Average ion moments in the plasma sheet boundary layer

Abstract

Using four months of tail data obtained by the three‐dimensional plasma instrument on board the AMPTE/IRM satellite in 1986, we have done a statistical survey on the behavior of ion moments and related plasma parameters in the plasma sheet boundary layer and the immediately adjacent outer central plasma sheet. The more than 40,000 spin averages of ion density, ion bulk velocity, ion temperature, ion temperature anisotropy and ion β were analyzed with respect to differences between their values in plasma sheet boundary layer and adjacent central plasma sheet. We also looked into their dependence on magnetic activity, and possible interrelations among them. As a primary result, we find that the plasma sheet boundary layer is not just a transition region where plasma parameters are gradually changing from plasma sheet to lobe values, but is indeed a unique layer where crucial parameters like ion density and ion β remain on nearly constant levels which are different from those in the central plasma sheet. Accordingly, there is a sharp gradient in ion density and ion β at the interface between the plasma sheet boundary layer and the central plasma sheet. A second important finding is that the average flow velocities in the plasma sheet boundary layer are surprisingly low, typically below 100 km/s. High‐speed flows do occur quite often and under all levels of magnetic activity, but in bursts of often less than one minute duration with intermittent intervals of nearly stagnant plasma. Their rate of occurrence is higher during high AE and, in particular, during plasma sheet expansions. All high‐speed flows are field‐aligned and directed toward the Earth.

Energy conversion regions as observed by Cluster in the plasma sheet

“…This is true for both CLRs and CGRs. [Baumjohann et al, 1988[Baumjohann et al, , 1989. Red and blue colors are used to indicate CLRs and CGRs, respectively, while light red and light blue are reserved for RAND-Ls and RAND-Gs.…”

“…[43] According to Baumjohann et al [1988Baumjohann et al [ , 1989, the ion b approaches small values in the PSBL, of the order of b ] 0.1, while the central plasma sheet corresponds to higher values, b^0.1. From Figure 5, we clearly see that the ratio between the number of CLRs and the number of CGRs is considerably larger in the rightmost bin (high b bin) as compared to the leftmost bin (low b bin).…”

Solar wind dependence of ion parameters in the Earth's magnetospheric region calculated from CLUSTER observations

“…Extensive statistical studies of the plasma within the near-Earth magnetosphere have previously revealed the behaviour of such parameters (e.g. Baumjohann et al, 1988;Newell and Meng, 1992;Borovsky et al, 1998;Wing and Newell, 1998;Měrka et al, 2002;Tysganenko and Mukai, 2003;Wing et al, 2005;Neagu et al, 2005;Denton et al, 2005Denton et al, , 2006Borovsky and Denton, 2006a, b;Lavraud et al, 2005Lavraud et al, , 2006Thomsen et al, 2007;Nagata et al, 2007;Åsnes et al, 2008) under a variety of conditions. The primary aim of the work outlined in the current paper is to reveal the averaged bulk parameters in the magnetosphere as a first step to a determination of how such parameters evolve during geomagnetic disturbances.…”

High-speed flowing plasmas in the Earth’s plasma sheet

“…In addition, observing a high speed-flow event does not necessarily mean there is a substorm in progress [9,10]. However, because most of the time the convection speed in the plasma sheet is relatively low [11,12], the general view is that high-speed flows play an important role in plasma convection and studies of high-speed flows have been closely tied to substorm research.…”