Aurora may be in the cards Sunday, Sept. 26, as streams of high-speed material from the sun arrive and jangle Earth's magnetic field. A minor storm will kick off the evening and develop into a moderate G2 storm between about 10 p.m. and 1 a.m. G2 events often feature bright arcs and rays covering at least half of the northern sky across the Upper Midwest. The moon shouldn't be too much of a bother either. It rises around 9:30-10 p.m. near last-quarter phase, not so bright that it would extinguish the aurora.
But first, a caution. Space weather forecasts like weather forecasts are subject to change. The past couple hoped-for displays of northern lights either never materialized or produced only anemic glows. That doesn't mean tonight's show won't happen, but remember it's always a possibility. Clear skies are predicted for my location this evening, so I'll be watching the northern horizon closely and posting updates on my Facebook page and Twitter.
You can always check the aurora's status before you hop in the car and drive to a dark-sky location. Go to the Aurora — 30-minute forecast to see the extent of the auroral oval, a doughnut-like ring of light centered over Earth's north magnetic pole (there's a southern version, too). If the oval lights up brilliant green and red, and its southern edge touches or spills over where you live, then there's a very good chance you'll see northern lights.
You can also check out NOAA's real-time solar wind site. There, you can watch data sent by NASA's Deep Space Climate Observatory (DSCOVR), a satellite stationed about a million miles (1.5 million kilometers) ahead of the Earth. It samples the incoming solar wind and provides about 90 minutes of notice before that cloud impacts the planet's magnetic field. If the wind is strong and has the right magnetic direction it can link into Earth's field and generate the aurora.
Without getting into the messy details, just look at the topmost graph labeled "Bz." If that red squiggle drops below the center black line to -5, there's a fair chance of seeing a minor aurora. If it's at -10, those chances are much better. And if plummets to -20, run outside right now!
Keep in mind that the graph is very close to what the DSCOVR senses in real time. If the squiggle hovers near the centerline and then suddenly drops to -10, you'll need to wait about an hour before the material that caused the plunge manifests itself in the sky as aurora. That's about how long it takes for the cloud to arrive at the Earth.
Here's what you need to do to prepare for tonight. First, assume the aurora will be a no-show. But don't let that stop you from making the attempt to see it. That way if it does appear, you'll be delighted. You don't need a perfect sky, just one where it's dark in the north direction. Watch the indicators I described above and check the most recent forecast, too. If you see a "6" near the top of the Sept. 27 column, that indicates a moderate G2 storm. A "5" is a minor storm.
If all looks good, head out and get set up for the show. Bring a camera and tripod. Set the lens wide open to f/2.8 (or 3.5, 4, whatever is the least number on the lens barrel), the ISO to 1600 and exposure time to 20 seconds. Remember to put both the lens and camera in manual (M) mode, then use your camera's live-view feature to focus on a bright star, so your pictures will be sharp. Check the back viewing screen after a few exposures and adjust the time and ISO as needed for best results.
Good luck! Check back this evening for updates on my Astro Bob's Astronomy for Everyone Facebook page.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.