The first official "report" of the event was published today at LFV's (the ATC operator) website:
LFV levererade full kapacitet 90 minuter efter radarbortfall
TL;DR: Cause: Solar flares combined with (un)favourable sun angle w.r.t. the radar stations (near sunset). Radar screens were back to normal after 45 minutes, full airspace capacity restored after 45 more minutes.
Google translate of webpage:
The investigation that LFV has done for radar interference in November 2015 shows that the reason was the radio radiation associated with solar flares. Radar shortfall came to affect parts of the air traffic in Sweden.
It was on Wednesday, November 4, 2015 at 16 o'clock that one of the control centers in Stockholm and Malmö were the first indications that LFV's radar did not deliver correct data to the flight line. The cause was quickly identified and action taken.
- When the disturbance occurred was the air traffic controllers could not use the information on the radar screens of the alternative ways to lead aircraft, says LFV's technical director Ulf Thibblin.
For LFV air safety a top priority. If the aircraft for various reasons, unfavorable weather or technical problems, can not be guaranteed reduced airspace capacity.
It has been discussed in recent days that the radar interference were not related to space weather, but that it instead was due to a cyber attack against Sweden.
- In our investigation we had early on with it as a hypothesis. There was nothing in our radardata- or Internet traffic logs to support or confirm a possible cyber attack. Also, we had the relationship in time with space weather, plus there were a few more technical reasons which excluded a cyber attack, says Ulf Thibblin.
After the radars had rebooted had LFV air traffic controllers correct radar presentation 45 minutes after the disturbance, and after another 45 minutes it was at full capacity in the airspace.
LFV radars have been affected in a similar way two times in the last 17 years - 1999 and 2003. The disturbance then and now occurred at sunrise or decline when the sun's angle is straight into, or virtually straight into the radar facility.
According to experts in the field it is possible to predict a solar flare but not its consequences, such as radio frequency radiation. As the shock travels at the speed of light from the sun to the earth, there is no opportunity to receive a notice in confirmed cases.
CAA radar facilities around Sweden continuously upgraded and will gradually be supplemented by a new type of monitoring, wide area multilateration, which means that LFV increases its robustness significantly.