Stormy
Space Weather Takes a Toll on Ozone
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on pic for animation of solar flare. | 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. 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.
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"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. 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. Back
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