Abstract. Equatorial F region plasma drift velocities measured by a digital ionosonde (CADI) that was recently installed in Fortaleza, Brazil, are used to investigate magnetospheric disturbance effects in the vertical (zonal) and zonal (vertical) velocities (electric fields). For the first time we report evidence of large fluctuations in irregularity zonal drift velocities (-50-180 m/s) associated with magnetospheric disturbances. The fluctuations in the zonal velocity, anti correlated with those in vertical velocity, are unlikely to be produced by prompt penetration of disturbance meridional electric field of high latitude/magnetospheric origin. A mechanism is proposed to explain the velocity fluctuations that involves: (1) Hall polarization vertical electric field in the E-layer that is field line mapped on to F-layer, and (2) electric field caused by vertical current arising from divergence in field line integrated zonal Pedersen current; both produced by the primary disturbance zonal electric field. Enhanced nighttime E region conductivity with possible spatial gradients, a requirement for the functioning of this mechanism, is observed to be present from other simultaneous measurements, whose source is suggested to be particle induced ionization in the south Atlantic Magnetic Anomaly (SAMA) zone, as known also from previous studies.
Abstract. Through the groundscatter process the Super Dual Auroral Radar (Super-DARN) has become a powerful tool for studying F region gravity waves. However, the measurement of the gravity wave position is not direct and relies on an assumption relating ground scatter distance to reflection distance. In previous studies it has been assumed that the tilting of the ionospheric reflecting layer was negligible. Hence the gravity wave distance has been calculated as if the reflecting layer was strictly horizontal. Using virtual height data from an ionosonde and ray tracing, we show that this assumption leads to a systematic error of about 16 % in the positioning of the ionospheric reflection point, with the error more than 30% on occasion. Using ray tracing, we obtained an improved relation between ionospheric reflection and ground scatter distances. With this improved distance calculation, we have found the direction and velocity for a number of gravity waves. These waves were found to be traveling equatorward, usually, with velocities between 50 and 280 m/s, in agreement with previous gravity wave observations and with the notion of filtering by the thermospheric wind. In some cases the source locations were determined by using gravity wave dispersion. These locations were found to be on the poleward side of the auroral oval during periods of weak, but observable, magnetic disturbance. Our raytracing studies found that the strongest features were due to gravity waves of 3-20 km amplitude.
Abstract. At Fortaleza, Brazil, in the equatorial zone about 400 km south of the magnetic equator a presunrise (secondary) maximum of spread F occurrence is observed during sunspot minimum and, in particular, during December solstice. The spread F takes the form of patches of irregularities that are convecting eastwards at-50 rn s '• . Most of the patches are collocated with bottomside bulges of the ionosphere. Our measurements indicate that these bottomside bulges are unstable due to a gradient-drift instability that is slowly growing and produces the spread F. The bulges themselves seem to be evidence of a Rayleigh-Taylor instability process.
Abstract. New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.
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