Latitudinal extension of low-latitude scintillations measured with a network of GPS receivers

Abstract

Abstract.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 scintillaCorrespondence to: C. E. Valladares (valladar@bc.edu) tions/TEC depletions mainly occur when the altitude of the magnetic equator F-region is above 500 km. Nevertheless, in many instances GPS scintillations and TEC depletions are observed to exist when the F-layer is well below 500 km 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 500 km 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 500 km, neither of the two requirements is needed, as the flux tube seems to be inherently unstable. We discuss these findings in terms of the RayleighTaylor instability (RTI) mechanism for flux-tube integrated quantities. We advance the idea that ...

Observations in equatorial anomaly region of total electron content enhancements and depletions

“…Sudden reductions in TEC, observed in the nighttime low-latitude Fregion, have been identified with the plasma density depletions of the equatorial origin (e.g. Valladaras et al, 2004). Consistent with this definition, the reductions in the TEC reported in Figs.…”

“…Accordingly, the TEC depletion consists of a sudden reduction in TEC followed by a recovery to a level near the TEC value preceding the depletion. As noted by Valladaras et al (2004), processes, such as the nighttime decay of the F layer and the density redistribution caused by the Fountain effect also lessen the TEC values. But these processes result in shallow TEC slopes and an absence of the recovery segment.…”

“…A TEC depletion associated with the equatorial plasma bubble has been defined by Valladaras et al (2004). Accordingly, the TEC depletion consists of a sudden reduction in TEC followed by a recovery to a level near the TEC value preceding the depletion.…”

“…Recent observations (Beach and Kintner, 1999;Bhattacharyya et al, 2000;Valladaras et al, 2004), using the GPS satellites in the equatorial and low-latitude zone, have revealed the occurrence of the UHF scintillations and the fluctuations in the total electron content (TEC). Since the TEC values are known to be highest around ±15 • magnetic latitude (more commonly known as the Appleton anomaly or the equatorial ionization anomaly region), any fluctuations in the TEC at these latitudes would result in severe scintillations, causing signal degradation.…”

Spread F – an old equatorial aeronomy problem finally resolved?

“…Networks of receivers deployed at different equatorial latitudes and at adjacent longitudes can be used to measure the location, extension and intensity of such depletions. Valladares et al (2004) has used a network of GPS receivers around the equatorial 75 • W meridian to study the occurrence of satellite scintillations and TEC depletions in the context of different developments of the equatorial anomaly. Using JULIA, the authors show a close correspondence between radar plumes, TEC depletions and the occurrence of scintillations.…”