The Aurora Borealis, or Northern Lights, occur when electrons collide with the upper reaches of Earth’s atmosphere, according to the NOAA Space Weather Prediction Center. The agency uses scales and indexes to communicate space weather conditions and the approximate locations where the aurora might be seen.
How the aurora forms
The lights begin when electrons are energized in the night side of Earth’s magnetosphere. These accelerated electrons follow the planet’s magnetic field down to the Polar Regions. There, they collide with oxygen and nitrogen atoms and molecules in the upper atmosphere.
According to the NOAA Space Weather Prediction Center, these collisions transfer energy to the atmosphere and excite the atoms to higher energy states. When the atoms relax back to lower energy states, they release light. This process works similarly to how a neon light operates.
The aurora typically forms between 80 and 500 kilometers above the surface of Earth. Earth’s magnetic field guides the electrons so that the aurora forms two ovals. These ovals are approximately centered at the magnetic poles.
Shapes and stages of the lights
The aurora appears in several different shapes. Often, it looks like a curtain made of folds of cloth, created by many tall rays. In the evening, these tall columns can create bands that reach completely across the visible sky.
Near midnight, the arcs often begin to twist and sway as if wind were blowing them. The arcs may then expand to fill the sky, moving rapidly and becoming very bright. The NOAA Space Weather Prediction Center calls this peak of activity an auroral substorm.
In the early morning, the aurora can take on a diffuse, cloud-like appearance. These patches often blink on and off repeatedly for hours. They finally disappear as the sun rises in the east.
Measuring storms and auroral locations
The NOAA Space Weather Prediction Center uses the NOAA Space Weather Scales to communicate space conditions to the public. These scales rate geomagnetic storms, solar radiation storms, and radio blackouts on numbered levels. Geomagnetic storms, which affect the aurora, are rated from G1 for minor events to G5 for extreme events.
The level of geomagnetic activity is driven by solar activity and is indicated by a measure called the Kp index. The Kp index ranges from zero to nine. At a Kp index of zero, the equatorward edge of the auroral oval is at approximately 66 degrees magnetic latitude.
For each level the Kp index increases, the aurora moves about two degrees toward the equator. When the Kp index is in the range of six to seven, the aurora might be visible from the northern edge of the United States. During an extreme G5 storm with a Kp of nine, the aurora can be seen as far south as Florida and southern Texas.
Conditions for viewing
To observe the aurora, skies must be clear, free of clouds, and dark. People must get away from city lights, and a full moon will diminish the apparent brightness of the display. During summer months at high latitudes, the midnight sun prevents observers from seeing the aurora.
The best time of night to see the most active aurora is usually within an hour or two of midnight. This typically falls between 10 p.m. and 2 a.m. local time. When geomagnetic activity increases, these active hours expand into the evening and morning.
The ideal times of year to view the displays occur close to the spring and autumn equinoxes. The solar wind interacts with Earth’s magnetosphere in a way that tends to produce larger geomagnetic storms near the equinoxes. However, the NOAA Space Weather Prediction Center notes that the number of dark hours decreases rapidly near the spring equinox.