Dynamic boundaries of the ionosphere variability Original Research Article

It has been shown that the conventional threshold of ±20% departures from monthly median cannot serve for reliably distinguishing quiet and disturbed ionospheric conditions at different latitudes/time-of-day/season/level of solar activity. After a 3 h filtering of daily-hourly foF2 critical frequency, for each 3 h UT bin new upper and lower variability boundaries are introduced, based on the extreme foF2 values normalized to the monthly median similar to assessments of warming-cooling of air temperature in meteorology. Application of so defined boundaries is made to long-term observations at 56 ionospheric stations world-wide for the period of 1942 to 1999 comprising in total more than 13,000,000 hourly foF2 values.