Prelude to the aurora

Note: aurora images from the next night (Tuesday) are in the previous post.

Monday night, 07 October, was clear, but the sky was again not very dark for Creede, with sqml=21.05–21.1.  I hate to waste a clear night, so my intention was simple: take one shot of the southern sky featuring Saturn, and then pack it in.  Here is that view of the southern sky:

Sony A7iii + 20mm FE f/1.8 G lens + Hoya Softon filter.

Nothing remarkable there.  Saturn is featured (the bright star just right of center) and some faint green airglow is evident near the horizon.  A full-size crop of the central region shows both Saturn and Neptune:

Mission accomplished.  Then out of habit I turned around and took what I thought would be a parting shot of the northern sky.  Wow!

Sony A7iii + 20mm FE f/1.8 G lens.  No filter. 10:41 pm MDT.

That broad red band arching up from right to left is called a Stable Auroral Red (SAR) arc.  This phenomenon is not actually auroral.  It is a misnomer dating back to the discovery in 1956, before the excitation mechanism was understood. It is caused by heat exchange between the ionosphere and Earth's ring current system (Van Allen radiation belts)  The diffuse red glow near the northern horizon, however, is auroral (i.e., caused by high-energy charged particles from the sun).

One important aspect of this image is that neither of these two features was visible to the unaided eye.   However, a long camera exposure (30 seconds in this case) easily picks up the red glow. 

Here is a more northerly view:

Same camera and lens.  Softon filter.

The SAR arc can be seen passing through Perseus and just over the top of the Little Dipper.  Polaris is the star that is just left of center.

SAR arc high in the northern sky.

After processing these images I decided to head back out with a wider-angle lens, and obtained the auroral images featured in the previous post.