NASA Science News

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

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.

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.

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.

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.

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 even brighter 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.

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."

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."

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

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.