Diurnal and seasonal variation of spreadFin the Arctic

The diurnal and seasonal variation in the occurrence of spread F is investigated for North American stations by means of hourly ionosonde data for the IGY period. Around the magnetic pole only very small diurnal and seasonal variations are found and explained by the existence of a permanent maximum over the Canadian archipelago (Foxe Basin). To the north of this area small diurnal variations exist during spring and fall months only, and a seasonal variation is noticed. The general level of spread-F occurrence, however, remains so high, that spread-F conditions exist regularly at local noon. For the rest of the Arctic, pronounced diurnal and seasonal variations exist; they are explained by a traveling maximum that moves along the auroral zone.