[FRC] Sunbathing at Solar Maximum
Karen & Ted n3zrx
n3zrx@arrl.net
Wed, 06 Sep 2000 20:35:27 -0400
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Is it safe to make one last
dash to the beach before the end of Northern summer.">
<TITLE>Sunbathing at Solar Maximum</TITLE>
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<H1><CENTER>Sunbathing at Solar Max</CENTER></H1>
<H4><CENTER>NASA scientists say solar maximum is in full swing.
So, is it safe to make one last dash to the beach before the
end of Northern summer?</CENTER></H4>
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<P><A HREF="../images/sunbathing/ssn_predict_l.gif"><B><IMG
SRC="../images/sunbathing/ssn_predict_med.jpg" ALIGN="RIGHT"
BORDER="1" ALT="see caption" WIDTH="290" HEIGHT="216" NATURALSIZEFLAG="3"></B></A><B>Sept.
5, 2000</B> -- For many Americans, Labor Day weekend was the
last chance to hit the beach during the Summer of 2000. With
autumn just around the corner and school starting around the
country, droves of sunbathers spent their holiday perfecting
a last-minute tan. But was it safe? After all, the Sun is near
the peak of its 11-year activity cycle. There are more sunspots,
solar flares and UV rays from the Sun than at any time since
1989.</P>
<P>"We're in the maximum phase of the solar cycle now,"
says Dr. David Hathaway, a solar physicist at the NASA Marshall
Space Flight Center, "and it will probably persist for another
year or more. This one is somewhat smaller than the last two
maxima in 1989 and 1979, but it's definitely bigger than average."</P>
<P><B><FONT SIZE="-1" FACE="Arial">Above</FONT></B><FONT SIZE="-1"
FACE="Arial">: The sunspot number is soaring in the year 2000,
which marks the peak of another 11-year sunspot cycle. A white
curve displaying monthly-averaged International Sunspot Numbers
is overlaid on an x-ray image of the Sun. [<A HREF="http://science.msfc.nasa.gov/ssl/pad/solar/predict.htm">more
information</A>]</FONT></P>
<P>Fortunately for beach-goers, elevated levels of solar activity
around the peak of the sunspot cycle <I>do not</I> substantially
increase the risk of sunburns on Earth. Our planet's atmosphere
acts as a shield against the most harmful forms of radiation
-- and the shielding actually increases slightly near the peak
of the solar cycle. So, if you failed to buy any super-protective
<I>Solar Max Sunscreen</I> for your Labor Day on the beach, you
can relax. There was nothing to worry about beyond the usual
forms of <A HREF="http://text.nlm.nih.gov/nih/cdc/www/74txt.html">skin
cancer and exposure-related maladies</A>.<BR>
<BR>
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</TABLE>"There really isn't much going on over the course
of a solar cycle that will affect you sitting at the ground...because
the atmosphere shields you," says Dr. Richard McPeters,
principle investigator for NASA's Total Ozone Mapping Spectrometer
(TOMS) at the NASA Goddard Space Flight Center. In space, radiation
levels during solar maximum can be much higher than they are
at solar minimum, posing hazards to astronauts and orbiting satellites.
But, "when you get to the radiation that gets through the
atmosphere, the variation [between solar maximum and solar minimum]
is very small, less than a percent."</P>
<P>Ultraviolet (UV) radiation comes from the Sun in a range of
wavelengths; some are completely blocked by the atmosphere, while
others pass through to the Earth's surface. <BR>
<BR>
The light that scientists call UV has wavelengths that range
between 400 nanometers (nm) and 100 nm. A nanometer is a billionth
of a meter, or one-millionth of a millimeter.<BR>
<BR>
In general, the shorter the radiation's wavelength, the more
energy it contains and the more damaging it can be to people,
plants and animals. UV radiation with a wavelength of 400 nm
-- which is close to visible light -- is not dangerous to people,
while 100 nm UV -- which is closer to X-rays (less than 1 nm)
-- is very dangerous. Fortunately, atmospheric ozone, oxygen
and other gases block out the most dangerous UV photons at wavelengths
shorter than 300 nm.</P>
<P><CENTER><IMG SRC="../images/sunbathing/eit304.gif" WIDTH="398"
HEIGHT="135" ALIGN="BOTTOM" BORDER="1" NATURALSIZEFLAG="3" ALT="see caption"></CENTER></P>
<P><B><FONT SIZE="-1" FACE="Arial">Above</FONT></B><FONT SIZE="-1"
FACE="Arial">: This image, courtesy of Dr. Judith Lean at the
US Naval Research Laboratory, shows three extreme ultraviolet
(30 nm) pictures of the Sun captured by the ESA/NASA Solar and
Heliospheric Observatory at different times during the current
solar cycle. In 1996, near solar minimum, the extreme ultraviolet
Sun was nearly featureless. Near the peak of the cycle, the Sun
is dotted by fiery regions of hot gas trapped in magnetic fields
above sunspots and plages. These active regions produce copious
numbers of extreme ultra-violet and X-ray photons that are absorbed
in outer layers of our atmosphere before they reach Earth's surface.
