A latitudinal-distributed network of GPS receivers has been operating withinColombia, Peru and Chile with sufficient latitudinal span to measure theabsolute total electron content (TEC) at both crests of the equatorialanomaly. The network also provides the latitudinal extension of GPSscintillations and TEC depletions. The GPS-based information has beensupplemented with density profiles collected with the Jicamarca digisondeand JULIA power maps to investigate the background conditions of thenighttime ionosphere that prevail during the formation and the persistenceof plasma depletions. This paper presents case-study events in which thelatitudinal extension of GPS scintillations, the maximum latitude of TECdepletion detections, and the altitude extension of radar plumes arecorrelated with the location and extension of the equatorial anomaly. Thenit shows the combined statistics of GPS scintillations, TEC depletions, TEClatitudinal profiles, and bottomside density profiles collected betweenSeptember 2001 and June 2002. It is demonstrated that multiple sights of TECdepletions from different stations can be used to estimate the drift of thebackground plasma, the tilt of the plasma plumes, and in some cases even theapproximate time and location of the depletion onset. This studycorroborates the fact that TEC depletions and radar plumes coincide withintense levels of GPS scintillations. Bottomside radar traces do not seem tobe associated with GPS scintillations. It is demonstrated thatscintillations/depletions can occur when the TEC latitude profiles aresymmetric, asymmetric or highly asymmetric; this is during the absence ofone crest. Comparison of the location of the northern crest of theequatorial anomaly and the maximum latitude of scintillations reveals thatfor 90% of the days, scintillations are confined within the boundaries ofthe 50% decay limit of the anomaly crests. The crests of the anomaly arethe regions where the most intense GPS scintillations and the deepest TECdepletions are encountered. In accord with early results, we observe thatGPS scintillations/TEC depletions mainly occur when the altitude of themagnetic equator F-region is above 500km. Nevertheless, in many instancesGPS scintillations and TEC depletions are observed to exist when the F-layeris well below 500km or to persist when the F-layer undergoes its typicalnighttime descent. Close inspection of the TEC profiles duringscintillations/depletions events that occur when the equatorial F-layer peakis below 500km altitude reveals that on these occasions the ratio of thecrest-to-equator TEC is above 2, and the crests are displaced 10° or morefrom the magnetic equator. When the equatorial F-layer is above 500km,neither of the two requirements is needed, as the flux tube seems to beinherently unstable. We discuss these findings in terms of theRayleigh-Taylor instability (RTI) mechanism for flux-tube integratedquantities. We advance the idea that the seeming control that the reversefountain effect exerts on inhibiting or suppressing GPS scintillations maybe related to the redistribution of the density and plasma transport fromthe crests of the anomaly toward the equatorial region and then to muchlower altitudes, and the simultaneous decrease of the F-region altitude.These two effects originate a decrease in the crest/trough ratio and areduction of the crests separation, making the whole flux tube more stableto the RTI. The correspondence between crest separation, altitude of theequatorial F-region, the onset of depletions, and the altitude (latitude)extension of plumes (GPS scintillations) can be used to track the fate ofthe density structures.
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