At the end of the last solar cycle in 2019, the official predictions were that the next cycle would be just as mild as its predecessor.
Those predictions were wrong. The current Solar Cycle 25 turned out far stronger than NASA and NOAA expected. Now, scientists say that the Sun's activity is on an escalating trajectory, outside the boundaries of the 11-year solar cycle. In fact, a new analysis of the data suggests that the activity of the Sun has been gradually rising since 2008.
"All signs were pointing to the Sun going into a prolonged phase of low activity," says plasma physicist Jamie Jasinski of NASA's Jet Propulsion Laboratory (JPL). "So it was a surprise to see that trend reversed. The Sun is slowly waking up."
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The Sun seems like a constant, reassuring presence day to day, but it's actually fairly tempestuous and changeable. One particular change it undergoes is what we call the solar cycle. Every 11 years or so, the Sun undergoes an increase in activity to solar maximum before subsiding again down to solar minimum.
This activity manifests as a significant increase in sunspots, solar flares, and coronal mass ejections at solar maximum, at which point the Sun's poles reverse their polarity. It's all very normal; if the number didn't give it away, we're currently in the 25th solar cycle on record, which means scientists have been watching this happen for centuries now, using sunspots as a proxy to record solar activity.
However, even with this wealth of solar cycle data, predicting how the Sun is going to behave is an inexact art. There's a lot more going on inside the Sun than we are privy to, driving changes that scientists are still trying to explain.
For example, for a 70-year period from 1645 to 1715, almost no sunspots appeared at all, a period known as the Maunder Minimum. A similar lull occurred between 1790 and 1830, known as the Dalton Minimum.
"We don't really know why the Sun went through a 40-year minimum starting in 1790," Jasinski says. "The longer-term trends are a lot less predictable and are something we don't completely understand yet."
Solar cycles 22 and 23, commencing in 1986 and 1996, respectively, were pretty average in terms of sunspot activity. In spite of this, the solar wind pressure steadily decreased over both cycles, leading scientists to believe that we might be headed towards something similar to the Maunder or Dalton events.
In 2008, Solar Cycle 24 commenced, becoming one of the weakest solar cycles on record in terms of sunspot and flare activity. Solar Cycle 25, scientists thought, was going to be more of the same, but its activity level turned out to be higher, more consistent with an average solar cycle.
Jasinski and his colleague, space physicist Marco Velli of JPL, have now analyzed long-term solar data and found something surprising. In 2008, at the start of Solar Cycle 24, the solar wind began growing stronger and has continued to increase steadily since then.
That strength is measured in speed, density, temperature, thermal pressure, mass, momentum, energy, and magnetic field magnitude, all of which showed an increase.
Now, the Sun is an extraordinarily complex engine, and one thing that's crystal clear is that it's really difficult to predict what it's going to do next. Jasinski and Velli believe their findings indicate that we could see an uptick in rowdy space weather, with powerful winds, solar storms, flares, and coronal mass ejections.
Their results are also consistent with another aspect of the Sun's behavior: the Hale cycle. This is represented by pairs of solar cycles, with each Hale cycle lasting 22 years, ending when the magnetic poles swap back to their original polarities. A growing body of evidence suggests that the Hale cycle is the primary cycle, and each solar cycle is only half of the full cycle.
At the moment, the solar wind pressure is still lower than it was at the turn of the 20th century. The only way we will know what it does from this point is to continue observing to see whether it continues to increase or remains stable.
The findings also have much broader implications, adding to the mounting evidence that sunspot numbers present, at best, an incomplete picture. We need to study a much wider catalog of solar behavior if we wish to understand the dynamics of our wild, beautiful star.
The research has been published in The Astrophysical Journal Letters.