<BR>
<BR>
"</FONT>The <I>extreme</I> ultraviolet photons that are
most intense during the peak of the solar cycle aren't the same
as the UV rays that give you sunburns," notes Dr. Judith
Lean, a physicist at the US Naval Research Laboratory. "Sunburns
come from the UV-A and UV-B bands around 300 nanometers. Extreme
ultraviolet photons from the Sun are at least 10 times more energetic
than UV-A and UV-B and <A HREF="../images/sunbathing/sunspectrum.htm">they
vary 100 times more</A> [between solar minimum and solar maximum].
It's a good thing they're all <A HREF="../images/reboost/atmosphere.gif">absorbed
by nitrogen and oxygen at high altitudes</A> -- otherwise a day
at the beach would be no fun."</P>
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<P>Heightened levels of solar ultraviolet radiation can actually
strengthen the shielding capacity of Earth's atmosphere by a
slight amount. <BR>
<BR>
The increase of UV radiation around 200 nm during Solar Max has
the effect of increasing ozone production in the stratosphere.
Ozone is formed in the stratosphere when high-energy UV radiation
splits oxygen molecules into two oxygen atoms. One of those atoms
then recombines with an oxygen molecule to form a three-atom
oxygen molecule: ozone. <BR>
<BR>
The exact amount of increase in stratospheric ozone is still
debated, but the total increase in the ozone above people's heads
is generally agreed to be about 1 or 2 percent during Solar Maximum.
This added ozone helps to offset the slight increase in UV radiation
at wavelengths that reach the ground. <BR>
<BR>
<A HREF="http://toms.gsfc.nasa.gov/ery_uv/ery_uv1.html"><IMG
SRC="../images/sunbathing/ee000903_med.gif" WIDTH="300" HEIGHT="232"
ALIGN="LEFT" BORDER="1" NATURALSIZEFLAG="3" ALT="see caption"></A>"The
combined effect of these opposite influences [heightened UV <I>plus</I>
heightened shielding] leads to a change in the 'erythemal weighting
function' -- a measure of the sunburn-causing power of the UV
striking the surface -- of nearly zero during Solar Max,"
says Dr. Jay Herman, a scientist at the Goddard Space Flight
Center who works with NASA's Total Ozone Monitoring Spectrometer
(TOMS). Erythema is the scientific word for skin reddening or
sunburn.</P>
<P>Spaceborne TOMS instruments are able to monitor erythemal
UV radiation levels on the ground. Every day a new map is published
at the TOMS web site. In areas of the globe marked by red, yellow
or pink (see above), prolonged exposure to the Sun can be dangerous.</P>
<P><B><FONT SIZE="-1" FACE="Arial">Above</FONT></B><FONT SIZE="-1"
FACE="Arial">: Daily updates of erythemal UV exposure around
the world can be found on the TOMS Web site. Colors indicate
the intensity of solar ultraviolet radiation reaching plant and
animal life after it has been filtered through stratospheric
ozone, clouds and aerosols multiplied by its biological damage
potential. Red, yellow and pink denote levels that can be especially
dangerous with prolonged exposure. [<A HREF="http://toms.gsfc.nasa.gov/ery_uv/ery_uv1.html">more
information</A>]</FONT><BR>
<BR>
"From the viewpoint of a beach-goer, the critical thing
is the degree of cloudiness," says Herman. "The reason
that August is so much more dangerous than April in the northern
hemisphere (the two months have approximately the same sun angle)
is because August usually has much less cloud cover than April.
