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Northern lights in - 40°C, FINLAND/RIISITUNTURI NATIONAL PARK, LAPLAND, This could be any mountain spruce forest north of the Polar circle. The Northern lights, or Aurora borealis is a phenomenon, originating in the uppermost part of the atmosphere and caused by solar wind particles that are speeded up along the Earth’s magnetic field lines. It happens all year round, but is only visible when nights are dark enough, and most frequently during the equinoxes, in September-October and March-April. Riisitunturi National Park covers 77 Sq km and is one of 25,000 protected sites under the EU Natura 2000 umbrella.