Solar Max is Over, Earth's Future Looks Brighter

But the Sun Experienced a Second Peak in 2002

August 28, 2001
By Robert Roy Britt, Senior Science Writer

A menacing peak in cyclical solar activity officially has passed, a NASA scientist says, but its impact on Earth's weather is far from over. In fact, the outlook is sunny in many ways.

The flurry of Sun flares and expulsions seen over the past two years has begun to ebb, and activity will continue to decline for the next five years or so. Now it has set into motion a series of salutary changes to the planet's long-term climate and perhaps even daily weather.

The outcomes are predicted to include fewer clouds over the United States in coming years and a southward shift in storm tracks. Other effects should include a deflation of the planet's atmosphere, which will make it easier for mission managers to keep the International Space Station in its proper orbit.

And the beleaguered ozone layer, infamous for growing that gaping hole each year, is about to get a slight breather thanks to less abuse from solar activity.

A final effect will be more noticeable from the ground. Anyone lucky enough to live at northern latitudes will find that Sun-fueled geomagnetic storms, which create the colorful Northern and Southern Lights, will actually increase in coming months.

The rhythm of the Sun

The Sun has rhythm. Beyond its propensity to show up in the East every 24 hours (a rhythm actually dictated by Earth's rotation) there is a deeply rooted cycle of activity within the Sun that increases and decreases every 11.3 years, on average.

This solar cycle, as it is called, is measured by the number of sunspots, tangles of magnetic energy that reflect the overall activity near the surface of the Sun. With more and stronger sunspots come increasingly furious coronal mass ejections -- bubbles of gas and charged particles that are hurled into space and sometimes threaten Earth satellites and power grids.

During the peak in activity, called the solar maximum or solar max, the Sun also releases energy through coronal holes, open magnetic fields that cause sharp increases in the amount of charged particles riding outward on the ever-present solar wind.

David Hathaway, a solar physicist at NASA's Marshall Space Flight Center, began predicting the recent peak in the Sun's activity back in the early 1990s. He had figured it would come in June or July of 2000. Only recently has he been able to look back at the data and figure out where the peak was.

"The maximum sunspot number occurred in July of 2000 and we expect that date to hold," Hathaway told SPACE.com. He said this peak was bigger than average but less dramatic than the previous two.

But as its temper settles, the Sun still has some punch in store.

"Solar flares and coronal mass ejections will decline in frequency with the sunspot number," Hathaway said. "However, geomagnetic storms will continue to increase in frequency due to the high speed solar wind streams from low latitude coronal holes that form late in the solar cycle."

The visible effect will at times be stunning. Earlier this year, a geomagnetic storm sparked aurora -- sheets and filaments of multicolored lights caused by the excitation of gas molecules high in the atmosphere -- that were seen as far south as Texas.

Space weather this weekend squares with the passage of the peak, Hathaway says. A powerful solar flare erupted Saturday at the Sun and sent a coronal mass ejection to Earth that triggered a strong radio blackout on the sunlit side of Earth.

Flares and coronal mass ejections like Saturday's will continue into the future but at a lower rate than a year ago during the solar maximum peak, Hathaway said. "The cycle doesn't show any evidence that we're going to peak again at a higher value than we did last year," he said. "It still looks to me like we passed the maximum."

Bloated atmosphere

Earth's atmosphere also undergoes a less visible but still dramatic change during the peak in the solar cycle, one that has a direct impact on anything orbiting the planet.

While more than half of Earth's atmosphere is huddled within 6 miles (10 km) of the planet's surface, the atmosphere extends several hundred miles up, getting ever thinner with height.

During solar maximum, extra doses of the most extreme wavelengths of ultraviolet light heat Earth's upper atmosphere, a region called the thermosphere, which starts at about 60 miles up (100 kilometers). The Space Station and some satellites orbit in the thermosphere.

Though the thermosphere is about a million times less dense than the atmosphere at sea level, it still creates drag on anything orbiting through it. Skylab and Mir are two famous victims of the effect.

During solar minimum, the gas temperature in the thermosphere is around 1,290 Fahrenheit (700° C). But during solar maximum, the temperature can more than double, Hathaway says. The extra heat causes the thermosphere to expand during solar maximum. Denser layers of atmosphere reach higher, and so the region where the Space Station orbits can become 50 times more dense, which increases drag.

In May of last year, amidst the peak in solar activity, the Space Shuttle Atlantis fired its jets while affixed to the Space Station and raised the 35-ton habitat's orbit by 27 miles (43 km). The orbit will decay more than a mile (about 2 kilometers) each year, NASA engineers say. Eventually the Station is to get its own booster system.

Because molecules are far apart in the thermosphere, astronauts cannot feel the incredible heat. In fact, it cannot be measured by normal means, Hathaway says. Instead, scientists use orbital decay to estimate the density of the air, and from this they infer the temperature.

The Sun, fueling clouds

All wind and clouds on Earth are directly or indirectly tied to the Sun. Heat from the Sun produces the temperature differences that lead to pressure differences. Air naturally moves from high to low pressure areas, and this creates the winds.

The oceans store solar heat for long periods, and watery currents move the energy around the globe, fueling everything from mild breezes and localized fog to ferocious storms.

Consistent troughs of wind carry hurricane seedlings from Africa to Florida in a process that can take weeks. Days-long interactions between warm, moist tropical air and cooler Arctic air fuels tornadoes in the South and Midwest that sometimes migrate to the Northeast. A buildup of afternoon heat can force air to rise and fuel mountain thunderstorms that billow out of nowhere on a moment's notice. Earth's tilt and orbit conspire to alter how much sunlight reaches each hemisphere, creating the seasons.

