High historical values of foEs: Reality or artefact?

• Published in: Journal of atmospheric and solar-terrestrial physics Vol. 74; pp. 51 - 54
• Main Authors: LASTOVICKA, J, BOSKA, J, BURESOVA, D, KOUBA, D
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier 2012
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Physics of the ionosphere; Physics of the magnetosphere; Sporadic-E layer; Mid latitude; diurnal variations; Radio wave; Sporadic E; Ionospheric measurements; Summer; Artefact; geomagnetic field; data bases; Radio sounding; Ionosphere
• ISSN: 1364-6826; 1879-1824
• DOI: 10.1016/j.jastp.2011.10.008

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 foEs, seven summers (June 2004-August 2010) from a midlatitude station Pruhonice are chosen. All hourly values of foEs greater than or equal to 6.0MHz are selected from this data set, altogether 282 values. In 90% of cases the "classical" way of evaluation of foEs provides values higher than "true" values from modern digisonde (average difference about 1MHz). 38% of "classical" foEs are oblique reflections, which however do not occur in direction perpendicular to geomagnetic field. The occurrence of high values of foEs varies very much from year to year between 0 and 130 events per year. As for diurnal variation, a pronounced maximum occurs at a arrow right 410:00LT and 16:00LT (secondary), and a minimum after midnight.