Estudos de micropulsacoes geomagneticas PC3-5 em latitudes muito baixas, no Brasil / PC3-5 study of geomagnetic micropulsations at very low latitude, in Brazil

AUTOR(ES)
DATA DE PUBLICAÇÃO

1998

RESUMO

This study is aimed at the understanding of source mechanisms and propagation modes of Pc3-5 micropulsations at very low and equatorial latitudes. It used simultaneous data measured with a chain of 10 geomagnetic stations in Brazil using high sensitive °uxgate magnetometers. Multiple tapered spectral estimation method based on Fast Fourier Transforms (FFT) was used to obtain micropulsation power spectra, polarization parameters and phases. It was observed an increase on the occurrence of highly polarized Pc3-5 pulsations, reaching maxima close to local noon (14:00 and 15:00 UT) in the stations nearest to the magnetic equator. The dynamic spectra showed the occurrence of 81 simultaneous events of Pc3-4 pulsations with high degree of polarization, mainly during daytime. The diurnal events showed increases in the polarized power density of 3,2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of the pulsations observed at station close of the magnetic equator showed a delay of 48± and 62± in relation to the most distant one. This e®ects is clearly related to the increase of ionospheric conductivity and equatorial electrojet intensity. The polarized power density curves for all the 10 stations showed a similar behaviour, the maximum peaks occurring at same frequency. This may indicate a possible link to the plasmaspheric cavity global oscillation mode. The ellipticity was quase-linear, with the arger axis in the north-south direction, due to the fact that the poloidal mode is almost uncoupled from the oscillation of magnetic ¯eld lines. The amplitude of north-south component is much larger than the west-east one. These results could be an evidence that field line resonance mechanism does not explain Pc3-5 micropulsations observed at very low and equatorial latitudes. The increase in the occurrence of Pc3-4 diurnal events is probably associated with incoming compressional waves resulted from bow shock ion cyclotron instabilities that propagate in the magnetosphere at low latitudes. The propagation of the compressional waves may excite the trapped compressional mode or the compressional global mode. The resulting phenomenon, field lines forced oscillations in the plasmasphere, may be a possible source mechanism for Pc3-4 pulsations at very low latitudes.

ASSUNTO(S)

micropulsacoes geomagneticas pulsacoes geomagneticas ondas de extrema baixa frequencia (ulf) variacoes magneticas ondas magnetohidrodinamicas geofisica espacial geomagnetismo

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