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| Titel |
Vlasov simulations of electron trapping on auroral field lines |
| VerfasserIn |
H. Gunell, I. Mann, J. De Keyser, L. Andersson |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062867
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| Zusammenfassung |
| In the auroral zone, electric fields that are parallel to the earth’s magnetic field are
known to exist and to contribute to the acceleration of auroral electrons. 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).
Stationary kinetic models have been used to study the current-voltage characteristics of
the auroral current circuit (Knight, Planet. and Space Sci., vol. 21, 741-750, 1973). Fluid and
hybrid simulations have been used to model parallel electric fields and Alfvén waves, and to
study the relationship between them (e.g., Vedin and Rönnmark, JGR, vol. 111, 12201, 2006).
Ergun, et al. (GRL, vol. 27, 4053-4056, 2000) found stationary Vlasov solutions over regions
extending several Earth radii, and Main, et al. (PRL, vol. 97, 185001, 2006) performed
Vlasov simulations of the auroral acceleration region. 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.
Gunell et al. (submitted to GRL, 2012) performed Vlasov simulations, using a model
that is one-dimensional in configuration space and two-dimensional in velocity
space, and found that about half of the potential drop is found in a thin double layer.
The other half is in a region, which extends a few earth radii above it. The double
layer itself is stationary, while there are oscillations in the longer low-field region.
The current-voltage characteristic approximately follows the Knight relation. The
altitude of the double layer decreases with an increasing field-aligned potential
drop.
Here, we use Vlasov simulations to study how the time dependent formation of the
potential drop can lead to trapping of electrons between the electric field that accelerates them
downward and the magnetic mirror, which reflects them back up again. The presence of a
trapped population influences the shape of the potential profile. Thus, it is important for the
understanding of auroral acceleration to also understand the processes that trap and release
particles. |
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