We may already be able to observe the beginnings of the next solar cycle, which is already visible through the current one.
It's been a wild ride. Solar cycle number 25 has been one of the strongest cycles in recent memory so far, producing several massive sunspot clusters. The current large region facing Earth (Active Region 3780) is now easily visible with eclipse glasses…no magnification needed. Cycle 25 began in 2019.
Massive sunspot comes into view. Image credit: NASA/SDO
A stormy year
The most recent solar cycle will undoubtedly go down in history as it heads toward an active maximum in 2025. But although Cycle 25 will last until the end of the current decade, there are already signs that Cycle 26 could be starting just beneath the simmering surface of the Sun. A study from the University of Birmingham, recently presented at the Royal Astronomical Society's National Astronomical Meeting in Hull, UK, shows that key indicators of the start of the next cycle may already be in place.
The numbering of the solar cycle according to the current convention goes back to the beginning of cycle 1 in 1755. The pattern for numbering the cycles was introduced in 1852 by the astronomer Rudolf Wolf.
We know that a new solar cycle has officially begun when sunspots appear at higher latitudes. These also typically have reversed polarity compared to the previous cycle. As the cycle progresses, they then push closer to the solar equator. Spots from two cycles can also merge as the transition begins.
A large sunspot cluster from May 2024. Image credit: NASA/SDO
Arranging the points from successive cycles relative to latitude creates a butterfly diagram that illustrates this effect, known as Spörer's Law.
A butterfly diagram (above) showing sunspots relative to latitude over time. Image credit: NASA/MSFC
A look inside the sun
But there's more to the Sun than meets the eye. A large ball of hydrogen and helium gas, the Sun doesn't rotate as a single solid mass. Instead, it rotates faster at the equator (25 days) than near the poles (34 days). Scientists can explore the Sun's interior using a method known as solar helioseismology, which studies waves that travel through the Sun's photosphere to model the interior.
These internal sound waves form bands in a phenomenon known as solar torsional oscillation. Faster rotating belts appear as harbingers of the next cycle. These move toward the solar equator as the cycle progresses, along with visible sunspots.
“The signs of Cycle 26 that we are seeing are that the Sun's rotation has been increasing at around 50 degrees latitude and now appears to be stabilising,” Rachel Howe (University of Birmingham) told Universe Today. “This is part of a pattern called torsional oscillation, where bands of slightly faster and slower rotation appear at mid-latitudes before the cycle officially begins, and move to lower latitudes as the cycle progresses, along with sunspot activity. In previous cycles, we have seen that the faster rotating band associated with the cycle can be traced back to around the maximum of the previous cycle, and we believe we are seeing the start of the pattern again. However, it will be several more years before we can expect sunspots belonging to the new cycle!”
A solar cycle map showing velocity and torsional oscillations over time versus latitude for the last three solar cycles… and the beginning of solar cycle 26 (top right). Image credit: Rachel Howe.
Keeping an eye on the sun around the clock
The science of helioseismology is enabled by the Global Oscillation Network Group (GONG), a worldwide network that continuously monitors the Sun. In space, the Helioseismic Magnetic Imager on board the joint ESA-NASA Solar and Heliospheric Observatory (SOHO) complements this effort. The Michelson Doppler Imager (MDI) on NASA's Solar Dynamics Observatory (SDO) also plays a key role in this campaign. This effort dates back to 1995 and covers the last three solar cycles.
Big Bear Lake and Solar Observatory, part of the GONG solar monitoring network.
This gives researchers insight into the beginning of the last two solar cycles. It also provides clues as to what we might expect at the start of the 26th solar cycle. “By understanding how this flow pattern relates to the sunspot cycle, we may be able to better predict how strong the next solar maximum will be and when it will occur,” says Howe.
Sunspots from July 31, 2024. Image credit: Eliot Herman.
Solar Cycle 25 has been extremely active so far, exceeding all expectations. This follows the historic lull that preceded it between Cycles 24 and 25. Observers saw few sunspots during this pronounced minimum. However, this was consistent with many predictions by astronomers who study the Sun, pointing to a stronger than usual cycle on the upswing.
Outlook for Cycle 26
“The Sun never ceases to amaze,” says Howe. “Some of the most exciting discoveries of recent times have come from spacecraft – Solar Orbiter and Parker Solar Probe – that are flying closer to the Sun than ever before, helping scientists unravel the connections between what we see on the Sun's surface and the 'space weather' events that affect us on Earth. We are studying the Sun's surface in more detail than ever before, but there is also a place for long-term studies (of which this work is a part) that track the large-scale patterns inside the Sun over decades.”
A magnetic view of the Sun, courtesy of SDO. Image credit: NASA/SDO
The solar storm of May 10 was the most impressive of the cycle so far. This storm sent auroras as far south as Spain and Mexico, where auroras are rarely seen. We were rewarded with a persistent red glow from central Germany, an unforgettable sight.
Solar cycles and more
Historically, the Wolf sunspot number defines the level of solar activity. Astronomers refer to this as the relative or Zurich sunspot number. A 2013 study suggested that the orientation and strength of the heliospheric current sheet is a better indicator of the state of the current solar cycle than the sunspot number.
We usually say it's an 11-year solar cycle from one minimum/maximum to the next… but it's actually twice as long. The Sun's magnetic field reverses every 11 years and returns to the same relative orientation every 22 years.
We see “starspot cycles” on other suns, too. It is also unclear why our Sun has an 11-year cycle “burned in.” Nor are we sure that this has always been the case throughout its entire 4.6 billion year lifespan.
This research provides a great model to test the next solar cycle as we struggle to understand and live with our stormy star.
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