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Sun Activity
The Sun provides the energy needed for life to exist on Earth. Every so often, sunspots and solar flares occur
on the Sun's surface and can cause disruptions in our daily lives. From the invention of the telescope in the
17th Century to NASA's Nimbus-7 satellite, innovations have allowed us to gaze into space and study the sun
and moon in amazing detail. The sun is constantly changing and we have been studying sunspots, solar flares
and other solar phenomena for hundreds of years.
The sun emits radiation across the entire electromagnetic
spectrum.
• Visible: This part of the spectrum, which we can detect
with our eyes, allows us to see and provides the energy
for plants to produce food by photosynthesis.
• Ultraviolet (UV): We cannot see this part of the
spectrum, but it can damage unprotected skin, producing
anything from a mild to severe burn to skin cancer.
• Infrared: This part of the spectrum is made up of
invisible rays that provide the heat that helps keep the
Earth warm.
• Charged Particles: The sun continuously emits energy
and particles that make up the solar wind. When the charged particles interact with the Earth's
magnetic field, particularly near the poles, the result is the aurora borealis, which is a spectacular
display of color in the night sky.
Sunspots - Sunspots are dark areas that form and disappear on the surface of the Sun over periods of days or
weeks. Sunspots are caused by concentrated magnetic fields that reduce the amount of energy flow to the
surface of the sun from its interior. The reduced energy flow causes the area to cool from about 10,800 °F
(the average temperature of the Sun's surface) to 7,600 °F. Because sunspots are cooler than the rest of the
Sun, they appear dark on the Sun's surface. Sunspots are so big that all of planet Earth would fit into a
supspot
Solar Flares - Solar flares are the release, in a single burst, of energy in many forms - electro-magnetic
(from radio waves through the visible spectrum to gamma rays and x-rays), energetic particles (protons and
electrons), and matter that is so hot it is in the form of plasma. Flares are characterized by their brightness in
x-rays. The National Oceanic and Atmospheric Administration monitors the x-rays from the Sun with
detectors on some of its satellites. Observations for the last few days are available at NOAA's website,
Today's Space Weather.
Flares are closely related to the cycles of the Sun's magnetic field, and they emerge from relatively cool,
intensely magnetic regions of the solar surface - sunspots.
The energy released during a flare is typically ten million times greater than the energy released from a
volcanic explosion. Even then, it only releases a fraction of the total energy emitted by the Sun every second.
The radiation and radioactive particles released during solar flare activity can damage satellites and interrupt
radio communications on Earth.
United States Environmental Protection Agency
Office of Radiation and Indoor Air (6608J) | EPA 402-F-07-049
June 2008
www.epa.gov/radtown/sun_activity.html
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RadTown USA
Coronal Mass Ejections (CMEs) - Coronal mass ejections are
the sudden release of large masses of plasma from the very hot
corona, which is the atmosphere just above the surface of the sun.
CMEs expand away from the Sun at speeds as high as 4 million
miles per hours! The light and x-rays accompanying a CME reach
earth in a few minutes. The mass of particles may take three to five
days to arrive. (The picture on the right, taken by the SOHO [Solar
and Heliospheric Observatory] spacecraft, shows a CME. A small
disk on the instrument blocks out the Sun itself so events in the
Sun's atmosphere can be studied.)
Coronal mass ejections are more likely to have a significant effect
on our activities than solar flares because they carry more material
into a larger volume of interplanetary space, increasing the
likelihood that they will interact with the Earth. CMEs typically
drive shock waves that produce energetic particles that can be damaging to both electronic equipment and
astronauts that venture outside the protection of the Earth's magnetic field.
Geomagnetic Storms - While a flare alone produces high-energy particles near the Sun, a CME can reach
the Earth and disturb the Earth's magnetosphere, setting off a geomagnetic storm. Often, these storms produce
surges in the power grid and static on the radio, and, if the waves of energetic particles are strong enough,
they can overload power grids and drown out radio signals. This type of activity can also affect ground to air,
ship to shore, and navigational communication, military detection, and early warning systems.
Observing the ejection of CMEs from the Sun provides an early warning of geomagnetic storms. Only
recently, with SOHO, has it been possible to continuously observe the emission of CMEs from the Sun and
determine if they are aimed at the Earth.
Potential Health Effects - Solar flares and coronal mass ejections result in the release of radiation across
the spectrum, from x-rays to light waves to fast-moving protons to plasma. We know that satellites can be
affected (even made non-functional) and astronauts need to be aware of the risk and seek shelter during these
storms. Astronauts on the Space Station receive increased exposure during these solar phenomena. The
energetic particles from a flare or CME would be dangerous to an astronaut on a mission to the Moon or
Mars. As for sunspots, they are merely cooler regions of the sun and do not cause any particular harm.
Out of all of the Sun's activities, it is actually the Sun's UV rays that pose the greatest risk to human health.
Who is protecting you?
Federal Aviation Administration (FAA)
Commercial airlines' navigational equipment can be affected by solar activity causing data on the location of
planes to be inaccurate. Fortunately, if navigators are alerted of the storm they can switch to a backup system
that does not depend on vulnerable technology. The Federal Aviation Administration routinely receives alerts
of solar flares so that they can anticipate communication and navigation problems and operate accordingly.
United States Environmental Protection Agency
Office of Radiation and Indoor Air (6608J) | EPA 402-F-07-049
June 2008
www.epa.gov/radtown/sun_activity.html
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RadTown USA
National Aeronautic and Space Administration (NASA)
NASA's Solar Particle Alert Network (SPAN) consists of multiple radio and optical telescopes that stream
continuous data on solar flare activity. While solar flare eruptions are difficult to predict, instruments, such as
those used in SPAN, can provide some warning as solar material makes its way from the Sun to the Earth.
This information also allows astronauts, who receive more radiation exposure in space, to move to well
shielded areas of their spacecraft.
U.S. National Oceanic and Atmospheric Administration (NOAA)
NOAA's Space Environment Center provides real-time monitoring and forecasting of solar and geophysical
events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and
geophysical disturbances.
What you can do to protect yourself
UV rays pose a much greater risk to human health than the radiation from the Sun's other activities. Here are
some of ways in which you can better protect yourself from the Sun's harmful UV rays:
• Cover Up: Wear tightly woven, loose-fitting, and full-length clothing.
• Wear Sunglasses that Block 99-100% of UV Radiation: Sunglasses that provide 99-100%
UVA and UVB protection greatly reduce sun exposure that can lead to cataracts and other eye
damage.
• Always Use Sunscreen: Apply a broad spectrum sunscreen with a Sun Protection Factor (SPF)
of 15 or higher liberally on exposed skin. Reapply every 2 hours, or after working, swimming,
playing, or exercising outdoors.
• Watch for the UV Index: The UV Index provides important information to help people plan
outdoor activities in ways that prevent overexposure to the sun. This information is commonly
found near weather predictions in newspapers and on the internet sites, like EPA's Sunwise UV
Index site.
Resources
You can explore this radiation source further through the resources at the following URL:
http://www.epa.gov/radtown/sun activitv.html#resources
We link to these resources to maintain up-to-date information.
United States Environmental Protection Agency Office of Radiation and Indoor Air (6608J) | EPA 402-F-07-049 June 2008
www.epa.gov/radtown/sun_activity.html
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