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Energy conversion and transfer in the plasma sheet
The dynamics of the solar wind–magnetosphere–ionosphere system depends critically on
energy conversion and transfer processes that take place in the plasma sheet. Schematically,
magnetic energy stored in the magnetospheric lobes is believed to be converted to kinetic
energy of bursty bulk flows (BBFs) by reconnection; BBFs carry the energy (as well as
magnetic flux and matter) towards the inner magnetosphere, where the flow is braked by the
interaction with the higher magnetic field; as a result, part of the energy is converted into
Poynting flux, that ultimately powers the aurora. While this scheme provides a reasonable
framework to describe the energy flow in the plasma sheet, numerous features are still far
from being completely understood.
In a given reference system, the local rate of the energy conversion between its mechanical
and electromagnetic forms is measured by E.J, with E the electric field and J the current
density. In load regions, where E.J>0, electromagnetic energy is converted into bulk kinetic
energy, similar to a motor, and/or dissipated as heat. In generator regions E.J<0 and
mechanical energy is converted into magnetic energy stored locally or radiated away as
electromagnetic energy flux (Poynting flux).
The Cluster mission allows, for the first time, a systematic examination of the energy
conversion, by the evaluation of the E.J term. In the plasma sheet, E can be estimated from
data provided by the ion spectrometry (CIS) and electric field (EFW) experiments, while J
can be derived from the four-point magnetic field data provided by the FGM experiment. The
examination of energy conversion regions (ECRs) performed under ECSTRA included
statistical investigations, development work towards a neural network technique to identify
ECR events, as well as case study explorations of ECRs' structure and of the waves
associated with ECRs.
Statistical investigations
ECR identification by neural networks
Case study examination of the ECR structure
Waves in the energy conversion regions
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