observed on a 10 min time scale, and composed of individual high-speed flow burst (^400 km/s) on shorter time scales, of the order of tens of seconds [Angelopoulos et al, 1992[Angelopoulos et al, , 1994. Subsequently, field aligned burst have been observed also in lower b plasmas outside the PSBL, i.e., where b < 0.5 [Raj et al, 2002].…”

“…These high-speed flows can be manifested as bursty bulk flows (BBFs) and other sporadic and intermittent phenomena [e.g., Scholer et al, 1984;Angelopoulos et al, 1992;Chen and Wolf, 1993;Angelopoulos et al, 2002]. For example, the large substorm current wedge is believed to be caused by the braking and diversion of earthward directed flows closer to the inner boundary of the plasma sheet, resulting in the generation of electromagnetic power, which eventually can power the aurora [Wygant et al, 2000] as well as cause ionospheric Joule heating.…”

“…[9] The concept of BBFs was first introduced by Angelopoulos et al [1992] who investigated the occurrence of bursty bulk flows in the inner CPS, as characterized by a large plasma b > 0.5. Typically, BBFs correspond to bursty high-speed flow events observed on a 10 min time scale, and composed of individual high-speed flow burst (^400 km/s) on shorter time scales, of the order of tens of seconds [Angelopoulos et al, 1992[Angelopoulos et al, , 1994.…”

“…(Note that we will use the denomination BBF for high-speed plasma flows throughout the rest of the article, independently of the detailed character of the flows.) BBFs are believed to play a major role for magnetic flux, mass, and energy transport in the plasma sheet [e.g., Angelopoulos et al, 1992Angelopoulos et al, , 1994Angelopoulos et al, , 1999Sergeev et al 1996b;Schödel et al, 2001], and it has been showed that BBFs have the largest capability of transporting energy during the substorm expansion phase, as compared to the growth and recovery phases [YuDuan et al, 2010]. In the literature there are many reports of the relation between BBFs and auroral phenomena at the ionospheric end of the M-I coupling system, e.g., auroral expansions, localized brightenings, and auroral streamers [e.g., Fairfield et al, 1999;Lyons et al, 1999;Ieda et al, 2001;Sergeev et al, 2001;Nakamura et al, 2001aNakamura et al, , 2001bNakamura et al, , 2005bMiyashita et al, 2003;Forsyth et al, 2008].…”