Navegando Centro de Rádio Astronomia e Astrofísica Mackenzie (CRAAM) por Assunto "amas"
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- TeseEstudo da propagação das ondas de rádio de VLF no trajeto Havaí (HI) – Atibaia (SP)Magalhães, António Amândio Sanches de (2019-07-24)
Escola de Engenharia Mackenzie (EE)The sounding technique using very low frequency waves (VLF) is the only enabling the continuous survey of the ionospheric region D. Although there are in-situ measurements with rockets, the altitude of the D-region in ~70km prevents the measurements with satellites or with balloons, as well as the weak electronic density does not permit the use of vertical soundings by radio waves. In this work we use the data base of the South America VLF Network (SAVNET) to study the propagation anomalies of the VLF waves observed as phase and amplitude variations indicating changes of the local electrical conductivity. We show the first observational evidence of the presence of the South Atlantic Magnetic Anomaly (SAMA) when comparing ionospheric disturbances observed in two parallel propagation paths between the NPM (Hawaii) transmitter and receivers in Peru (PLO) and Brazil (ATI). Thus, it has been stated that the altitude of the quiescent region-D is 2-3 km lower inside the SAMA region. There was done a monitoring of ~7 years of the Terminator times, which allowed the estimation of the modal interference distance D along the same period, and the study of the variability of the nightly quiescent ionospheric altitude. Characteristic periods of seasonal effects have been found, as well as phenomena as the Quasi Biennial Oscillation (QBO) and one connection with the solar cycle activity. The ionospheric sensitivity has been also investigated by estimating the minimum of fluence Fmin necessary to produce an ionospheric disturbance during daylight. The monitoring of Fmin along the solar cycle 24 indicates a clear correlation with the solar cycle activity, which suggests being possible to obtain an ionospheric index characteristic of the Lyman-α solar flux radiation.