Don’t Fear the Minimum
By Mike Liemohn
I think that we can officially call Solar Cycle 24, the one we’re in now, a wimpy cycle. Only one monthly average of the sunspot number went above 100. Normally, meaning over the last few solar cycles, 30-40 months cross this somewhat arbitrarily chosen barrier into the realm of “highly active Sun.” See for yourself; here is a plot of the monthly-averaged sunspot progression over the last 15 years, from NOAA’s Space Weather Prediction Center:
While we have had a few very good space storms over the last five years, including two large ones on St. Patrick’s Days (in 2013 and 2015), the number of intense storms is about a factor of five lower than what it was at this point in Solar Cycle 23.
Still, we have much reason to hope for excellent auroral viewing over the next few years. This is because we are progressing into the declining phase of the solar cycle. It sounds strange, but it’s true: the declining phase is the best time for regular auroral displays. Again, from the NOAA SWPC, here is the monthly-averaged progression of the Ap index:
Ap is a compilation of magnetic perturbations from high-latitude monitoring stations. Comparing it with the sunspot progression above, you can clearly see that Ap peaks after the sunspot number. This is because sunspots, and their large energy releases into the solar system known as coronal mass ejections, are not the only thing that cause disturbed space weather here at Earth.
There is another primary contributor to activity in our space environment, a phenomenon at the Sun called a high speed stream. In addition to photons, the Sun also emits a magnetized, electrified, supersonic gas called the solar wind. There are two types of solar wind, fast and slow. The slow solar wind is moving at about 400 kilometers per second, which means it can zip past the diameter of the Earth in 2 minutes. The fast solar wind, however, typically goes over 600 kilometers per second. The fast wind comes from a particular feature on the Sun called a coronal hole, a dark spot in an image of the Sun viewed in ultraviolet light.
In the right image from the Solar Dynamics Observatory, you can see a coronal hole near the southern pole of the Sun. That’s where they usually are, emitting fast wind away from the ecliptic plane of the solar system, where all of the planets are orbiting. During the declining phase of the solar cycle, however, it is very common for these coronal holes to migrate to lower latitudes on the Sun, sometimes coming close enough to the equator to spray the planets with fast solar wind. This is called a high speed stream.
A high speed stream usually rams into the slow solar wind ahead of it, creating a snowplow effect that builds up a plug of particles and magnetic field that can cause a space storm here at Earth. The high speed stream itself, however, is also a source of activity for our space environment, constantly shaking our magnetic bubble just enough to trigger aurora displays for several days in a row.
The best part about equatorial coronal holes and high speed streams is that they usually last for months. It takes the Sun about a month to rotate once, so the same coronal hole will often come around again and shoot out another high speed stream right at us. It’s the closest thing we have to long-term predictability in forecasting space weather.
So, don’t fear the minimum that’s approaching. Embrace it and look forward to many great nights of brilliant light displays in the sky. We’re just entering the best part of the solar cycle for auroral enthusiasts.
Mike Liemohn is part of the extended Aurorasaurus scientist network and a professor of Space Science at the University of Michigan. His research uses simulations to study electrically charged particles moving in planetary space environments, especially the inner planets of Earth, Mars, and Venus.