High historical values of foEs-Reality or artefact?

► We compare high values of foEs evaluated by digisonde and in classical ionosonde way. ► 90% of historical high foEs seem to be overestimated, in average by 1 MHz. ► 38% of “classically” evaluated high foEs appear to be oblique reflections. ► Annual occurrence frequency of high foEs varies very much in 2004-2010.