Minimalist Sun May Stoke Aurora Tonight

Today’s sun is blank like it’s been for much of 2018 as the solar cycle heads toward a 2019-2020 minimum. The fine texture you see is caused by solar granules which are the tops of rising cells of hot plasma. A typical granule is about 1,500 km across and lasts around 10 minutes before cooling and sinking back into the sun. NASA / SDO

It’s been a long time, but skywatchers in the northern U.S. states have a shot at seeing the aurora beginning at nightfall tonight through about 1 a.m. (Central Time). We’ve been in a dry spell for northern lights for some time now. That’s not surprising considering that the sun is slouching toward the next solar minimum expected in 2019-2020.

Graph showing the last three solar cycle plotting sunspot numbers. The double peak of the current cycle is shown. You can see there’s been a general decline in solar activity over the past 25 years or so. NASA

Solar activity bounces back and forth between high and low during a cycle that lasts about 11 years. The last maximum occurred in spring 2014 and was one of the wimpiest on record. In fact, it was the least active maximum since 1906. A key marker of solar activity is the number of sunspots visible on the sun’s surface, called the photosphere. That number drops as minimum approaches. This year, according to Spaceweather.com, the sun’s been spotless 112 days. Just this weekend, we ended 24 days in a row of zero spots when a small sunspot finally grew into view … then dissolved a day later!

You can see several active coronal holes in this photo taken in far ultraviolet light by NASA’s Solar Dynamics Observatory on July 22. Holes that cross the center of the disk are more likely to affect Earth. NASA / SDO

But just because there are no spots doesn’t mean the sun has other ways of tossing its aurora-inducing weight about. Our star is surrounded by a magnetic field that keeps hot electrons and protons rising from the surface bound to the sun like the thread that binds a button to a sweater. Except in certain areas called coronal holes. Here, the magnetic field opens up and allows particles to flow freely into space at high speed. Coronal holes aren’t fixed features but change shape, size and position over time.

Tonight’s aurora chances — a G1 minor geomagnetic storm is forecast — stem from just such a hole that’s been aimed directly toward Earth the past couple days. Let’s hope it’s enough to “strike the match” and get a storm going. Before you get too excited, there’s a mitigating factor here, the moon. It’s big and gibbous-y and sets after 2:30 a.m. and may easily wash out this spare opportunity to see that welcome green glow in the northern sky. Still, it’s worth taking a look before you go to bed. Should the storm grow, we have an hour or so of dark skies before dawn tomorrow to watch, too.