Well, the 'Northern Lights' (Aurora Borealis) are always there. But daylight makes them impossible to see. Therefore, winter is the best time to catch them – around September through April.
While the Interior is probably the best place to view them, they can sometimes be visible even in the "Lower 48". The further out of town you get, the better the view because there are no city lights to reflect up into the sky. Fairbanks is a favorite tourist destination in the winter for aurora viewing.
Because of the long summer days in Alaska, it is almost impossible to see them during the summer months. In the winter, the further north you go the better the view. This reverses in the summer months due to the near constant sunlight. In the summer, any hope of catching the lights is confined to southern Alaska and the lower 48 on days when the auroral index is high.
How high is the Aurora?
The bottom of the Aurora is an average of 60 miles up.