Explainer: Aurora Borealis (Northern Lights)

Aurora Borealis seen from Cut Bank on October 12, 2021
Aurora Borealis
Aurora Borealis in Montana (March 30, 2022)
Aurora Borealis photo by Blaise Poitra
Posted at 8:37 AM, Feb 17, 2023
and last updated 2023-02-17 10:37:04-05

GREAT FALLS — Some people in Montana were fortunate to see the Aurora Borealis on Tuesday and into Wednesday - but what causes this mesmerizing natural phenomenon?

It starts with the sun. The Earth is surrounded by magnetic fields. During solar storms, large masses of charged particles, or protons and electrons, are forced towards the Earth at speeds of 250 to 500 miles per second by the solar wind.

Generally, the particles are directed towards the area with the greatest magnetic activity, the poles. Upon reach the North or South Pole, these particles interact with atmospheric gases, mostly oxygen and nitrogen. The collisions between the two cause heat, which is released in the form of light.

The color visible depends on the height of the collision. Higher altitude oxygen created a red hue, while green hues are indicative of lower altitude oxygen molecules. Pink and blue hues correlate to nitrogen molecules, the most abundant gas in the atmosphere.

Aurora Borealis

In order for the Northern Lights to be visible at mid-latitudes such as Montana, the solar storm needs to be particularly intense. Brightness is directly related to the solar storm's strength. The planetary K index (Kp) is the most accurate scale for geomagnetic activity. The Kp index goes from 1 (dim, visible only near the poles) to 9 (very bright, visible overhead in northern U.S. states). A Kp of 5 is generally seen as the threshold for a solar storm and that is when the Northern Lights are usually visible near the Canadian border.

Prime viewing for the Aurora Borealis is during the wintertime months due to the lack of daylight hours, however it can technically be viewed anytime of the year. A full moon or city lights can prohibit the brightness of the Northern Lights.

Two good resources for knowing when the Aurora Borealis might be visible in our area are the Space Weather Prediction Center and Soft Serve News, which posts frequent updates to let people know how likely it is that the Northern Lights may be visible.