Nighttime enhancements in ionospheric electron content in the northern and southern hemispheres

Abstract

Nighttime enhancements in ionospheric electron content (IEC) observed at conjugate stations in the equatorial anomaly and mid-latitude regions under moderate to high solar activity conditions are studied. The observations at equatorial anomaly latitudes show that when an enhancement occurs in one hemisphere then an enhancement usually occurs in the conjugate hemisphere. The enhancement characteristics (frequency of occurrence, time of occurrence, amplitude, and duration) and their seasonal and solar activity variations are in agreement with the fact that the primary source of an enhancement is the post-sunset increase in the equatorial fountain. It is suggested that the north-south differences 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 enhancement occurs in one hemisphere then either no enhancement or only a weak enhancement occurs in the conjugate hemisphere; on no observed night does a strong enhancement occur in both hemispheres. The occurrence and other characteristics of the enhancements demonstrate that the primary 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.