Morfologia e dinâmica das irregularidades ionosféricas de pequena escala e imageamento ionosférico por GPS / Morphology and dynamic of small-scale ionospheric irregularities and ionospheric imaging by GPS

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The signals received from the Global Positioning System (GPS) satellites were used to study the behavior of three important geophysical parameters of the ionosphere: small-scale irregularities, their zonal drift velocities and the Total Electron Content (TEC). From the fluctuations (scintillations) in the amplitude of the GPS L1 signal (1575.42 MHz), detected by receivers located in the Brazilian longitudinal sector and distributed at different latitudes, the morphology, the occurrence and the zonal drift of the irregularities with the Fresnel-length were analyzed as a function of the solar cycle, the universal time and the seasonal period. The results show that the irregularities causing scintillations are more frequent during the pre-midnight hours and, the scintillation levels, are larger near the crests of the equatorial ionization anomaly, where the TEC gradients are more intense. The magnitude of the irregularity mean zonal drifts and the occurrence and intensity of the scintillations are higher during the summer solstice months and also tend to enhance with the increase in the solar activity. Depending on the latitude of the observation sites, the dynamics of these irregularities may present a different behavior, such as, large magnitudes and secondary peaks in their velocities. The geomagnetically spaced receiver technique used to estimate the irregularity zonal drifts have also showed to be efficient in the estimations of the scattering height causing scintillations. Simultaneous observations conducted at magnetically conjugate stations showed that GPS signal are not sensitive to F region bottomside sinusoidal irregularities (BSS), but the signals received from VHF geostationary satellites revealed strong scintillations caused by this type of plasma density irregularities. The observations at the conjugate points also showed that a relation between the scintillation level and the size of the TEC depletion (plasma bubble signature) is not straightforward. In addition, analyses of the ionospheric slab thickness did not reveal much about the intensity/variability of the scintillations. A multi-instrumental data assimilation system (named MIDAS), using inverse problem formulations and TEC data as input, was adopted in the tomographic reconstructions of the ionospheric electron density. A good agreement between the reconstruction results and the F2 peak parameters scaled from the ionograms revealed that the technique is capable to reproduce the ionosphere at the south crest of the equatorial ionization anomaly. In the magnetic equator, at that time when an intense plasma vertical gradient is expected to occur, the tomographic reconstructions fail to reproduce the ionosphere. Two-dimensional maps with coupling TEC and scintillation data showed that the scintillations are more intense at the boundaries of the anomaly peak, where strong spatial gradients of the TEC are observed. It suggests that the electron density gradients can be used as an important precursor to the development of GPS L band scintillations.

ASSUNTO(S)

cintilação ionosférica tomografia ionosférica total electron content sistema de posicionamento global deriva de irregularidades irregularity drifts conteúdo eletrônico total ionospheric tomography ionospheric scintillation global positioning system irregularidades ionosféricas ionospheric irregularities

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