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| Titel |
Generation of mesoscale F layer structure and electric fields by the combined Perkins and Es layer instabilities, in simulations |
| VerfasserIn |
R. B. Cosgrove |
| 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 ; 25, no. 7 ; Nr. 25, no. 7 (2007-07-30), S.1579-1601 |
| Datensatznummer |
250015881
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| Publikation (Nr.) |
 copernicus.org/angeo-25-1579-2007.pdf |
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| Zusammenfassung |
| The generic equilibrium configuration of the nighttime midlatitude
ionosphere consists of an F layer held up against gravity by winds and/or electric fields, and a
sporadic E (Es) layer located by a sheared wind field, which experiences the same electric
fields as the F layer. This configuration is subject to two large-scale (e.g. >10 km)
"layer instabilities": one of the F layer known as the Perkins instability, and another of the
Es layer which has been called the Es layer instability. Electric fields on scales
larger than (about) 10 km map very efficiently between the Es and F layers, and the two
instabilities have a similar geometry, allowing them to interact with one another. As shown
through a linear growth rate analysis, the two most important parameters governing the interaction
are the relative horizontal velocity between the Es and F layers, and the integrated
conductivity ratio ΣH/ΣPF, where ΣH and ΣPF are the field
line integrated Hall conductivity of the Es layer, and the field line integrated Pedersen
conductivity of the F layer, respectively. For both large and small relative velocities the
growth rate was found to be more than double that of the Perkins instability alone, when
ΣHΣPF=1.8.
However, the characteristic eigenmode varies considerably
with relative velocity, and different nonlinear behavior is expected in these two cases. As a
follow up to the linear growth rate analysis, we explore in this article the nonlinear evolution
of the unstable coupled system subject to a 200 km wavelength initial perturbation of the F
layer, using a two-dimensional numerical solution of the two-fluid equations, as a function of
relative horizontal velocity and ΣHΣPF.
We find that when
ΣHΣPF⪝0.5
the Perkins instability is able to control
the dynamics and modulate the F layer altitude in 2 to 3 h time. However, the electric
fields remain small until the altitude modulation is extremely large, and even then they are not
large enough to account for the observations of large midlatitude electric fields. When
ΣHΣPF⪞1
the Es layer becomes a major contributor
to the F layer dynamics. The Es layer response involves the breaking of a wave, with
associated polarization electric fields, which modulate the F layer. Larger electric fields
form when the relative velocity between the Es and F layers is large, whereas larger
modulations of the F layer altitude occur when the relative velocity is small. In the latter
case the F layer modulation grows almost twice as fast (for ΣHΣPF=1)
as when no Es layer is present. In the former case the electric fields associated with the
Es layer are large enough to explain the observations (~10 mV/m) , but occur over
relatively short temporal and spatial scales. In the former case also there is evidence that the
F layer structure may present with a southwestward trace velocity induced by Es layer
motion. |
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