Night-time electron density of the lower ionosphere in the South Atlantic Geomagnetic Anomaly region obtained with a LF/VLF oblique ionosonde

Studies of the reflectivity of the night-time lower ionosphere in the region of the South Atlantic Geomagnetic Anomaly have been carried out for the period August 1980 to July 1981, using data collected by an oblique ionosonde at low/very low frequency, in the southern part of Brazil, near the centre of the Anomaly. From these studies monthly average behaviour of heights and reflection coefficients of the lower ionosphere between 15 and 60 kHz have been deduced. Assuming an exponential model of the electron density distribution in the lower ionosphere the appropriate numerical parameters were calculated using a trial-and-error approach with ‘full wave’ calculations and iterative computational techniques. For the period considered three sets of parameters could satisfactorily represent the lower ionosphere in the anomalous region and in the period analysed: one valid from August to November 1980, one valid from December 1980 to March 1981 and the other valid from May to July 1981. April 1981 seemed to present anomalous behaviour.