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| Titel |
Origin of type-2 thermal-ion upflows in the auroral ionosphere |
| VerfasserIn |
L. M. Kagan, J.-P. St.-Maurice |
| 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 ; 23, no. 1 ; Nr. 23, no. 1 (2005-01-31), S.13-24 |
| Datensatznummer |
250015117
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| Publikation (Nr.) |
 copernicus.org/angeo-23-13-2005.pdf |
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| Zusammenfassung |
| The origin of thermal ion outflows exceeding 1km/s in the high-latitude F-region
has been a subject of considerable debate. For cases with strong
convection electric fields, the "evaporation" of the ions due to frictional
heating below 400-500km has been shown to provide some satisfactory
answers. By contrast, in the more frequent subclass of outflow events
observed over auroral arcs, called type-2, there is no observational
evidence for ion frictional heating. Instead, an electron temperature
increase of up to 6000° K is observed over the outflow region. In this
case, field-aligned electric fields have long been suspected to be involved,
but this explanation did not seem to agree with expectations from the ion
momentum balance. In the present work we provide a consistent scenario for
the type-2 ion upflows based on our case study of an event that occurred on
20 February 1990. We introduce, for the first time, the electron energy
balance in the analysis. We couple this equation with the ion momentum
balance to study the salient features of the observations and conclude that
type-2 ion outflows and the accompanying electron heating events are indeed
consistent with the existence of a field-aligned electric field. However,
for our explanation to work, we have to require that an allowance be made for
electron scattering by high frequency turbulence. This turbulence could be
generated at first by the very fast response of the electrons themselves to
a newly imposed electric field that would be partly aligned with the
geomagnetic field. The high frequencies of the waves would make it
impossible for the ions to react to the waves. We have found the electron
collision frequency associated with scattering from the waves to be rather
modest, i.e. comparable to the ambient electron-ion collision frequency.
The field-aligned electric field inferred from the observations is likewise
of the same order of magnitude as the normal ambipolar field, at least for
the case that we have studied in detail. We propose that the field-aligned
electric field is maintained by the north-south motion of an east-west arc.
The magnetic perturbation associated with the arc itself converts a small
fraction of the perpendicular electric field into a field parallel to the
total magnetic field, while the north-south motion ensures that the
conversion never stops. |
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