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| Titel |
Latitudinal extension of low-latitude scintillations measured with a network of GPS receivers |
| VerfasserIn |
C. E. Valladares, J. Villalobos, R. Sheehan, M. P. Hagan |
| 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 ; 22, no. 9 ; Nr. 22, no. 9 (2004-09-23), S.3155-3175 |
| Datensatznummer |
250014997
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| Publikation (Nr.) |
 copernicus.org/angeo-22-3155-2004.pdf |
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| Zusammenfassung |
| A latitudinal-distributed network of GPS receivers has been operating within
Colombia, Peru and Chile with sufficient latitudinal span to measure the
absolute total electron content (TEC) at both crests of the equatorial
anomaly. The network also provides the latitudinal extension of GPS
scintillations and TEC depletions. The GPS-based information has been
supplemented with density profiles collected with the Jicamarca digisonde
and JULIA power maps to investigate the background conditions of the
nighttime ionosphere that prevail during the formation and the persistence
of plasma depletions. This paper presents case-study events in which the
latitudinal extension of GPS scintillations, the maximum latitude of TEC
depletion detections, and the altitude extension of radar plumes are
correlated with the location and extension of the equatorial anomaly. Then
it shows the combined statistics of GPS scintillations, TEC depletions, TEC
latitudinal profiles, and bottomside density profiles collected between
September 2001 and June 2002. It is demonstrated that multiple sights of TEC
depletions from different stations can be used to estimate the drift of the
background plasma, the tilt of the plasma plumes, and in some cases even the
approximate time and location of the depletion onset. This study
corroborates the fact that TEC depletions and radar plumes coincide with
intense levels of GPS scintillations. Bottomside radar traces do not seem to
be associated with GPS scintillations. It is demonstrated that
scintillations/depletions can occur when the TEC latitude profiles are
symmetric, asymmetric or highly asymmetric; this is during the absence of
one crest. Comparison of the location of the northern crest of the
equatorial anomaly and the maximum latitude of scintillations reveals that
for 90% of the days, scintillations are confined within the boundaries of
the 50% decay limit of the anomaly crests. The crests of the anomaly are
the regions where the most intense GPS scintillations and the deepest TEC
depletions are encountered. In accord with early results, we observe that
GPS scintillations/TEC depletions mainly occur when the altitude of the
magnetic equator F-region is above 500km. Nevertheless, in many instances
GPS scintillations and TEC depletions are observed to exist when the F-layer
is well below 500km or to persist when the F-layer undergoes its typical
nighttime descent. Close inspection of the TEC profiles during
scintillations/depletions events that occur when the equatorial F-layer peak
is below 500km altitude reveals that on these occasions the ratio of the
crest-to-equator TEC is above 2, and the crests are displaced 10° or more
from the magnetic equator. When the equatorial F-layer is above 500km,
neither of the two requirements is needed, as the flux tube seems to be
inherently unstable. We discuss these findings in terms of the
Rayleigh-Taylor instability (RTI) mechanism for flux-tube integrated
quantities. We advance the idea that the seeming control that the reverse
fountain effect exerts on inhibiting or suppressing GPS scintillations may
be related to the redistribution of the density and plasma transport from
the crests of the anomaly toward the equatorial region and then to much
lower altitudes, and the simultaneous decrease of the F-region altitude.
These two effects originate a decrease in the crest/trough ratio and a
reduction of the crests separation, making the whole flux tube more stable
to the RTI. The correspondence between crest separation, altitude of the
equatorial F-region, the onset of depletions, and the altitude (latitude)
extension of plumes (GPS scintillations) can be used to track the fate of
the density structures. |
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