From July 14 to 16, 2000, the
surface of the sun exploded. Huge, bright flares spewed out into space like
powerful fountains colorfully lit from beneath. Within a few hours, the solar
storm bombarded Earth with a shower of positively-charged hydrogen atoms, called
protons, causing scientific and communications satellites to short-circuit.
Through a series of chemical reactions in our atmosphere, the protons
drastically diminished the upper-most areas of the ozone layer, a protective
blanket mostly in the stratosphere that blocks life-threatening ultraviolet
radiation from reaching the Earth. This shower of protons, known by solar
science insiders as the Bastille Day event, was the third largest of its kind in
the last 30 years.

Atmospheric scientist Charles
Jackman and a team of researchers from NASA’s Goddard Space Flight Center and
Hampton University in Virginia recognized a rare opportunity to gather further
proof that solar storms destroy ozone. They already knew that when protons
bombard the upper atmosphere, they break up molecules of gases like nitrogen and
water vapor. Once freed, those products readily react with ozone molecules and
reduce the ozone layer. So, Jackman and his colleagues recalled specific
Northern Hemisphere atmospheric data from NASA and National Oceanic and
Atmospheric Administration (NOAA) satellites that continuously monitor the
composition of gases and molecules that surround our planet. Their findings,
published in the August 1, 2001, issue of Geophysical Research Letters, show
that less than one percent of total atmospheric ozone in the Northern Hemisphere
can be quickly reduced by one of these events. “It is an indication of the power
of the sun to actually affect the atmosphere in a sudden, cataclysmic way,”
Jackman says. While the results do not show a significant impact on human
health, especially considering that most of the ozone loss documented in this
study occurs over the northern polar region, they are important scientifically.
The study gives detailed and quantified knowledge of how a solar storm affects
upper-level ozone. As scientists race to better understand humankind’s role in
ozone loss, they must first be able to tease out the natural causes.

“A lot of impacts on
ozone, like those caused by humans, are very subtle and happen over long periods
of time,” says Jackman.“But when these solar proton events occur you can see
immediately a change in the atmosphere.”

Jackman explains that ultraviolet
radiation from the sun continuously strikes the upper atmosphere. These harmful
ultraviolet rays would make life on Earth impossible if it weren’t for the ozone
layer that absorbs the radiation. Ozone forms when three atoms of oxygen bind
together to create a single molecule (O3), but when ozone absorbs
ultraviolet radiation, the molecules split apart into a single free oxygen atom,
and an oxygen molecule of two tightly bound oxygen atoms (O2). O2
is the oxygen we breathe, and it makes up about 21 percent of the Earth’s air.

“The free oxygen atom is so
reactive and there is so much O2 around, that out of 1001 times that
this reaction occurs, 1000 times it will reform,” Jackman says. As a result,
day-to-day break down of ozone by ultraviolet radiation doesn’t affect the
overall amount of it in the atmosphere.“The only way to destroy ozone is for
that free atom of oxygen to reform with something else,” Jackman says.

That’s where solar proton events
enter the picture. When protons from the sun hit the atmosphere they break apart
both water vapor and nitrogen gas, which accounts for 78 percent of our
atmosphere. The nitrogen gas molecules (N2) disconnect and leave two
free nitrogen atoms. Nitrogen atoms are highly reactive with O2,
creating oxides of nitrogen. Once formed, these molecules can last for weeks to
months depending on where they end up in the atmosphere before they get
destroyed. Protons also break up water vapor (H20) into a hydroxide
molecule (OH) and a free-floating single atom of hydrogen. Both of these
products also react easily with ozone and reduce its levels in the atmosphere.
Fortunately, oxides of hydrogen are short-lived and only stay together as long
as the rain of protons keeps coming.

The atmosphere’s layers include
the troposphere, the stratosphere, the mesosphere and the thermosphere. The
troposphere lies closest to Earth at zero to nine miles up, and the thermosphere
floats furthest away at about 60 miles up. Ozone is distributed over these
layers. About 90 percent lies within the stratosphere, and most of that
amount-over 80 percent of the total ozone-stays in the middle and lower
stratospheric regions. Around 9 percent can be found in the troposphere, and the
remaining one percent sits in the mesosphere.

By observing the Bastille Day
solar event, Jackman and his colleagues found that the short-term effects of
hydrogen oxides destroyed up to 70 percent of the ozone in the middle
mesosphere. “The mesosphere was really shaken,” Jackman says. At the same time,
ozone loss caused by longer-term nitrogen oxides cut out close to nine percent
of the ozone in the upper stratosphere. But, Jackman says, only a few percent of
total ozone resides in the mesosphere and upper 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 that doesn’t sound like a lot,
scientifically speaking the numbers for the specific atmospheric regions are
quite significant.

“This is an instance where we
have a huge natural variance,” Jackman says. “The ultimate goal of a lot of our
work is to understand the human impacts on ozone. In order to do that, you have
to first be able to separate the natural effects on ozone.”

Jackman, C. H.; McPeters, R. D.; Labow, G. J.; Fleming, E. L.; Praderas,
C. J.; Russell, J. M., Northern Hemisphere atmospheric effects due to the
July 2000 solar proton event,Geophysical Research Letters, August
1, 2001 (Vol. 28, No. 15, p. 2883)

Credit :NASA Earth Observatory