Huge light source visible below Salo, just right of Kirjakkala (I think). What might it be?

Saw this strong light coming from a really wide area actually while on a plane flying towards Helsinki from Stockholm. So the picture is facing North and that should be Salo if I’m correct. It looked like it was made of multiple big rectangles. (A bit visible in the flare shape of the flare).

Clearly visible that the light is way stronger than the city lights or normal street lights.