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| Titel |
Vlasov simulations of auroral flux tubes |
| VerfasserIn |
Herbert Gunell, Johan De Keyser, Ingrid Mann |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075816
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| Zusammenfassung |
| Electric fields that are parallel to the earth’s magnetic field are known to exist in the
auroral zone, where they contribute to the acceleration of auroral electrons. Thus,
parallel electric fields form an integral part of the auroral current circuit. Transverse
electric fields at high altitude result in parallel electric fields as a consequence of the
closure of the field-aligned currents through the conducting ionosphere (L. R. Lyons,
JGR, vol. 85, 1724, 1980). These parallel electric fields can be supported by the
magnetic mirror field (Alfvén and Fälthammar, Cosmical Electrodynamics, 2nd ed.,
1963).
The current-voltage characteristics of an auroral flux tube has been studied using
stationary kinetic models (Knight, Planet. and Space Sci., vol. 21, 741-750, 1973).
Observations have shown that field-aligned potential drops often are concentrated in electric
double layers (e.g. Ergun, et al., Phys. Plasmas, vol. 9, 3685-3694, 2002). In the upward
current region, 20-50% of the total potential drop has been identified as localised. How the
rest of the potential is spread out as function of altitude is not yet known from observations
(Ergun et al., J. Geophys. Res., vol. 109, A12220, doi:101.1029/2004JA010545,
2004).
We have performed Vlasov simulations, using a model that is one-dimensional in
configuration space and two-dimensional in velocity space. In the upward current region,
most of the potential drop is found in a thin, stationary, double layer. The rest is in a region,
which extends a few earth radii above it. The current-voltage characteristic approximately
follows the Knight relation. The altitude of the double layer decreases with an increasing
field-aligned potential drop.
In the downward current region, the voltage is significantly lower than in the upward
current region for the same value of the current. Double layers have been observed also in the
downward current region (Andersson et al., Phys. Plasmas, vol. 9, 3600–3609,
doi:10.1063/1.1490134, 2002). It has been shown that the double layer position is stationary
only when electrons are accelerated into the stronger magnetic field (Song et al., Physica
Scripta, vol. 45, 395–398, doi:10.1088/0031-8949/45/4/019, 1992), whereas in
the downward current region the polarity of the voltage is such as to accelerate
electrons towards weaker magnetic fields. We use Vlasov simulations to follow
the motion of the double layer in this region and to examine the current-voltage
relationship. |
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