The current solar cycle appears to be double-peaked

Dr David Hathaway

The resurgence comes just two years after the Sun reached a maximum in its 11-year cycle of behaviour. Astronomers say a second peak of activity in so short a space a time is unusual but not unprecedented.

Although the cycle is thought to occur because of gas motions in the Sun, scientists admit they still have much to learn about the mechanisms at work.

A peak in activity is marked not just by more sunspots but also huge explosions, or solar flares, which erupt close to them. And coronal mass ejections - billion-tonne clouds of magnetized gas - will buffet the planets, including Earth.

According to sunspot counts, the most recent maximum peaked in the middle of 2000. Indeed, one energetic eruption on 14 July of that year - the so-called Bastille Day Event - resulted in spectacular aurorae as far south as Texas, US, and caused electrical brown-outs, temporarily disabling some satellites.

Spot counts

After that, sunspot counts slowly declined and the Sun was relatively quiet for month-long stretches. It seemed that the star was winding down. Or so astronomers had thought.

But as 2002 unfolds, the Sun is back on a high and once again littered with spots. Eruptions are also frequent. Dr David Hathaway, a solar physicist at the American space agency's Marshall Space Flight Center, said: "The current solar cycle appears to be double-peaked," and the second peak has arrived.

The increased activity will please skywatchers (Image by Juha Kinnunen)

After analysing past 11-year cycles, some astronomers said the sunspot count in 2000 peaked a few months earlier than had been expected; perhaps indicative of an extended maximum to come.

In this apparent second maximum, the count is only a few percent smaller than the first.

A "double-dip" is unsurprising say scientists. The maximum of 11 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.

Chaotic behaviour

During solar maximum, the intense magnetic fields that reach above the Sun's surface become tangled, particularly near the sunspots. The twisted magnetic fields can snap back and explode, powering solar flares and coronal mass ejections.

Dr 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.

The source of all this variability is the turbulent outer layer of the Sun. Occupying its outermost third is a convective zone where gas is rising and falling in planet-sized cells.

Below the convective zone lies a radiative zone, which is a calmer region, where photons transport the Sun's energy outward. The Sun's changeable magnetic field is generated at the boundary between these two layers, where strong electric currents flow.

Last year, scientists using a technique called helioseismology, probed the Sun's interior, revealing that currents of gas at the base of the convective zone change velocity every 16 months.

"That's about the same as the time between the double peaks of recent solar maxima," said Dr Hathaway. But are the two connected? "It's hard to be sure," he said, because the detailed inner workings of the Sun were a mystery.

But whatever the cause, a resurgent Sun is welcomed by many skywatchers. Solar eruptions trigger aurorae on Earth, filling the night sky at high latitudes with dancing lights.