Mid‐latitude spread F occurrence related to geomagnetic activity at preferred local times

Abstract

Superposed‐epoch methods have been used to investigate the level of spread F occurrence relative to high geomagnetic activity. The controls used were determined from high values of the AE index during two 6‐hour periods representing early morning intervals at two high midlatitude stations separated by 10 hours of local time. Delayed spread F occurrences at these stations (Lannion in France and Canberra in Australia) were considered using these controls. The results show that spread F occurrence is enhanced (delayed, however, sometimes by as much as 3 days) by using controls for the early morning periods of the station being investigated, but not when controls for the early morning periods of the other station are used. It is suggested that in examining relationships between other geophysical phenomena and geomagnetic activity, it may be profitable to consider geomagnetic activity at specific local times rather than employing the use of a daily index such as ΣKp.