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| Titel |
Spread F – an old equatorial aeronomy problem finally resolved? |
| VerfasserIn |
R. F. Woodman |
| 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 ; 27, no. 5 ; Nr. 27, no. 5 (2009-05-04), S.1915-1934 |
| Datensatznummer |
250016510
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| Publikation (Nr.) |
 copernicus.org/angeo-27-1915-2009.pdf |
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| Zusammenfassung |
| One of the oldest scientific topics in Equatorial Aeronomy is related to
Spread-F. It includes all our efforts to understand the physical mechanisms
responsible for the existence of ionospheric F-region irregularities, the
spread of the traces in a night-time equatorial ionogram – hence its name –
and all other manifestations of the same. It was observed for the first time
as an abnormal ionogram in Huancayo, about 70 years ago. But only recently
are we coming to understand the physical mechanisms responsible for its
occurrence and its capricious day to day variability. Several additional techniques
have been used to reveal the spatial and temporal characteristics of the F-region
irregularities responsible for the phenomenon. Among them we have, in
chronological order, radio star scintillations, trans-equatorial radio
propagation, satellite scintillations, radar backscatter, satellite and
rocket in situ measurements, airglow, total electron content techniques
using the propagation of satellite radio signals and, recently, radar
imaging techniques. Theoretical efforts are as old as the observations.
Nevertheless, 32 years after their discovery, Jicamarca radar observations
showed that none of the theories that had been put forward could explain
them completely. The observations showed that irregularities were detected
at altitudes that were stable according to the mechanisms proposed. A
breakthrough came a few years later, again from Jicamarca, by showing that
some of the "stable" regions had become unstable by the non-linear
propagation of the irregularities from the unstable to the stable region of
the ionosphere in the form of bubbles of low density plasma. A problem
remained, however; the primary instability mechanism proposed, an extended
(generalized) Rayleigh-Taylor instability, was too slow to explain the rapid
development seen by the observations. Gravity waves in the neutral
background have been proposed as a seeding mechanism to form irregularities
from which the instability would grow, but the former are difficult to
observe as a controlling parameter. Their actual role still needs to be
determined. More recently, radar observations again have shown the existence
of horizontal plasma drift velocities counter streaming the neutral wind at
the steep bottom of the F-region which produces a fast growing instability
from which a generalized Rayleigh-Taylor instability can grow. The
mechanisms proposed would explain the rapid development of the large and
medium scale irregularities that have been observed, including some seen
only by radars. Nevertheless, a proper quantitative theoretical mechanism
that would explain how these irregularities break into the very important
meter scale ones, responsible for the radar echoes, needs to be developed.
This paper makes a selective historical review of the observations and
proposed theories since the phenomenon was discovered to our current
understanding. |
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