The northern lights may be visible from parts of the U.S. on Tuesday.
According to the National Oceanic and Atmospheric Administration (NOAA), the arrival of a so-called “corotating interaction region” could trigger “G2”-class geomagnetic storms.
Such storms are known to bring the aurora borealis as low as Idaho and New York—although current NOAA predictions are more conservative.
The agency’s forecasts suggested that the phenomena may appear directly over Alaska, Idaho, Minnesota, Montana, North Dakota, Washington and Wisconsin, as well as be visible above the northern horizon in parts of Maine and South Dakota.
What Causes the Northern Lights?
The northern (and southern) lights form when particles from the solar wind excite atoms in the Earth’s upper atmosphere, causing them to glow.
Appearing as curtains of light that trace along the geomagnetic field lines, the aurorae vary in color depending on the particular species of atoms being excited.
As Earth’s atmospheric composition is dominated by oxygen and nitrogen, aurorae tend to give off a green hue (from the former) or a blue–pink–purple light (from the latter.)
The geomagnetic storms forecast for this week are the result of a corotating interaction region—a transition zone produced by the meeting of fast and slow-moving streams in the solar wind.
The interaction of the two streams creates a compression region in the solar wind that appears to rotate in step with the sun, hence the name.
The fast stream causing this week’s corotating interaction region is being powered by a massive hole in the southern hemisphere of the sun’s corona, which is presently facing toward Earth.
How To See the Aurora
To have the best chance of spotting the northern lights Tuesday night—and in general—NOAA recommends looking skyward either just after sunset, or just before sunrise.
“The aurora does not need to be directly overhead but can be observed from as much as a 1,000 km [621 miles] away when the aurora is bright and if conditions are right,” the agency explains on its website.
The northern lights are easiest to spot in clear weather and in places where the skies are darker—away from sources of light pollution like cities.
At this time of year, aurora-chasers are recommended to dress up warmly, in layers, to fend off the night chill.
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