Lecturer: Maria Hamrin (UmU) Date: 2011-12-01 10:30 Place: Aniara
Plasma sheet energy conversion and energy transfer - Cluster observations
Maria Hamrin
Dept. of Physics
Umeå University
Abstract
Using Cluster estimates of the power density, E.J, where E is the
electric field and J the current density, we can analyze energy
conversion and transfer processes in the Earth's plasma sheet. Cluster
data from the years 2001-2004 at altitudes of 15-20RE have been used
to show that the plasma sheet exhibits a high level of fine structure
with the occurrence of localized Energy Conversion Regions
(ECRs). Approximately three times as many Concentrated Load Regions
(CLRs, E.J>0) as Concentrated Generator Regions (CGRs, E.J<0) have
been identified, confirming the average load character of the plasma
sheet. We show that a southward IMF Bz is favourable for plasma sheet
energy conversion, and that there is an increased particle and
Poynting flux towards the Earth at times when Cluster observes an
enhanced energy conversion in the plasma sheet. Some ECRs are found to
relate to auroral activity. While ECRs are relevant for the energy
conversion between the electromagnetic field and the particles, bursty
bulk flows (BBFs) play a central role for the energy transfer in the
plasma sheet. We show that ECRs and BBFs are likely to be related,
although details of this relationship are yet to be explored. The
plasma sheet energy conversion increases rather simultaneously with
increasing geomagnetic activity in both CLRs and CGRs. Consistent with
large scale magnetotail simulations, most of the observed ECRs appear
to be rather stationary in space, but varying in time. We estimate
that the ECR life time and scale size are a few minutes and a few RE,
respectively. It is conceivable that ECRs rise and vanish locally in
significant regions of the plasma sheet, possibly oscillating between
load and generator character, while some energy is transmitted as
Poynting flux to the ionosphere.
Created 2011-11-21 22:48:00 by Mats Holmström Last changed 2011-11-22 20:44:53 by Mats Holmström