The ISS seen 242 miles (389 km) above Earth. As the solar cycle wanes, Earth's atmosphere will shrink, creating less drag for the station. Image is from the Space Shuttle Endeavour on April 29, 2001.

The relationship between sunshine and weather is an odd one, whereby the energy from the Sun brings about the very clouds that obscure the Sun, says Petra Udelhofen. She is a NASA-funded researcher at the Institute for Terrestrial and Planetary Atmospheres at the State University of New York at Stony Brook.

Recent research shows that during the peak in the 11-year cycle of solar activity, at least some parts of the United States experience more cloudiness.

Udelhofen studied data from 1900 to 1987 collected by observers who noted the percentage of cloudiness several times each day. She also studied the records from dozens of automatic stations around the country that record the amount of sunshine each day.

Udelhofen then combined these data sets and compared them to the solar cycle, as measured by the number of sunspots.

Her work, published in the July 1, 2001, issue of Geophysical Research Letters, shows that cloudiness in much of the United States is about 2 percent greater during years of solar maximum compared to the solar minimum years.

Storm tracks

Udelhofen suspects the increase is caused by a shift in the jet stream, a river of high-speed, high-altitude winds that circle the globe, west-to-east, in an undulating manner, sometimes staying well north and sometimes dipping deep into the United States.

Previous studies have indicated that the jet stream shifts north during periods of high solar activity. The shift is tied to increased energy in the atmosphere created by the absorption of the Sun's additional output.

And meteorologists know that storms track along the jet stream. Previous work by Joanna Haigh of Imperial College in London found that in the Mediterranean, storm tracks shift north by some 400 miles during solar maximum. It's uncertain though if there were more clouds created or if clouds were pulled north, stolen from the rest of North American and the Caribbean, as Udelhofen only looked at U.S. data.

Still, Udelhofen said she's provided the first observational data to connect the Sun's cycle to increased cloudiness that might be associated with this shift in the jet stream.

"Most of the continental United States shows an increase in cloudiness, but on the West Coast and also over the Great Lakes there is a decrease," Udelhofen said.

The study did not consider cloud type, and so no inferences can be made about possible changes in precipitation associated with the additional clouds.

Heat wave

In recent years, a growing number of scientists have suggested that changes in the Sun's output, and more precisely changes in its overall brightness, could be responsible for some or most of the global warming that has been measured.

The average surface temperature around the globe has risen by about 1 degree Fahrenheit since 1880. While much evidence suggests the rise is due largely to the output of carbon dioxide from cars and factories, many scientists have reserved judgement on whether, or at least how much humans have contributed to the temperature rise.

Might the Sun be the real culprit?

Since the 1970s, researchers have known that when there are more sunspots, the Sun is brighter. And mounting evidence shows a connection between this brightness and the overall warmth of Earth. The connection is related not just to the 11-year solar cycle, but to much longer periods of high and low solar activity.

Studying tree rings and ancient layers of glacial ice for clues to past global temperatures, researchers have found curious links to records of the solar cycle. Most interesting is what scientists call the Little Ice Age, a temperature drop that began in the 13th Century, bottomed out at 2 degrees below the long-term average, and did not reach previous levels until the late 19th Century.

Solar activity was persistently high prior to the 13th Century, when things were warmer, according to Sallie Baliunas, a researcher at Harvard-Smithsonian Center for Astrophysics.

"Activity then dropped to low levels during the Little Ice Age, and recovered by the early 20th century," Baliunas says. "The period of least solar activity coincided with the coldest century of the last millennium -- the 17th century."

Critics frequently charge that the Sun's total output does not change enough to affect Earth's climate so strongly. Baliunas says that's a good argument. "But that leaves unanswered the fact that the Sun's signal is so strong in the climate records."

Still, Baliunas doubts that the Sun is the sole cause of global climate change. She said current research seeks to figure out the magnitude of the Sun's influence so that the human effect can be better assessed.

Zapping ozone

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.

"If you look at the total atmospheric column, from your head on up to the top of the atmosphere, this solar proton event depleted less than one percent of the total ozone in the Northern Hemisphere," said Charles Jackman, a researcher at NASA's Goddard Space Flight Center Laboratory and lead author of the study.

That hole above our heads

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."

Last year, the hole above Antarctica reached record proportions. Herman said it's too early to tell what will happen this year.

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.

Last year, researchers learned that the planet's surface temperature might affect ozone levels. Scientists at NASA's Jet Propulsion Laboratory found that the unusually cold temperatures in the stratosphere, 10 to 30 miles (16 to 48 kilometers) up, are related to balmy winters at ground level. The cold stratosphere was in turn blamed for fueling ozone depletion.

Relief for astronauts, satellites, power grids

The reduced solar activity also means less radiation that would threaten astronauts on spacewalks. And in coming years, fewer storms will bombard satellites, which can be damaged by severe space weather.

Even power grids on Earth can be affected, as happened in 1989 when a power surge triggered by solar energy damaged transformers of the Hydro-Quebec power system, leaving 6 million people in Canada and the Northeast United States without power for more than nine hours. No such damage has occurred during this peak, part of what scientists call Sunspot Cycle #23.

But experts caution that even though the peak is past, severe solar storms can still crop up. In fact, the strongest solar flare of this cycle came after the peak, in April 2001.

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."

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