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| Titel |
Bi-layer structure of counterstreaming energetic electron fluxes: a diagnostic tool of the acceleration mechanism in the Earth's magnetotail |
| VerfasserIn |
D. V. Sarafopoulos |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 28, no. 2 ; Nr. 28, no. 2 (2010-02-09), S.455-477 |
| Datensatznummer |
250016777
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| Publikation (Nr.) |
 copernicus.org/angeo-28-455-2010.pdf |
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| Zusammenfassung |
| For the first time we identify a bi-layer structure of energetic electron
fluxes in the Earth's magnetotail and establish (using datasets mainly
obtained by the Geotail Energetic Particles and Ion Composition (EPIC/ICS)
instrument) that it actually provides strong evidence for a purely spatial
structure. Each bi-layer event is composed of two distinct layers with
counterstreaming energetic electron fluxes, parallel and antiparallel to the
local ambient magnetic field lines; in particular, the tailward directed
fluxes always occur in a region adjacent to the lobes. Adopting the X-line
as a standard reconnection model, we determine the occurrence of bi-layer
events relatively to the neutral point, in the substorm frame; four (out of
the shown seven) events are observed earthward and three tailward, a result
implying that four events probably occurred with the substorm's local
recovery phase. We discuss the bi-layer events in terms of the X-line model;
they add more constraints for any candidate electron acceleration mechanism.
It should be stressed that until this time, none proposed electron
acceleration mechanism has discussed or predicted these layered structures
with all their properties. Then we discuss the bi-layer events in terms of
the much promising "akis model", as introduced by Sarafopoulos (2008). The
akis magnetic field topology is embedded in a thinned plasma sheet and is
potentially causing charge separation. We assume that as the Rc
curvature radius of the magnetic field line tends to become equal to the ion
gyroradius rg, then the ions become non-adiabatic. At the limit
Rc=rg the demagnetization process is also under way and the
frozen-in magnetic field condition is violated by strong wave turbulence;
hence, the ion particles in this geometry are stochastically scattered. In
addition, ion diffusion probably takes place across the magnetic field,
since an intense pressure gradient is directed earthward; hence, ions are
ejected tailward of akis. This way, in front of akis an "ion capsule region"
is formed with net positive charge. In between them a distinct region with
an electric field E⊥ orthogonal to the magnetic field is emerged;
E⊥ in front of akis is directed earthward. The field-aligned and
highly anisotropic energetic electron populations have probably resulted via
spatially separated antiparallel and field-aligned electric fields being the
very heart of the acceleration source. We assume that the ultimate cause for
the field-aligned electric fields are the net positive capsule charge and
the net negative charge trapped at the tip of akis; both charges will be
eventually neutralized through field aligned currents, but they remain
unshielded for sufficient time to produce the observed events. |
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