Abstract
Abstract. In this article, and in a companion paper by Hamrin et al. (2009) [Occurrence and location of concentrated load and generator regions observed by Cluster in the plasma sheet], we investigate localized energy conversion regions (ECRs) in Earth's plasma sheet. From more than 80 Cluster plasma sheet crossings (660 h data) at the altitude of about 15–20 RE in the summer and fall of 2001, we have identified 116 Concentrated Load Regions (CLRs) and 35 Concentrated Generator Regions (CGRs). By examining variations in the power density, E·J, where E is the electric field and J is the current density obtained by Cluster, we have estimated typical values of the scale size and life time of the CLRs and the CGRs. We find that a majority of the observed ECRs are rather stationary in space, but varying in time. Assuming that the ECRs are cylindrically shaped and equal in size, we conclude that the typical scale size of the ECRs is 2 RE≲ΔSECR≲5 RE. The ECRs hence occupy a significant portion of the mid altitude plasma sheet. Moreover, the CLRs appear to be somewhat larger than the CGRs. The life time of the ECRs are of the order of 1–10 min, consistent with the large scale magnetotail MHD simulations of Birn and Hesse (2005). The life time of the CGRs is somewhat shorter than for the CLRs. On time scales of 1–10 min, we believe that ECRs rise and vanish in significant regions of the plasma sheet, possibly oscillating between load and generator character. It is probable that at least some of the observed ECRs oscillate energy back and forth in the plasma sheet instead of channeling it to the ionosphere.
Highlights
It is well known that the plasma sheet, on the average, behaves as a load
Hamrin et al (2009) [Occurrence and location of concentrated load and generator regions observed by Cluster in the plasma sheet], hereafter referred to as H09A, in situ data are used for investigating energy conversion in the plasma sheet
In H09A we find that Concentrated Generator Regions (CGRs) often prefer locations closer to the Plasma Sheet Boundary Layer (PSBL), while Concentrated Load Regions (CLRs) are located closer to the central plasma sheet
Summary
It is well known that the plasma sheet, on the average, behaves as a load. Conversion from electromagnetical to mechanical energy occurs in load regions where the power density E·J >0, where E is the electric field and J the current density. The ECRs previously investigated (Marghitu et al, 2006, 2009; Hamrin et al, 2006), as well as the ECRs discussed here and in the companion paper H09A, are labelled concentrated load or generator regions (CLRs and CGRs) to distinguish them from more extended energy conversion regions which may exist in the plasma sheet. No experimental efforts have been made to try to characterize the scale size and life time of the CLRs and CGRs observed It is not known whether the ECRs are spatial entities moving around and crossing the spacecraft path, or if they are a consequence of time variations in the power density with e.g. alternately oscillating load and generator characteristics. We argue that it is probable that the CLRs and CGRs observed in the plasma sheet data from 2001 are the consequence of the power density varying in time rather than in space
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