According to Luftfartsverket, solar flares disturbed the radar antennas which led to the cancellation of flights as the planes could not be seen on radar screens.
Solstorm stoppade flygtrafiken ? radarbevakningen kollapsade - DN.SE
Scientists are mildly sceptical since the flares didn't even qualify for a Class 1 on the 5-grade scale.
Forskare: solstorm dålig förklaring till radarstopp - DN.SE
Only southern Sweden was affected, including Arlanda and Landvetter.