J. R. de Souza

Equatorial ionospheric vertical plasma drift model over the Brazilian region

Comparison between equatorial inospheric F region vertical plasma drift from satellite measurements [Fejer et al., 1995], for the Brazilian longitude sector, and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods. Using ionosonde measurements around sunset and satellite measurements at other local times, we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the Brazilian longitudes, where the magnetic declination is around −20°. The so derived drift model, here called IDM (ionosonde drift model), is used as an input to the Sheffield University plasmasphere-ionosphere model (SUPIM). It is shown that the F layer heights given by SUPIM with IDM are in good agreement with ionosonde measurements over the Brazilian longitudes and that IDM better simulates the F layer heights than the averaged drifts given by the satellite drift model [Fejer et al., 1995].

An overview of IRI-observational data comparison in American (Brazilian) sector low latitude ionosphere

Abstract This paper presents an overview of the existing results of comparisons between the IRI-90 predictions and the observational data for the Brazilian longitude sector and attempts to place the major findings in a global perspective. The observational data subjected to comparison with the IRI model include NmF2/foF2 (the peak density/critical frequency of the F2-layer), hmF2/ hpF2 (the height of NmF2/foF2), TEC (total ionospheric electron content) and the electron density-height profiles. The Brazilian stations for which results are presented are: Fortaleza near the magnetic equator and Cachoeira Paulista near the equatorial anomaly crest. The results are compared with those from Asian and Indian longitude sectors. An attempt is made to identify points of agreement/disagreement between the observational data and the IRI. Seasonal and solar activity dependent persistent trends of discrepancies have been found with respect the NmF2, hmF2 and TEC. We have tried to highlight the global nature of such discrepancies aiming at providing an eventual basis for incorporating needed modification for improving the predictive capability of the IRI for the equatorial and low latitude ionosphere.

Total electron content at low latitudes and its comparison with the IRI90

Abstract We used total electron content (TEC) data measured by Faraday rotation technique over Cachoeira Paulista (22.5°S, 45°W), in Brazil, to study the TEC variations with the solar flux at 10.7 cm (F10.7) and to compare the results with the IRI90 predictions. The data were divided into summer, equinox and winter. During the analysed period F10.7 varied from 66 up to 330. Our data shows that the observed TEC at 1600 LT (around the diurnal maximum) and at 0500 LT (around the diurnal minimum) increases with F10.7 until saturation is reached which occurs at F10.7≈210 to 220 for equinox and summer, and at F10.7≈180 for winter months. Comparison with the IRI90 predictions shows that IRI overestimates the TEC at 0500 LT for all solar flux values. At 1600 LT, IRI overestimates the observed TEC for low solar flux but underestimates it for high solar flux values.

Wave structure and polarization electric field development in the bottomside F layer leading to postsunset equatorial spread F

In this paper we present the results of a study on the characteristics of large-scale wave structure in the equatorial ionospheric F region that serve as precursor to postsunset development of the spread F/plasma bubble irregularities. The study is based on analysis of Digisonde data from three equatorial sites in Brazil (Fortaleza, Sao Luis, and Cachimbo) for a period of about 2 months at a medium solar activity phase. Small-amplitude oscillations in the F layer heights, extracted at a number of plasma frequencies, present characteristics as them being generated from upward propagating gravity waves. They represent wave structures in polarization electric field having zonal scale of a few hundred kilometers. Their amplitudes in the afternoon hours undergo amplification toward evening, leading to postsunset development of equatorial spread F/plasma bubble irregularities, on a statistical basis. On the days of their larger amplitudes they appear to occur in phase coherence on all days, and correspondingly, the evening prereversal vertical drift velocities are larger than on days of the smaller amplitudes of the wave structure that appear at random phase on the different days. The sustenance of these precursor wave structures is supported by the relatively large ratio (approaching unity) of the F region-to-total field line-integrated Pedersen conductivities as calculated using the Sheffield University Plasmasphere-Ionosphere Model simulation of the low-latitude ionosphere. The significant amplification in the wave structure toward sunset and the “phase coherent” nature of their occurrences on different days are explained tentatively on the basis of the spatial resonance mechanism.

Ionospheric Electron Content over Brazilian low latitude and its comparison with the IRI and SUPIM models

Abstract Ionospheric Electron Content (IEC), measured at the Brazilian low latitude station of Cachoeira Paulista (22.5°S, 45°W, dip latitude 14°S) was compared with IEC calculated using the IRI-90 and Sheffield University Plasmaphere-Ionosphere (SUPIM) models for a large range of solar fluxes at 10.7 cm (F10.7). The analysis of the F10.7 influence on IEC showed that, for Cachoeira Paulista (CP) at 05 LT (around the diurnal minimum), there is a good agreement between the IEC measurements and the models. At 16 LT (around the diurnal maximum), substantial discrepancies were observed in the IRI-90 results while SUPIM presents a good agreement up to F10.7 ≈ 200. The measured IEC diurnal variation is better represented by SUPIM, mainly during higher F10.7. At low latitudes two critical parametric inputs for the SUPIM mathematical model are the equatorial ExB plasma drifts and thermospheric neutral winds. Therefore, updated values of these parameters should be introduced in the model calculations everytime they are available. At CP, a station located close to the southern Appleton anomaly peak due to the fountain effect, where large discrepancies between IRI-90 and observations were found mainly with high F10.7, this model should include adequately this effect, which we believe is the main reason for the discrepancies.