Important solar cycle landmarks that are influenced by variations in the sun’s magnetic field might predict changes in weather patterns and risks to telecommunications.
We have been attempting to define the solar cycle using sunspots ever since humanity was able to notice them for the first time around 400 years ago. Solar activity, including sunspots and solar flares, ebbs and flows about every 11 years, changing Earth’s weather patterns and sometimes posing a hazard to communications. Everyone from farmers to the military would benefit if these shifts could be accurately predicted.
A “solar minimum,” or period of decreased solar activity, has traditionally been used by scientists to indicate the start of each cycle. However, R Robert Leamon, a research scientist at the Partnership for Heliophysics and Space Environment Research (PHaSER), a University of Maryland Baltimore County partnership with NASA, argues that the “solar minimum” framework is rather arbitrary and inaccurate.
A recent study conducted by Leamon demonstrates that many significant changes during the solar cycle can be accurately described and predicted using a “solar clock” based on the magnetic field of the sun rather than the presence or absence of sunspots. The new approach improves the classic sunspot technique by predicting surges in dangerous solar flares or shifting weather patterns years in advance.
The new research, which was published in Frontiers in Astronomy and Space Sciences, specifically demonstrates that the solar cycle functions as a separate sequence of events. At each fifth of a cycle, noticeable and sometimes abrupt changes take place. That is true regardless of how long a given cycle really is, which can range from a few months to a year. Leamon and colleagues refer to it as a “circle of fifths” as a tribute to music enthusiasts.
Finding the landmarks
The new study by Leamon and colleagues Scott McIntosh, at the National Center for Atmospheric Research (NCAR), and Alan Title, at the Lockheed Martin Advanced Technology Center, builds on work by Leamon, McIntosh, and Daniel Marsh, also at NCAR, published in 2020. That paper demonstrated the existence of a solar cycle phenomenon the research team dubbed “the terminator.”
The sun’s magnetic field changes direction each solar cycle, but there is overlap between consecutive cycles. The sun’s magnetic field is also called the polar field because it either points to one of the sun’s poles or the other. A terminator marks when the previous cycle’s polar field has completely disappeared from the sun’s surface and is quickly followed by a dramatic rise in solar activity.
The new paper points to additional landmarks along the journey through a full solar cycle from terminator to terminator. These landmarks are clearer and more consistent than using sunspots as a guide to cycle length. For example, “The max number of sunspots doesn’t quite align with when the polar field reverses, but the polar field reversal happens at exactly one-fifth of the cycle going from terminator to terminator,” Leamon says.
At two-fifths of a cycle, dark areas called “polar coronal holes” re-form at the sun’s poles. At three-fifths of a cycle, the last X-flare, a class of very large and potentially dangerous solar flares, occurs. At four-fifths, sunspots are at a minimum—but this landmark is less consistent. And then the sun passes through another terminator, after which solar activity rapidly picks up again. Other phenomena, such as UV emissions, also line up nicely on the fifths.
Symptoms and causes
The team picked out patterns in data collected daily by two ground-based observatories. The Dominion Radio Astrophysical Observatory in Penticton, Canada has measured solar radio flux, which serves as a useful proxy for solar activity, daily since 1947. The Wilcox Solar Observatory at Stanford University has collected daily measurements of magnetic fields on the sun’s surface since 1975.
Once the team realized the changes that occur at exactly one-fifth of a cycle, they asked, “How many different solar things can we look at? And then we realized they all overlap on this same set of fifths,” Leamon says. Different parameters shift at different points on the cycle, but “everything is tied to these five landmarks.”
This new theory of a solar clock changes the focus from sunspots to shifts in the magnetic fields. “It’s almost like symptoms and causes,” Leamon says. While sunspots are an important symptom, the magnetic field is the underlying cause driving the solar cycle.
The longest threads
This shift in framework improves researchers’ ability to predict events in the solar cycle more precisely and further in advance, which gives people like satellite operators time to make preparations as needed based on predicted solar activity. Once observatories detect an initial polar field reversal, the precise length of the first fifth of the cycle is set. That means the timing of the other fifths (and their associated events) is a simple matter of multiplication.
The new framework also puts tighter bounds on the period within the cycle when severe flares are expected, which is useful information for people on Earth. Rather than a gradual shift from minimum to maximum activity, the period from terminator to about three-fifths of a cycle seems to be the peak period for flares, with a rapid drop-off after that point until the next terminator. The current cycle began after a terminator in December 2021, and the new framework predicts the last major flares should occur in mid-2027.
Leamon points to a quote by physicist Richard Feynman to explain the value of a theory like this one, that accounts for many variables within a system. “Nature uses only the longest threads to weave her patterns so that each small piece of her fabric reveals the organization of the entire tapestry,” Feynman said. Leamon and colleagues’ new theory is an example of one of these long threads—precisely predicting many aspects of the solar cycle with a single, simple parameter, and making it easier for humans to be ready for changes driven by the sun.
Reference: “Deciphering Solar Magnetic Activity: The Solar Cycle Clock” by Robert J. Leamon, Scott W. McIntosh and Alan M. Title, 10 May 2022, Frontiers in Astronomy and Space Sciences.
The study was funded by NASA Headquarters and the National Center for Atmospheric Research.