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| Titel |
Spiral structures and regularities in magnetic field variations and auroras |
| VerfasserIn |
Y. I. Feldstein, L. I. Gromova, M. Forster, A. E. Levitin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
2190-5010
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| Digitales Dokument |
URL |
| Erschienen |
In: History of Geo- and Space Sciences ; 3, no. 1 ; Nr. 3, no. 1 (2012-02-21), S.1-31 |
| Datensatznummer |
250000853
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| Publikation (Nr.) |
 copernicus.org/hgss-3-1-2012.pdf |
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| Zusammenfassung |
The conception of spiral shaped precipitation regions, where solar corpuscles
penetrate the upper atmosphere, was introduced into geophysics by C. Störmer
and K. Birkeland at the beginning of the last century.
Later, in the course of the XX-th century, spiral distributions were disclosed
and studied in various geophysical phenomena.
Most attention was devoted to spiral shapes in the analysis of regularities
pertaining to the geomagnetic activity and auroras.
We review the historical succession of perceptions about the number and positions
of spiral shapes, that characterize the spatial-temporal distribution of magnetic
disturbances.
We describe the processes in the upper atmosphere, which are responsible for the
appearance of spiral patterns.
We considered the zones of maximal aurora frequency and of maximal particle
precipitation intensity, as offered in the literature, in their connection
with the spirals.
We discuss the current system model, that is closely related to the spirals
and that appears to be the source for geomagnetic field variations during
magnetospheric substorms and storms.
The currents in ionosphere and magnetosphere constitute together with
field-aligned (along the geomagnetic field lines) currents (FACs) a
common 3-D current system.
At ionospheric heights, the westward and eastward electrojets represent
characteristic elements of the current system.
The westward electrojet covers the longitudinal range from the morning
to the evening hours, while the eastward electrojet ranges from afternoon
to near-midnight hours.
The polar electrojet is positioned in the dayside sector at cusp latitudes.
All these electrojets map along the magnetic field lines to certain plasma
structures in the near-Earth space.
The first spiral distribution of auroras was found based on observations
in Antarctica for the nighttime-evening sector (N-spiral),
and later in the nighttime-evening (N-spiral) and morning (M-spiral)
sectors both in the Northern and Southern Hemispheres.
The N- and M-spirals drawn in polar coordinates form an oval, along which one
observes most often auroras in the zenith together with a westward electrojet.
The nature of spiral distributions in geomagnetic field variations was
unabmibuously interpreted after the discovery of the spiral's existence in the auroras had been
established and this caused a change from the paradigm of the auroral zone
to the paradigm of the auroral oval.
Zenith forms of auroras are found within the boundaries of the auroral oval.
The oval is therefore the region of most frequent precipitations of corpuscular
fluxes with auroral energy, where anomalous geophysical phenomena occur most
often and with maximum intensity.
S. Chapman and L. Harang identified the existence of a discontinuity at
auroral zone latitudes (Φ ∼ 67°) around midnight between the westward
and eastward electrojets, that is now known as the Harang discontinuity.
After the discovery of the auroral oval and the position of the westward
electrojet along the oval, it turned out, that there is no discontinuity at
a fixed latitude between the opposite electrojets, but rather a gap, the
latitude of which varies smoothly between Φ ∼ 67° at midnight
and Φ ∼ 73° at 20:00 MLT.
In this respect the term ''Harang discontinuity'' represents no intrinsic phenomenon,
because the westward electrojet does not experience any disruption in the
midnight sector but continues without breaks from dawn to dusk hours. |
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