Pinning Down the Sun-Climate Connection; Solar Influence Extends beyond Warm, Sunny Days
Perkins, Sid, Science News
Second in a two-part series on the sun-Earth connection
The solar flares that spew massive amounts of energy and particles earthward are notorious for the havoc they can wreak on satellites, power grids, and our planet's magnetic field (SN: 1/13/01, p. 26). The charged particles that slam into the outer fringes of the atmosphere also ionize the air and stimulate shimmering auroras. During periods of increased solar activity, particularly during high-points of the 11-year sunspot cycle, these breathtaking sky shows often appear far south of their normal Arctic venues.
Strong solar activity can also have substantial short-term influences; by cooling the atmosphere in some places and heating it in others. These meteorological effects typically last days or weeks. But many scientists propose that changes in the sun's magnetic field and radiation output during its 11-year cycle of activity also have longer-term effects. They influence the movement of weather systems and other aspects of atmospheric and climate patterns.
It's tough to discern the subtle climatic effects of solar variation amidst a cacophony of strong earthly influences--greenhouse gases, volcanoes, sulfate aerosols, to name a few. But using sophisticated statistical analyses of an ever-growing stockpile of climate and weather data, scientists say they're uncovering ways in which even small variations in solar activity could have big effects down at ground level.
Intense solar activity has substantial short-term effects that can shape Earth's weather. Consider the magnetosphere, which swaddles the planet at altitudes between 1,000 and 6,000 kilometers. It's composed of thick layers of charged particles--primarily protons and electrons--trapped in space by Earth's magnetic field. When a strong solar flare delivers a jolt to these belts, it can shake loose a torrent of these charged particles. Auroras, which at times of increased solar activity sprawl across the sky more often than average, are merely one result of this ionic rain upon the upper atmosphere.
An especially powerful type of solar eruption, known as a coronal mass ejection, can inject protons directly into Earth's atmosphere. As these high-energy solar protons rain down, they pummel the air like subatomic bullets. The kinetic energy of these charged particles may warm the outer layers of the polar atmosphere by several degrees, says Charles H. Jackman, an atmospheric scientist at Goddard Space Flight Center in Greenbelt, Md. This heating--which typically lasts just a few days, Jackman says--primarily affects the air at heights above 50 km.
At altitudes from 30 to 90 km, the energetic particles also split molecules of nitrogen into nitrogen atoms, which then react with oxygen in the air to form ozone-destroying compounds. This occurs mainly at polar latitudes, where most of the charged particles have spiraled in along lines of the Earth's magnetic field. Because ozone absorbs ultraviolet (UV) light, the depletion of ozone can cause the atmosphere to cool.
During a large solar storm, a portion of the upper stratosphere in the polar regions can lose up to 20 percent of its ozone and cool as much as 3[degrees]C, an effect that can last for several weeks. Jackman is quick to emphasize that such solar activity is not the cause of the infamous "ozone holes," which occur at lower altitudes and are aggravated by chemicals such as chlorofluorocarbons.
Not only do solar storms influence day-to-day weather, but longterm, subtle variations in solar activity drive Earth's climate, scientists propose. Satellite data show that, overall, the amount of radiation Earth actually receives from the sun varies little between the maximum and minimum phases of the 11-year solar cycle.
When measured across all wavelengths of light, the total radiation reaching the planet changes only about 0.1 percent during the cycle, says Rodney A. Viereck, a space physicist at the National Oceanic and Atmospheric Administration in Boulder, Colo. …