NIGHTTIME ENHANCEMENTS IN IONOSPHERIC ELECTRON-CONTENT IN THE NORTHERN AND SOUTHERN HEMISPHERES

Nighttime enhancements in ionospheric electron content (IEC) observed at conjugate stations in the equatorial anomaly and mid-latitude regio ns under moderate to high solar activity conditions are studied. The o bservations at equatorial anomaly latitudes show that when an enhancem ent occurs in one hemisphere then an enhancement usually occurs in the conjugate hemisphere. The enhancement characteristics (frequency of o ccurrence, time of occurrence, amplitude, and duration) and their seas onal and solar activity variations are in agreement with the fact that the primary source of an enhancement is the post-sunset increase in t he equatorial fountain. It is suggested that the north-south differenc es in the enhancement characteristics, e.g. the enhancement being more frequent and stronger in the southern hemisphere than in the northern hemisphere, are due to the north-south differences in the neutral air wind velocity. At mid-latitudes, on the other hand, when an enhanceme nt occurs in one hemisphere then either no enhancement or only a weak enhancement occurs in the conjugate hemisphere; on no observed night d oes a strong enhancement occur in both hemispheres. The occurrence and other characteristics of the enhancements demonstrate that the primar y source for the nighttime enhancements in IEC at mid-latitudes (i.e. the downward flow of plasma from the protonosphere to the ionosphere) is asymmetric; a strong downward flow occurs in only one hemisphere on any one night.