The Aurora Borealis is active all year but can only be seen when the Alaskan night sky is dark from August to April. The Aurora is a natural light display caused by charged particles from the Sun interacting with the Earth's Atmosphere. When these particles collide with oxygen, yellows and greens are produced; with nitrogen they produce reds, violets and rarely blues. The colors are also affected by altitude- green/red around 150 miles; purple/violet at 60 miles.
As you can tell, this wonder photo captures the rare blues and violets mentioned above.