Occurrence and location of concentrated load and generator regions observed by Cluster in the plasma sheet

M Hamrin,O Marghitu,L M Kistler,P Norqvist,B Klecker,I Dandouras,S Buchert

doi:10.5194/angeo-27-4131-2009

Abstract

Abstract. Here, and in a companion paper by Hamrin et al. (2009) [Scale size and life time of energy conversion regions observed by Cluster in the plasma sheet], we investigate localized energy conversion regions (ECRs) in the Earth's plasma sheet. In total we have studied 151 ECRs within 660 h of plasma sheet data from the summer and fall of 2001 when Cluster was close to apogee at an altitude of about 15–20 RE. Cluster offers appropriate conditions for the investigation of energy conversion by the evaluation of the power density, E·J, where E is the electric field and J the current density. From the sign of the power density, we have identified more than three times as many Concentrated Load Regions (CLRs) as Concentrated Generator Regions (CGRs). We also note that the CLRs appear to be stronger. To our knowledge, these are the first in situ observations confirming the general notion of the plasma sheet, on the average, behaving as a load. At the same time the plasma sheet appears to be highly structured, with energy conversion occurring in both directions between the fields and the particles. From our data we also find that the CLRs appear to be located closer to the neutral sheet, while CGRs prefer locations towards the plasma sheet boundary layer (PSBL). For both CLRs and CGRs, E and J in the GSM y (cross-tail) direction dominate the total power density, even though the z contribution occasionally can be significant. The prevalence of the y-direction seems to be weaker for the CGRs, possibly related to a higher fluctuation level near the PSBL.

Highlights

Energy conversion within the plasma sheet plays an important role for the Earth’s magnetosphere and its energy budget
Hamrin et al (2009) [Scale size and life time of energy conversion regions observed by Cluster in the plasma sheet], we investigate the life time and scale size of the ECRs
In H09B we show that the ECRs are concentrated in space as well and time, and typical scale sizes of Concentrated Load Regions (CLRs) and Concentrated Generator Regions (CGRs) are obtained

Summary

Introduction

Energy conversion within the plasma sheet plays an important role for the Earth’s magnetosphere and its energy budget. Assuming a scalar pressure p and using the equation of motion from one-fluid theory, we obtain an equation for the energy conservation of the bulk motion by multiplying with the plasma bulk velocity, v, ∂ Wk ∂t = −∇. The right hand side of the equation corresponds to the source terms for the bulk kinetic energy density: the divergence of the bulk kinetic energy flux, −∇·(Wkv), the work done by the pressure forces on the plasma, −∇p·v, and the work done by the electromagnetical forces on the plasma, E·J. When E·J (or −∇p·v) is positive, work is done on the plasma and the kinetic energy increases. If E·J is negative, the particles are losing energy to the electromagnetic field, as can be seen from the Poynting theorem (equation of energy conservation for the electromagnetic field)

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Conclusion