High historical values of foE s—Reality or artefact?

Abstract

Very high values of foEs had sometimes been reported in the past. These values, as well as all other ionogram-scaled values, had been derived from ionograms under the assumption of the vertical reflection of ordinary mode of sounding radio waves. In the past it was impossible to check the validity of this assumption. However, modern digisondes determine clearly the oblique or extraordinary mode reflections. To test the assumption of the vertical reflection of ordinary mode for high values of foE s, seven summers (June 2004–August 2010) from a midlatitude station Pruhonice are chosen. All hourly values of foE s≥6.0 MHz are selected from this data set, altogether 282 values. In 90% of cases the “classical” way of evaluation of foE s provides values higher than “true” values from modern digisonde (average difference about 1 MHz). 38% of “classical” foE s are oblique reflections, which however do not occur in direction perpendicular to geomagnetic field. The occurrence of high values of foE s varies very much from year to year between 0 and 130 events per year. As for diurnal variation, a pronounced maximum occurs at ∼10:00LT and 16:00LT (secondary), and a minimum after midnight.