Supercooled ion temperatures observed in the topside ionosphere at dawn meridian during storm periods

A few rare events of supercooled ion temperature near the magnetic equator at dawn meridian have been observed by the first satellite of the Republic of China, ROCSAT-1, at 600 km altitude. The ion cooling event occurs within a narrow longitude region where a large density reduction together with a large downward, eastward, and field-aligned flow is observed. Although ion temperature troughs around the magnetic equator have been observed by many satellites before, and the interhemispheric plasma transport of adiabatic cooling effect for ions near the equator has been modeled to explain the past observations, the current observation enunciates some new facts of the ion cooling effect that have not been reported. First, ion cooling events are observed by ROCSAT at an unprecedented low altitude of 600 km during the solar maximum years of 2000 and 2001. Second, the ion temperature trough is observed, together with a large density reduction and a large downward and eastward drift that were absent in the past observations. Third, the present events are only observed near the dawn meridian during the disturbed periods. Comparing the observational facts with the existing models, we conclude that the model of interhemispheric plasma transport [Bailey et al, 1973; Heelis et al., 1978; Bailey and Heelis, 1980] seems to fit best with the current observations. However, instead of resorting to the quiet time neutral wind to induce the quasi steady state plasma transport as modeled before, a storm time transient dynamical process could have occurred within a limited longitude region where ions at high latitudes are forced to move along the field lines to the dip equator to cause ion cooling observed at 600 km lower than predicted in the model. This storm time ionospheric process could be related to the creation of a westward electric field in the inner magnetosphere near the dawn meridian from a sudden enhancement of the convection field during a disturbed period modeled in the comprehensive ring current model [Fok et al., 2003].