Occurrence Rates of Spread‐F Echoes in Australasian VI Ionograms

Abstract

AbstractObservations of frequency spreading (FS) and range spreading (RS) in ionogram traces for Australian vertical incidence ionosondes for 1975 through 2013 have been analyzed to provide diurnal, seasonal, latitudinal, and solar‐cycle variations of the occurrence rates of FS and RS traces. For midlatitude ionosondes, both FS and RS rates were found to be negatively correlated with solar activity. FS was found to have maximum occurrence rates in June and July (JJ), with sub‐maximum in December and January during solar minimum years. The FS occurrence rates in JJ months were highest just after midnight. RS echoes had lower occurrence rates than FS echoes overall, but in general had seasonal and diurnal variations similar to those of FS echoes. The FS and RS occurrence rates of the midlatitude Townsville ionosonde are highly correlated those of the Akita ionosonde in the conjugate hemisphere. The equatorial station Vanimo and the subauroral latitude station Hobart have different FS and RS occurrence statistics when compared to the midlatitude ionosondes. Vanimo RS occurrence rates are consistent with bottomside spread‐F instabilities associated with the Rayleigh‐Taylor instability at sunset during solar maximum. During solar minimum Vanimo spread‐F rates are similar to midlatitude vertical incidence (VI) ionosondes. Station Hobart is at subauroral latitudes. It observes high rates of FS echoes throughout the night for all seasons. Hobart spread‐F traces are consistent with plasma instabilities associated with convection dynamics of the subauroral latitudes.