However, a clear day in either month will have the same effect
on health."</P>
<P>"The day of the year is also important because the largest
variation of UV irradiance (after clouds) is caused by the slant
path of the radiation through the atmosphere. The slant path
is at a minimum around the time of the summer solstice,"
which leads to the largest UV exposure, explained Herman.<BR>
<BR>
Herman says that daily and weekly variations in the ozone layer
are much larger than the effect of the solar cycle. "Because
of wave activity in the stratosphere and troposphere, there is
a considerable variation as a function of longitude (at a fixed
latitude). The entire ozone field rotates about the Earth, relative
to a fixed point on the ground, in about 2 to 3 weeks. From the
viewpoint of an observer on the ground, it appears that the ozone
field overhead varies on a daily basis by a few percent at latitudes
of 30 degrees and much more at latitudes above 50 degrees,"
he explained.</P>
<P><A HREF="http://toms.gsfc.nasa.gov/ozone/today.html"><IMG
SRC="../images/sunbathing/product_sample_oz.gif" WIDTH="300"
HEIGHT="225" ALIGN="RIGHT" BORDER="1" NATURALSIZEFLAG="3" ALT="Map of global ozone distribution"></A><B><FONT
SIZE="-1" FACE="Arial">Right</FONT></B><FONT SIZE="-1" FACE="Arial">:
Readers with an internet connection can monitor ozone concentrations
for themselves, thanks to near-realtime global maps of ozone
column density published at the Earth Probe TOMS web site. In
this map, completed on Sept 4, 2000, the <A HREF="http://www.wmo.ch/web/arep/00/ozbull2.html">Antarctic
ozone hole</A> is prominent as a purple region surrounding the
south pole. [<A HREF="http://toms.gsfc.nasa.gov/ozone/today.html">more
information</A>]</FONT><BR>
<BR>
The largest changes at middle latitudes are those driven by the
Quasi Biennial Oscillation or QBO effect. "This effect is
caused by fairly low level winds that first blow eastward and
then westward on an approximately 2.3 year cycle," says
Herman. "This wind system causes ozone amounts at low and
middle latitudes to vary far more than the solar cycle effect."
A paper by Herman and collaborators just accepted for publication
in the <I>Journal of Geophysical Research</I> describes how QBO
oscillations cause multi-year changes in UV irradiance of ±15%
at 300 nm and ±5% at 310 nm at the equator and at middle
latitudes.</P>
<P><I>TOMS-EP and other ozone-measurement programs are important
parts of a global environmental effort of NASA's Earth Science
enterprise, a long-term research program designed to study Earth's
land, oceans, atmosphere, ice, and life as a total integrated
system.</I></TD>
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<B>Web Links</B></TD>
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<TR>
<TD BGCOLOR="#fffff0" VALIGN="TOP" WIDTH="100%">
<P><B><FONT SIZE="-1" FACE="Arial,helvetica"><A HREF="http://toms.gsfc.nasa.gov/">TOMS
Web Site</A></FONT></B><FONT SIZE="-1" FACE="Arial,helvetica">
-- data and information about the Total Ozone Mapping Spectrometer
(TOMS) instrument aboard NASA's Earth Probe (TOMS-EP) satellite.<BR>
<BR>
<B><A HREF="http://www.spaceweather.com/">SpaceWeather.com</A></B>
-- Daily updates and news about solar activity and all forms
of space weather.</FONT></P>
<P><A HREF="ast30may_1m.htm"><B><FONT SIZE="-1" FACE="Arial,helvetica">Solar
S'Mores</FONT></B></A><FONT SIZE="-1" FACE="Arial,helvetica">
-- As a result of the solar maximum, Earth's atmosphere is "puffed
up" like a marshmallow over a campfire leading to extra
drag on Earth-orbiting satellites.</FONT></P>
<P><A HREF="http://science.msfc.nasa.gov/ssl/pad/solar/predict.htm"><B><FONT
SIZE="-1" FACE="Arial,helvetica">Sunspot Cycle Predictions</FONT></B></A><FONT
SIZE="-1" FACE="Arial,helvetica"> -- from the NASA Marshall
Space Flight Center</FONT></P>
<P><A HREF="http://see.gsfc.nasa.gov/edu/SEES/strat/class/S_class.htm"><B><FONT
SIZE="-1" FACE="Arial,helvetica">Stratospheric Ozone: An Electronic
Textbook</FONT></B></A><FONT SIZE="-1" FACE="Arial,helvetica">
-- a comprehensive tutorial about the chemistry and dynamics
of the ozone layer</FONT></P>
<P><A HREF="http://code916.gsfc.nasa.gov/Space_based/sbuv/sbuv.html"><B><FONT
SIZE="-1" FACE="Arial">The Solar Backscatter Ultraviolet (SBUV)
instrument on NASA's Nimbus-7 satellite</FONT></B></A><FONT SIZE="-1"
FACE="Arial"> --Operating from November 1978 to June 1990, the
SBUV sensor measured the intensities of various UV wavelengths
backscattered from the Earth's atmosphere over the course of
approximately one solar cycle. Based on SBUV data scientists
concluded that over the course of an 11-year solar cycle, most
of the variation in radiation intensity occurs at wavelengths
shorter than 280 nm.</FONT></TD>
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