Scientists are struggling to explain the Sun’s bizarre recent behavior. Is this solar cycle a fluke, or a sign of a deeper trend?
The Sun is acting weird. It typically puts on a pageant of magnetic activity every 11 years for aurora watchers and sungazers alike, but this time it overslept. When it finally woke up (a year late), it gave the weakest performance in 100 years.
What’s even weirder is that scientists, who aren’t usually shy about tossing hypotheses about, are at a loss for a good explanation. Three scientists, David Hathaway (NASA / Marshall Space Flight Center), Giuliana de Toma (High Altitude Observatory), and Matthew Penn (National Solar Observatory) presented possible explanations at this month’s meeting of the American Astronomical Society’s Solar Physics Division, but their results sparked a lively debate rather than a scientific consensus.
A Weak and Weird Solar Cycle
A well-behaved Sun flips its north and south magnetic poles every 11 years. A cycle starts when the field is weak and dipolar—basically, a giant bar magnet. But the Sun’s rotation is faster at its equator than at its poles, and this difference soon stretches the field lines like distended rubber bands around the solar surface. Frenetic activity ensues, with magnetic tangles producing sunspots, prominences, and sometimes flares and plasma explosions. All of that dies down when the Sun-wide magnetic field lines finally snap into simpler configurations, re-establishing the dipole field and beginning the next cycle.
The Sun has been doing all of that, just to a lesser degree. “Not only is this the smallest cycle we’ve seen in the space age, it’s the smallest cycle in 100 years,” says Hathaway, who took part in the Solar Cycle 24 Prediction Panel back in 2007.
The panel members were split at the time on whether the next solar activity cycle would be strong or weak, but their middle-of-the-road estimate anticipated 90 sunspots as a peak value near August 2012. Instead, the peak sunspot number seems to be less than 70, and the maximum arrived later than expected. Cycle 24 should have peaked in 2012, 11 years after its last minimum in 2001, but the Sun overslept by a full year, waking up in 2013 instead.
And its waking has been asymmetric: the north pole has led the cycle since 2006, with the south pole lagging behind. “It’s not uncommon to see hemispheres going out of phase . . . Usually this [asymmetry] lasts a year or so and then the hemispheres synchronize,” de Toma explains. “We don’t know why this is lasting for so long.”
It’s possible that, weak and weird as it is, Cycle 24 is still part of the Sun’s normal variation, even if it’s one of the weakest cycles yet recorded.
In fact, both Hathaway and de Toma think the 11-year cycle might be part of a larger one. Historical records show weak cycles at the turn of the 19th and 20th centuries, so it could be that the solar cycle tapers every 100 years or so in what’s known as the Gleissberg Cycle. It’s not easy to establish the existence of a cycle that turns over on such a long timescale, and even Hathaway admitted, “Certainly I don’t understand how it works.”
Doug Biesecker (NOAA), chair of the most recent prediction panel, says, “I remain highly skeptical . . . [Even] if you believe there is a 100-year cycle, then that still doesn't tell us why. Just that it is.”
Penn offered another, more catastrophic option: the sunspot cycle might die altogether. His team uses sunspot spectra to measure their magnetic fields, and his data show a clear trend: the magnetic field strength in sunspots is waning.
“If this trend continues, there will be almost no spots in Cycle 25, and we might be going into another Maunder Minimum,” Penn states. The first Maunder Minimum occurred during the second half of the 17th century. Almost no spots were seen on the Sun during this time, which coincided with Europe’s Little Ice Age.
But Penn acknowledges that magnetic field measurements from other studies don’t always see the same trend he sees. Some observations show that sunspots’ magnetic field strength varies with the solar cycle, and others (including de Toma’s) show that sunspots’ magnetic fields aren’t changing with time. De Toma was even able to reproduce Penn’s results by excluding small sunspots, suggesting Penn’s trend might result from the way his team selects the sunspots they measure.
Another word of caution came from Hathaway, who notes that the Maunder Minimum might have been a catastrophic event rather than a gradual trend. “Many of my colleagues are poring over historical records to find out . . . what did lead up to the Maunder Minimum?” he says. “New observations suggestion that the cycle before the Maunder Minimum wasn’t particularly small.”
Regardless of what’s causing the Sun’s strange behavior, Hathaway and Penn, who are both in the solar prediction business, anticipate that Cycle 25, expected to peak in 2024, will be the weakest yet.
Penn’s prediction is based on the weakening magnetic field he sees within sunspots; Hathaway’s are instead based on measurements of the Sun’s polar field and the meridional flow, the flow of magnetic flux from the Sun’s equator to the poles. A stronger flow would help strengthen weak fields, but meridional flows have been completely absent in Cycle 24 so far. We might have a long wait ahead of us to see if and when the Sun recovers.