In panel (a), the red line represents the hourly values of foF2, recorded at Juliusruh ionospheric station from 12 to 19 December 1987. The green line represents the foF2 running median here considered as background value of the ionospheric parameter. In panel (b) the same parameters, recorded by the Rome ionospheric station, are reported, considering the same time period. In panel (c) is reported the variation in percentage of the foF2 for the two ionospheric stations, for the considered period.