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| Titel |
The Alfvén resonator revisited |
| VerfasserIn |
A. M. Hamza, W. Lyatsky |
| 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 (2010-02-02), S.359-366 |
| Datensatznummer |
250016767
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| Publikation (Nr.) |
 copernicus.org/angeo-28-359-2010.pdf |
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| Zusammenfassung |
| Two models for a magnetosphere-ionosphere coupling feedback
instability in the lower magnetosphere are studied. In both models the
instability arises because of the generation of an Alfvén wave from growing
arc-like structures in the ionospheric conductivity. The first model is based on
the modulation of precipitating electrons by field-aligned currents of the
upward moving Alfvén wave (Modulation Model). The second model takes into consideration the
reflection of the Alfvén wave from a maximum of the Alfvén velocity at
about 3000 km altitude (Reflection Model). 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 to
their centers. Such a shift arises because the structures move along the
ionosphere at a velocity different from the E×B drift velocity. As a
result, field-aligned currents of upward propagating Alfvén wave at some
altitude appear shifted with respect to the edges of the structures.
Although both models may work, the growth rate for the first model, as
based on the modulation of the precipitating accelerated electrons, for
typical conditions, may be tens or more times larger than that for the
second model based on the Alfvén wave reflection. The proposed models
can provide the growth of both single and periodic structures. When applied
to auroral arc generation the studied instability leads to high growth rates and
narrow arcs. The physical mechanism is mostly suitable for the
generation of auroral arcs with widths of the order of 1 km and less. The growth rate
of the instability for such structures can be as large as 0.3 s−1. In the
case of periodic structures, their motion must lead to the generation
of magnetic pulsations with periods of about 1–6 s, which is close to the
expected period of Alfvén resonant oscillations in the lower magnetosphere.
However, these oscillations (for the first and most effective model MM) are not
exactly Alfvén resonant oscillations. These oscillations are modulations in the
ionospheric density, which propagate along the ionospheric currents and not along the
magnetic field lines. |
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