Weatherz School: Northern lights

The night sky is a place of wonders. Of all that it has to offer, there’s nothing quite as breathtaking as the aurora. The cause of this incredible phenomenon is the interaction between tiny particles in the far reaches of our atmosphere.

The two key players behind the aurora are the Earth’s magnetic field and the solar wind. Charged protons and electrons are emitted from the sun. As these particles approach the earth, they get course corrected by the magnetic field and directed to the points on the planet where the field is the weakest, which is the poles.

Protons cause a faint aurora that we can’t quite see, but electrons are the special ingredient. When the electrons enter our atmosphere, they collide with molecules. In these collisions, the electrons excite the molecules and give off light, giving us a stunning aurora.

The color we see depends on height and the type of molecules involved in the collision, either nitrogen or oxygen. Green auroras are the most common, but the full spectrum ranges from pink auroras within 100 kilometers of the surface to red auroras more than 200 kilometers above the surface.

The two main factors in viewing the aurora are geomagnetic activity and your location. The Planetary K-index (Kp-index) rates geomagnetic activity on a scale of zero to nine.  A large swath of Canada can view the aurora on days with low geomagnetic activity. The Northland needs a Kp-index of 5 for a shot at the northern lights, and that requires a minor geomagnetic storm.

The aurora reaches Chicago with a strong geomagnetic storm of Kp7, and in extreme cases, St. Louis can see the northern lights. Geomagnetic storms can happen year round, but winter is the best time for viewing the aurora simply because night is longer.

Even if a Kp5 storm is expected, we aren’t guaranteed a light show. You have to get away from the light pollution of a city, we need the weather to cooperate, and even a full moon can interfere with your view of the aurora. When the stars align, it’s a sight to behold.