The K or Kp index is a measure of magnetic activity in Earth’s magnetic field and a good indicator of aurora. The farthest right red bar was the reading at 7 p.m. (CDT); the yellow bar after that showed a drop starting around 10 p.m. Credit: NOAA

If the sky had only been dark around 4 or 5 o’clock this afternoon, I think we would have seen a fine aurora here in the U.S.  Darkness finally arrived around 11, right about the time the northern lights started to wane.

From north of Duluth, we saw a low greenish arc at 11:15 p.m. followed at midnight by a few faint rays. Soon after, the moon came up and what was left of the lights was too faint to see.

The estimated 3-hour Kp index at right gives you an idea of the aurora potential through 10 p.m. July 9. I’ve included descriptions for the different levels. Keep in mind these are not hard and fast but based on my own observations over the years. Sometime after midnight, the levels dropped back down to 2.

This was the peak of the display from Duluth at around 11:50 p.m. Monday night. A low greenish arc and a few faint rays showed. The camera also picked up the flashing light of an airplane. Photo: Bob King

We have another shot at seeing auroras tonight (July 10) before entering a quiet period of about two days, according to the space weather forecast. Even if the northern lights show meekly, the sight of the Milky Way is worth any drive away from the city. It’s not only magnificent to the naked eye, but binoculars resolve so many clusters, nebulas and starry clumps within the hazy band, you’ll be tempted to stay up all night. We’ll have more on how to find some of these gems very soon.