The Earth's ozone layer is destroyed not only by human use of ozone depleting chemicals, but also by large solar storms, new research confirms.
A solar flare with an associated coronal mass ejection sent positively charged protons streaming to Earth from July 14 to 16, 2000. The bombardment of protons, called a solar proton event, was the third largest in the last 30 years.
Now a year later, new evidence from U.S. Earth orbiting satellites is helping scientists better understand how solar events such as this one affect the planet.
Large solar storms rain electrically charged particles down on Earth's atmosphere and deplete the upper-level ozone for weeks to months according to a study in the August 1 issue of the journal "Geophysical Research Letters."
"A lot of impacts on ozone are very subtle and happen over long periods of time," said Charles Jackman, a researcher at NASA Goddard Space Flight Center's Laboratory for Atmospheres and lead author of the study. "But when these solar proton events occur you can see immediately a change in the atmosphere, so you have a clear cause and effect."
Jackman and his colleagues examined impacts of a series of huge solar explosions on the atmosphere in the Northern Hemisphere with the help of data gathered by satellites operated by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA).
Jackman said, "You have to first be able to separate the natural effects on ozone, before you can tease out humankind's impacts."
Chlorine and bromine are linked to ozone decline which allows more of the Sun's UV rays to hit the Earth, resulting in more skin cancers and optical damage in humans and animals. Most of the chlorine and bromine comes from human produced compounds such as chlorofluorocarbons (CFCs) and halon gas.
Solar storms consist of coronal mass ejections and solar flares, the researchers explain. Coronal mass ejections are huge bubbles of gas ejected from the Sun and are often associated with these flares. Solar flares are explosions on the Sun that happen when energy stored in twisted magnetic fields, usually above sunspots, is suddenly released.
When protons like these bombard the upper atmosphere, they break up molecules of gases like nitrogen and water vapor. Once freed, those atoms react with ozone molecules and reduce the ozone layer.
When atmospheric winds blow nitrogen oxides down into the middle stratosphere, they can stay there for months, and continue to keep ozone at a reduced level.
When bombarded with protons from the Sun, water vapor molecules break up into forms where they react with ozone. These molecules, called hydrogen oxides, only last during the time period of the solar proton event.
But these short-term effects of hydrogen oxides can destroy up to 70 percent of the ozone in the another atmospheric layer known as the middle mesosphere.
Jackman and the other investigators used measurements from the Halogen Occultation Experiment (HALOE) instrument aboard the Upper Atmosphere Research Satellite (UARS) and the Solar Backscatter Ultraviolet (SBUV/2) instrument aboard the NOAA-14 satellite to obtain data on amounts of atmospheric gases like ozone and oxides of nitrogen in different layers of the atmosphere in the Northern Hemisphere.
The investigators then compared readings before and during the July 2000 solar event.
"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," Jackman said. While the solar event's impact on humans was negligible, it helped scientists verify their computer models.
On July 23, an advanced environmental satellite equipped with instruments to monitor Earth's weather and with a telescope that will be used to detect solar storms was launched from Cape Canaveral Air Force Station, Florida.
NOAA's Geostationary Operational Environmental Satellite (GOES-M) is the first of the GOES satellites equipped with a Solar X-ray Imager which will be used to forecast Earth space weather due to solar activity.
