Jan 18, 2002

The Resurgent Sun




Evidence is mounting that some solar cycles are double-peaked. The ongoing solar maximum may itself be a double -- and the second peak has arrived.


Marshall Space Flight Center


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January 18, 2002: Every 11 years solar activity reaches a fever pitch: Solar flares erupt near sunspots on a daily basis. Coronal mass ejections, billion-ton clouds of magnetized gas, fly away from the Sun and buffet the planets. Even the Sun's awesome magnetic field -- as large as the solar system itself -- grows unstable and flips.


It's a turbulent time called Solar Max.

Right: Sunspot counts for the current solar cycle peaked in mid-2000 and again in late 2001. Image courtesy David Hathaway, NASA/MSFC.

The most recent (and ongoing) Solar Max crested in mid-2000. Sunspot counts were higher than they had been in 10 years, and solar activity was intense. One remarkable eruption on July 14, 2000 -- the so-called "Bastille Day Event" -- sparked brilliant auroras as far south as Texas, caused electrical brown-outs, and temporarily disabled some satellites.




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After that, sunspot counts slowly declined and the Sun was relatively quiet for month-long stretches. Solar Max was subsiding.

But now, as 2002 unfolds, it's back. The Sun is again peppered with spots, and eruptions are frequent. Says David Hathaway, a solar physicist at the NASA Marshall Space Flight Center: "The current solar cycle appears to be double-peaked," and the second peak has arrived.

Scientists track solar cycles by counting sunspots -- cool planet-sized areas on the Sun where intense magnetic loops poke through the star's visible surface. Hathaway is an expert forecaster of sunspot numbers. "Sunspot counts peaked in 2000 some months earlier than we expected," he recalls. The subsequent dip toward solar minimum seemed premature to Hathaway, and indeed it was. Before long, sunspot counts reversed course and began to climb toward a second maximum that now appears to be only a few percent smaller than the first.


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Solar Max eleven years ago was much the same. A first peak arrived in mid-1989 followed by a smaller maximum in early 1991. In fact, if the ongoing cycle proves to be a double, it will be the third such double-peaked cycle in a row.


Left: International sunspot counts between 1975 and 1995 show that the last two sunspot cycles also had double-featured maxima.

During solar maximum, magnetic fields above the Sun's surface become impressively tangled, particularly near sunspots. Twisted magnetic fields -- stretched like taut rubber bands -- can snap back and explode, powering solar flares and coronal mass ejections.

Sunspots are the most visible sign of those complex magnetic fields -- but not the only one. Another sign is solar radio emissions, which come from hot gas trapped in magnetic loops. "The radio Sun is

now than it was in 2000," says Hathaway. By the radio standard, this second peak is larger than the first.

Hathaway notes a widespread misconception that solar activity varies every 11 years "like a pure sinusoid." In fact, he says, solar activity is chaotic; there is more than one period.

Earth-directed solar explosions, for instance, tend to happen every 27 days -- the time it takes for sunspots to rotate once around the Sun. There is also an occasional 155-day cycle of solar flares. No one knows what causes it. And the double peaks of recent solar maxima are separated by approximately 18 months.


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The source of all this variability is the turbulent Sun itself. The outermost third of our star -- the "convective zone" -- is boiling like hot water on a stove. California-sized bubbles rise 200,000 km from the base of the zone to the Sun's surface where they turn over and "pop," releasing heat (generated by nuclear reactions in the core) to space. Below the convective zone lies the "radiative zone" -- a calmer region where photons, not mass motions, transport the Sun's energy outward. Says Hathaway: "The Sun's magnetic field is generated at the boundary between these two layers where strong electric currents flow."


Right: This artist's concept of the solar interior reveals the boiling convective zone, the interface layer (where the Sun's magnetic field is generated), and the relatively calm radiative zone. [more]

Magnetic fields are produced by electric currents -- that is, charges in motion. The Sun itself is a conducting fluid. Our star is so hot that the atoms within it are mostly ionized; their nuclei are separated from their electrons. As a result, relative motions between neighboring layers of ionized gas carry currents and spawn magnetic fields. "The rotational velocity of the Sun changes suddenly near the convective-radiative boundary," says Hathaway. "The velocity shear is what drives the so-called solar magnetic dynamo."

Below: The false colors in this cutaway diagram of the Sun represent different gas velocities inside our star. Click on the image to view a 3MB movie of the 16-month "pulse" at the base of the convection zone. [more]


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Last year, scientists using a technique called helioseismology, which can probe conditions within the Sun much like seismic waves reveal the interior structure of our planet, announced that currents of gas at the base of the convective zone speed and slacken every 16 months.


"That's about the same as the time between the double peaks of recent solar maxima," notes Hathaway. Perhaps the two are connected. "It's hard to be sure," he cautions, because the detailed inner workings of stellar magnetic dynamos remain a mystery. "Helioseismology of the Sun, which can probe beneath its visible surface, is still a young field. We need more time to understand completely how the internal rhythms of our star affect the solar cycle."

Whatever the cause, a resurgent Sun is welcome news for many sky watchers. Solar eruptions can trigger one of the most beautiful spectacles on our planet: Northern Lights. If the Sun continues to storm, the skies could be alight, off and on, for many months to come.


Web Links

The Sunspot Cycle -- predictions, history, and in-depth information from David Hathaway and the NASA/MSFC solar physics group. See also, What is the Solar Cycle? from NASA/Goddard's "StarChild" and Sunspot Numbers from

Up & Downs: Solar maxima come every 11 years, but the ferocity of solar activity isn't the same each time. The 11-year peaks seem to be modulated by an even longer cycle, perhaps 100 years or more in length. During one notable trough called the "Maunder Minimum" very few sunspots were seen for 6 consecutive solar cycles between 1645 and 1715 -- and Europe was plunged into the "Little Ice Age." Click to view sunspot counts recorded during the past 400 years.

NOAA sunspot numbers and 10.7 cm radio fluxes -- see for yourself the double-peaked character of the current solar cycle.



The Most Powerful Solar Flares ever Recorded -- ( For reasons scientists don't understand, the most powerful solar flares erupt most often during the waning phases of sunspot cycles. Two of the strongest flares on record happened in early-2001 when the sunspot number was temporarily declining.

Sun's got the Beat -- (Science@NASA) Helioseismologists discover a curious 16 month rhythm at the base of the Sun's convection zone.

More helioseismology links: Surface Waves and Helioseismology, from Science@NASA; Helioseismic Holography, from,

Watching the Angry Sun -- (Science@NASA) Solar physicists are enjoying their best-ever look at a solar maximum thanks to NOAA and NASA satellites.

Origin of Earth's Magnetism -- like the Sun, our planet has a magnetic dynamo.

The Dynamo Process -- the basics

Planetary Dynamo on a Desk -- ( magnetic dynamos are hard to study because they are hidden deep inside stars and planets. An innovative device allows one scientists to study a dynamo on his desktop.


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