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| Titel |
Possible role of magnetosphere-ionosphere coupling in auroral arc generation |
| VerfasserIn |
W. Lyatsky, A. M. Hamza |
| 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 ; 18, no. 9 ; Nr. 18, no. 9, S.1108-1117 |
| Datensatznummer |
250014061
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| Publikation (Nr.) |
 copernicus.org/angeo-18-1108-2000.pdf |
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| Zusammenfassung |
Three models for the magnetosphere-ionosphere
coupling feedback instability are considered. The first model is based on
demagnetization of hot ions in the plasma sheet. The instability takes place in
the global magnetosphere-ionosphere system when magnetospheric electrons drift
through a spatial gradient of hot magnetospheric ion population. Such a
situation exists on the inner and outer edges of the plasma sheet where
relatively cold magnetospheric electrons move earthward through a radial
gradient of hot ions. This leads to the formation of field-aligned currents. The
effect of upward field-aligned current on particle precipitation and the
magnitude of ionospheric conductivity leads to the instability of this earthward
convection and to its division into convection streams oriented at some angle
with respect to the initial convection direction. The growth rate of the
instability is maximum for structures with sizes less than the ion Larmor radius
in the equatorial plane. This may lead to formation of auroral arcs with widths
about 10 km. This instability explains many features of such arcs, including
their conjugacy in opposite hemispheres. However, it cannot explain the very
high growth rates of some auroral arcs and very narrow arcs. For such arcs
another type of instability must be considered. In the other two models the
instability arises because of the generation of Alfven waves from growing
arc-like structures in the ionospheric conductivity. One model is based on the
modulation of precipitating electrons by field-aligned currents of the upward
moving Alfven wave. The other model takes into consideration the reflection of
Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000
km. The growth of structures in both models takes place when the ionization
function associated with upward field-aligned current is shifted from the edges
of enhanced conductivity structures toward their centers. Such a shift arises
because the structures move at a velocity different from the E×B
drift. Although both models may work, the growth rate for the model, based on
the modulation of the precipitating accelerated electrons, is significantly
larger than that of the model based on the Alfven wave reflection. This
mechanism is suitable for generation of auroral arcs with widths of about 1 km
and less. The growth rate of the instability can be as large as 1 s-1,
and this mechanism enables us to justify the development of auroral arcs only in
one ionosphere. It is hardly suitable for excitation of wide and conjugate
auroral arcs, but it may be responsible for the formation of small-scale
structures inside a wide arc.
Key words: Ionosphere (auroral ionosphere) -
Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere
interactions)
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