Abstract :
A brief review of sources of information on the occurrence and nature of sporadic-E (or Es) ionisation is followed by a survey of the principal features encountered at different latitudes. Successive former efforts to model empirically the temporal and locational variations of the vertical-incidence characteristics fEs, foEs and fbEs are described. Numerical representations of median Es values adopted by the Radiocommunication Sector of the International Telecommunication Union (ITU-R) are complimented with specifications of the statistical day-to-day variations. Alternative maps of low probability Es characteristics, for use in the estimation of the likelihood of interference to VHF radio circuits, are discussed, and current approaches to estimating sporadic-E reflection and obscuration losses are addressed. Potential ways of improving existing Es maps, particularly using recently available information from low-Earth orbiting occulation probes, are outlined. Finally, the way ahead for IRI in incorporating an Es model into the electron-density height profile is examined, and suggestions are offered for the approach to follow.
