A new study confirms a long-held theory that large solar storms rain electrically charged particles down on Earth's atmosphere and deplete the upper-level ozone for weeks to months thereafter. New evidence from NASA and NOAA satellites is helping scientists better understand how man and nature both play a role in ozone loss.

The study, appearing in the August 1 issue of Geophysical Research Letters, examined impacts of a series of huge solar explosions on the atmosphere in the Northern Hemisphere. A solar flare with an associated coronal mass ejection sent positively-charged protons streaming to Earth from July 14 to 16th, 2000. The bombardment of protons, called a solar proton event, was the third largest in the last 30 years.

The sudden onset of a high energy proton blast from a solar flare is captured by the LASCO C3 instrument on board the SOHO spacecraft on July 14, 2000. The particles reached the spacecraft in less than one half hour and continued pelting it for days. The snow-like white spots are the protons hitting the spacecraft's imager. Photo: NASA/ESA

Solar storms consist of coronal mass ejections and solar flares. 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, and once freed, those atoms react with ozone molecules and reduce the layer.

"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."

The study's 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 event.

When the sun's protons hit the atmosphere they break up molecules of nitrogen gas and water vapor. When nitrogen gas molecules split apart, they can create molecules, called nitrogen oxides, which can last several weeks to months depending on where they end up in the atmosphere. Once formed, the nitrogen oxides react quickly with ozone and reduce its amounts. When atmospheric winds blow them down into the middle stratosphere, they can stay there for months, and continue to keep ozone at a reduced level.

On July 14, 2000, an active region of the sun (called AR9077) produced a massive flare. This striking close-up of AR9077 was made by the orbiting TRACE satellite shortly after the flare erupted. Suspended in an arcade of magnetic loops, the image shows a one million degree hot solar plasma cooling down. Plasma is a gas that has been heated to a state where it contains ions and free-floating electrons. Photo: NASA/Stanford-Lockheed Institute for Space Research

Protons similarly affect water vapor molecules by breaking them up into forms where they react with ozone. However, these molecules, called hydrogen oxides, only last during the time period of the solar proton event. These short-term effects of hydrogen oxides can destroy up to 70 percent of the ozone in the middle mesosphere. At the same time, longer-term ozone loss caused by nitrogen oxides destroys a maximum of about nine percent of the ozone in the upper stratosphere. Only a few percent of total ozone is in the mesosphere and upper stratosphere with over 80 percent in the middle and lower stratosphere.

"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 impacts to humans are minimal, the findings are important scientifically.

"Solar proton events help us test our models," Jackman said. "This is an instance where we have a huge natural variance. You have to first be able to separate the natural effects on ozone, before you can tease out human-kind's impacts."

Chlorine and bromine are major culprits in ozone decline. Most of the chlorine and bromine comes from human-produced compounds such as chlorofluorocarbons (CFCs) and halon gas.

NASA's HALOE was launched on the UARS spacecraft September 15, 1991 as part of the Earth Science Enterprise Program. Its mission includes improvement of understanding stratospheric ozone depletion by measuring vertical profiles of ozone, hydrogen chloride, hydrogen fluoride, methane, water vapor, nitric oxide, nitrogen dioxide, aerosols, and temperature. The SBUV/2 instrument was launched aboard the NOAA-14 satellite on December 30, 1994 and its mission is to observe the ozone layer.