Aurora Alert Tonight And Tuesday Sept. 10-11

(Go to the end of the blog for updates.)

Cross your fingers. The space weather forecast looks promising with a moderate G2 geomagnetic storm expected to develop overnight. Northern lights could surge as far south as the the upper Midwest, New York, Idaho and Washington. The action is expected to begin early this evening with minor storming (G1) at nightfall blooming into a more extensive display between 10 p.m. and 1 a.m. Central Time.

Minor storms are forecast for Tuesday night too. In my experience G1 auroras are modest, appear low in the northern sky and need dark skies to really appreciate. G2s are brighter and livelier and can fill the northern sky. Our last good aurora occurred about two weeks ago at full moon — not exactly optimum conditions! Tonight, the moon is just past new, making this about as optimum as you can get.

Solar activity has been very low with only a few sunspots reported in the past month, but even during these “quiet” times, features such as coronal holes can spark an aurora. A coronal hole is an opening in the sun’s magnetic field that allows strong gusts of charged particles  — electrons and protons — to stream freely from the sun at speeds around a million miles an hour (500 km/sec). These can plug into Earth’s magnetic domain, called the magnetosphere, and stream rapidly down into the polar region to fire up auroras.

We’ve got that going for us tonight plus something called a CIR or co-rotating interactive region. Sorry for all the technical terms, but they’re just so useful for distinguishing one thing from another. The speed of the particle wind from the sun constantly varies. CIRs are regions where a faster-moving wind from the sun slams into an earlier, slower flow. Material can pile up in a CIR to increase the strength of the magnetic field bound up in the wind. When it arrives at Earth, the CIR buffet the magnetosphere, making auroras possible.

I’ll be watching tonight and try to get the word out early if the northern lights show. If you have a digital SLR camera, you can take pictures of the aurora. It’s not difficult. Here’s a quick guide:

1. Use a wide-angle lens — 16mm to 35mm is ideal — and set the lens to M or manual focus.
2. Attach your camera to a tripod.
3. Set the camera exposure mode to M (manual), the lens to its widest aperture (f2.8-f/4), ISO to 800 or 1600 and the exposure to 20 seconds
4. Center a bright star in the field of view, click the Live View button and magnify the star image 5x or 10x then focus it down to a point of light.
5. Press live view again to turn it off and then compose a scene with the aurora.
6. Press the shutter button and take the picture.
7. Check the back of the LCD viewfinder screen to see if your exposure looks good. If dark, increase the exposure time. If too bright, decrease it.

** UPDATE 7:30 p.m. Central Time: Earth’s geomagnetic field has been touched by the storm since late morning, but so far, it’s been a minor one with a Kp index of 5. In the past couple hours, activity has declined a bit. Where it’s dark, observers in the northern regions of the northern border states might see — at least for the moment — a glowing arc low in the northern sky. Still twilight here and partly cloudy.

** UPDATE 9:30 p.m.: Mostly clear here but no aurora visible.

** UPDATE 11 p.m.: The aurora is out! Not a big deal though. I’m seeing a green glow (arc) partially hidden by clouds a fist (10°) high in the northern sky. No rays visible. See photo.