The Sun plays a role in another environmental issue that human activity influences.

As the Sun winds down its activity, it delivers less frequent and less punishing blows of various forms of radiation to Earth's atmosphere. That's good news for ozone, which gets zapped by the Sun's high-energy storms.

Ozone occurs naturally in the stratosphere and filters out much of the Sun's ultraviolet radiation.

In a study released in the August 1 issue of Geophysical Research Letters, researchers presented new evidence confirming a long-held theory that large solar storms deplete the upper-level ozone for weeks to months.

NASA researchers studied the effects of a solar storm that hit Earth between July 14 and 16 last year, smack in the middle of the solar cycle's peak. During such storms, protons bombarded the upper atmosphere, breaking up molecules of gases like nitrogen and water vapor. Once freed, those atoms react with ozone molecules and break them down into other substances.

Using satellites to examine ozone before and after the event, the researchers found a small but measurable effect.

Jay Herman, another Goddard scientist, also uses satellites to study ozone. Herman said the 11-year solar cycle alters the amount of ozone roughly 2 percent from the peak to the low point in activity.

Other causes, such as Earth's seasons and the resulting change in sunlight at the poles, create greater fluctuations.

"To put this in perspective, the global average seasonal variation is 5-8 percent," Herman said. And in both the Northern and Southern hemispheres, where seasonal holes develop above the poles, ozone can vary as much as 25 percent each year.

The ozone holes are bounded by rings of high-altitude winds that circle each pole.

"There always has been a springtime reduction of ozone in the Antarctic," Herman said. "In recent years, the ozone hole has expanded to fill in the maximum area available within the polar vortex winds and has removed almost all of the ozone that is possible."

The relatively small populations of humans who live beneath the thinned layers of ozone can be exposed to higher doses of ultraviolet radiation, which studies suggest can lead to cancer and premature aging of the skin. Most researchers agree that the increased depletion is caused by the human production of chlorofluorocarbons, which means the human impact far exceeds that of the solar cycle.

But even this relationship is not so simple.

And the Sun's rhythm guarantees that another peak is roughly three presidential elections away.

With more and more satellites in space, and Earth's power grids operating under greater stress all the time, scientists and engineers are eager to know how strong the next maximum will be, and when it is due (the roughly 11-year cycle has been known to range anywhere from eight to 15 years).

Given recent predictability, it's a fair bet the next peak will occur around 2012. But its potential impact is unknown.

"It is still too early to reliably predict the size of the next cycle," says Hathaway, the solar physicist. "We won't have good estimates until near solar minimum, around 2006."