First off, I want to thank everyone who sent along a comment or observation of the aurora last night. It was thrilling to read how spectacular the display was and how far and wide it was seen. People wrote from Arkansas, Kansas, Maryland, Ontario and Tennessee and even Minnesota, where most of the state was under a heavy blanket of clouds. I stepped outside and checked the sky often during the night, but the clouds never budged. Just the same, your excitement was so palpable, it almost felt like being there.
According to the folks at the National Oceanic and Atmospheric Agency or NOAA, the geomagnetic storm that gave rise to the northern lights was a class G2 or moderate storm. Disturbances to Earth’s magnetic field caused by the sun are rated from G1 – minor with aurora visible in along the borders of northern U.S. states – to G5, which can seriously affect power grids, cause damage to satellite components, blank out radio communications and send auroras all the way down to Florida and southern Texas.
In last night’s storm, a stream of plasma shot connected to a solar flare or other activity on the sun belted a cloud of high-speed electrons and protons in the Earth’s direction. The average speed of one of these clouds or sprays is over 300 miles per second. When it reached the Earth’s vicinity yesterday afternoon, it strongly compressed the big magnetic bubble around the planet called the magnetosphere, squeezing billions of electrons straight into our atmosphere some 60 to 200 miles overhead.
When the electrons strike the oxygen and nitrogen atoms at that altitude, they excite them into a higher energy state. As the atoms return to their normal “ground” state, they emit light of different colors. The most commonly seen hue is green from excited oxygen. Because many auroras are faint, this color frequently appears pale white to the eye, but a time exposure with a camera will clearly reveal their green color. Red also stems from oxygen but it occurs higher up, which is why green rays are often topped by red. The strongest color emissions from nitrogen are in the deep violet end of the rainbow spectrum and invisible to the human eye. Another nitrogen excited state creates the red lower border to the aurora.
Once inside the bubble and on their way down, electrons follow the invisible lines of magnetic force in Earth’s magnetic field. They’re the same ones you see when you sprinkle iron filings around an ordinary magnet. Electrons spiraling around the many approximately parallel field lines in the polar regions create the multiple parallel rays that are so characteristic of the northern lights.
After a big auroral display, there will often be some “leftovers” the next night or two. I checked the NOAA space weather forecast this morning to see what might be in store. Although solar activity is presently low, lingering effects of the sun’s coronal mass ejection should continue through today and possibly into tonight. Keep an eye on the forecast and the Kp index, which is a gauge of changes in Earth’s magnetic field as measured by surface instruments. Make note of the rightmost bar in the graph at the Planetary Kp-index Monitor. This is the most recent plot of activity. If it’s green, meaning the index is below 4, no storm is in progress. A yellow bar at K = 4 indicates activity is picking up and a red bar (K = greater than 4) signals that’s it’s time to put on your coat and go out for a look!