Guest post by Vincent Ledvina
The doldrums of solar minimum are a difficult time for many. While high-latitude aurora chasers still experience the beauty of the aurora on most nights, skywatchers at mid-latitudes like myself rely on geomagnetic storms to see the ephemeral lights. During solar minimum these geomagnetic storms are far less frequent and powerful than during solar maximum, and huge aurora displays are few and far between. But if you have been wondering when this solar minimum will end, and about the upcoming solar cycle 25, then here is some exciting news. New research by R.J. Leamon et al. (2020) has revealed that the end of solar cycle 24 may be right around the corner and the upcoming solar cycle 25 may be among the strongest we’ve ever seen. Let’s explore this fascinating new research and what it means for the aurora chasing community.
Before we dive into the research, let’s cover a bit of solar physics background. While the Sun has an 11-year sunspot cycle, did you know there is also a 22-year magnetic cycle? The Sun’s magnetic field completely flips every 22 years, and scientists are able to observe bands of magnetic activity on the Sun that can be used to study these cycles. Studying so-called “bright points” (BP) on the Sun allows scientists to study these bands regularly. BP’s are small-scale magnetic loops that appear as brighter areas in extreme-ultraviolet (EUV) images. Tracking them as they migrate from the Sun’s poles to the equator reveals the Sun’s magnetic activity bands.
Terminator events and the Birth of Solar Cycles
When magnetic bands reach the equator, they cancel each other out in what has been dubbed a “terminator” by Scott McIntosh and his colleagues. This signals the death of one solar cycle and the start of another, and when a terminator event occurs, a sort of “solar tsunami” is triggered. This solar tsunami rushes from the equator to the Sun’s poles, and when it reaches around 30 degrees in latitude, it causes changes in the buoyancy of the Sun’s atmosphere which help generate sunspots. This tsunami, when it occurs, causes rapid sunspot growth, often within approximately one 27-day solar rotation of the terminator occurring. The scientists even predict at least one X-class flare within two solar rotations (about 54 days) of the terminator.
Leamon and his colleagues plotted the magnetic activity bands using large datasets of bright points and were able to directly see how well terminator events correlated with strong upticks in solar activity from a new cycle. In the first figure, the magnetic bands are shown as the green, red, blue, and purple stripes (bottom-most panel). The blue magnetic bands are what we’re concerned with, because when both those magnetic bands converge and terminate, we should expect to see the start of solar cycle 25.
But wait, there’s more! Chapman et al. (2020) propose the adoption of a “sun clock” that shows how solar and geomagnetic activity time with terminator events.
What we can see from these two figures is that solar activity may be very well organized by identifying these so-called “terminator” events. In the first figure, we see how proxies for solar activity rapidly increase after the terminator onset, observing the last two solar cycles. In the second figure, we see how solar flares and geomagnetic activity also increase sharply after terminators, looking at data from the past 12 solar cycles.
Solar Cycle 25 is coming!
Scientists predict this terminator event will occur between mid-February and September 2020, and many are certain we are getting close. In the transition between solar cycles, sunspots belonging to the next solar cycle (in this case, solar cycle 25) will appear with opposite magnetic polarity and at higher latitudes than old cycle sunspots. Over the past months, scientists have noticed an increasing number of these “new cycle” sunspots, indicating that solar cycle 25 is coming. These new cycle sunspots always occur during solar minimums when magnetic cycles “mix,” but after the terminator, we should start seeing a large increase in the number of these new sunspots. If McIntosh and his colleagues are correct, we should start seeing a ramp up in solar activity sometime later this year.
Solar Cycle 25—Stronger Than Predicted?
But besides predicting the timing of the next solar cycle, the same authors also propose the next solar cycle 25 will be one of the strongest on record. By analyzing the time between terminator events, they found a high correlation between short terminator time-separations and high-activity subsequent solar cycles. Plotting the sunspot number maximum vs. the time separation of the terminators (seen below), it is possible to draw a line of best fit to more clearly see this correlation. Using the terminator prediction of 2020 and its time separation between it and the last terminator event, it is possible to estimate the strength of solar cycle 25. The separation between these two terminators was very short, meaning that solar cycle 25 will be very strong— possibly the strongest we have seen in decades. Averaged over centuries, the solar cycle length is about 11 years, but there can be interesting variations! This prediction significantly differs from forecasts made by NOAA and NASA that claim cycle 25 will have an amplitude similar to or lower than that of solar cycle 24. Who is right? Only time will tell for sure.
What does it all mean?
Ultimately, as an aurora chaser, I am most interested in geomagnetic storms, not necessarily the complex physics that governs the Sun. But remember, if solar cycle 25 does shape up to be stronger than predicted, that would mean more sunspots and a higher chance for coronal mass ejections, which can cause geomagnetic storms and aurora. In other words, the more active the solar cycle, the more chances we will have to experience auroras, especially at mid-latitudes. Also, if you’re like me and are sick of solar minimum conditions, then this research is reassurance that there is light at the end of the tunnel; solar cycle 25 is in sight, and the Terminator theory predicts our active sun will be back soon (R.J. Leamon et al. (2020))!
These results are very new.
While it’s good to be optimistic about the years ahead, official confirmation and acceptance of this theory will require additional research and observation, which will take years. You may hear about it first on Twitter though—follow @swmcintosh and @leamonrj for more details.
Vincent Ledvina is a rising junior at the University of North Dakota (UND) studying physics. He is a 2020 Aurorasaurus summer intern, and an Aurorasaurus ambassador for North Dakota. Vincent has been aurora chasing since he was 15 and enjoys astrophotography. His latest project is leading a citizen science effort to build an aurora camera facility at UND’s Martens Observatory in North Dakota, incorporating Aurorasaurus citizen science. Last summer, Vincent interned at the National Solar Observatory and presented his research at the fall 2020 AGU conference. A paper on that research is in the works. Vincent also runs a YouTube channel, Apalapse, where he teaches photography, and is the president of North Dakota’s largest astronomy club, the Northern Sky Astronomical Society.