Astro Bob: Monday night's northern lights 'incredible' -- if you could see them
That was the reaction of Shawn Malone of Marquette, Mich., to the show put on by the aurora borealis late Monday into early Tuesday -- a show of brilliant red and green arcs, spires and rays that was visible over much of the country, as far south as Arkansas and Tennessee.
The Northland and most of Minnesota were 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, Astro Bob readers' excitement was so palpable, it almost felt like being there. See some of their photos posted on the Astro Bob blog at astrobob.areavoices.com.
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 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 Monday 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 Monday 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, the most exciting color in auroras and not often seen, was widespread in Monday night's display. 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. Check the Astro Bob blog for updates and more information about how to gauge the chances that the northern lights will appear.
Bob King, photo editor of the Duluth News Tribune, is an avid amateur astronomer. He can be reached at firstname.lastname@example.org.