Investigation by ray-tracing of the effect of a summer-winter asymmetry on whistler ducting

A ray-tracing model of the inner magnetosphere (L < 6) is constructed for a plasma distribution asymmetric about the equatorial plane, thus representing summer and winter conditions in the two hemispheres. At the reference height of 900 km, the oxygen ion concentration and electron density are taken to vary by factors of ten and two respectively between the hemispheres. The concentrations of hydrogen and helium ions at the reference level are chosen to ensure electron density continuity across the equatorial plane. The altitude at which ducts terminate is modelled to differ between the two hemispheres in accordance with the numerical simulations of Bernhardt and Park (1977).It is shown that the different plasma distributions in the two hemispheres affect the paths of ducted rays and consequently the likelihood of the reception of one-hop whistlers in the conjugate hemisphere. The difference in final latitude between propagation in the symmetric and asymmetric models for the same initial latitude is largest when ducts extend down to 300 km altitude in the conjugate hemisphere. When ducts terminate at greater altitude, the effect of a difference in termination heights between the two hemispheres generally has a larger effect than that of the plasma asymmetry. Both these effects may play a role in determining the seasonal variation of whistler occurrence.