During the Northern Hemisphere’s winter solstice, the phenomenon within the Arctic Circle is polar night.
Polar night occurs when the Sun remains below the horizon for at least 24 hours. The December solstice is the point at which the Northern Hemisphere is tilted farthest away from the Sun, so areas inside the Arctic Circle can experience a period without direct sunrise.
The Arctic Circle lies at roughly 66°34′ north, though its precise position shifts slightly over time. At the circle itself, polar night is theoretically associated with the winter solstice; farther north, the Sun stays below the horizon for many more days or even weeks. Murmansk, Russia, for example, experiences about 40 days of polar night each winter.
Polar night does not necessarily mean complete darkness every hour. Twilight can still brighten the sky, and atmospheric refraction may allow a small portion of the solar disk to appear near the boundary. Midnight sun is the opposite phenomenon, occurring in the Arctic during the Northern Hemisphere’s summer.