SOUNDS OF THE SUN / RELEVANT GLOSSARY
* DOPPLER EFFECT: The Doppler
effect occurs whenever a source of light or sound waves moves with
respect to the observer, and results in a change in the wavelength
and frequency of the waves. Sound waves are "pressed
together" when the source of the sound approaches and
"stretched out" when the source of the sound moves away.
It was named for J. Christian Doppler, an Austrian physicist, who in
1842 explained why the whistle of an approaching train had a higher
pitch than the same whistle when the train was going away. The
MDI instrument records small periodic changes in the wavelength of
light, measuring the oscillation speeds on the Sun using the Doppler
effect.
* HELIOSEISMOLOGY: Refers to
the science of using waves to learn about the Sun. Helios is Greek
for ‘sun’ or ‘light’; seismos means ‘tremor’; and logos
means ‘reasoning’ or ‘discourse’.
Just as seismologists learn about the interior of the Earth by
listening and watching earthquakes, helioseismologists study waves
from the Sun, although there are millions of times more waves to
observe and listen to.
* MDI INSTRUMENT: Sound waves
from active regions propagate through the Sun's interior and bounce
off the Sun's outer surface. The Michelson Doppler Imager (MDI)
instrument on the SOHO spacecraft can detect the extremely subtle
distortions these sound waves create on the Sun's surface. This data
helps deduce the location of an active region on the far side of the
sun.
* SOHO SATELLITE: The Solar
and Heliospheric Observatory (SOHO) orbits the Sun at a location
approximately one million miles from Earth to gain an unobstructed
view of the Sun. It carries 12 instruments and is a joint NASA
/ European Space Agency (ESA) mission.
* SOLAR FLARES & CME’s:
Active regions on the Sun produce explosions called solar flares,
and eruptions of plasma (hot, electrically charged gas) called
coronal mass ejections (CME’s). The radiation and plasma from
these events sweep past the Earth with the potential of disrupting
spacecraft, radio communications, and power systems. The Michelson
Doppler Imager (MDI) instrument on SOHO allows a week or so advance
warning of these potential solar storms by creating a window through
the Sun to its far side. (Note: CME’s are almost always what is
projected toward the Earth; solar flares typically remain on the
Sun)
* SPACE WEATHER: A branch of
science that watches activity on the surface of the Sun (like solar
flares) to track radiation in space. This radiation can come as
plasma (particles) or electromagnetic radiation (light). A typical
space weather forecast takes into account activity like the solar
flares and sunspots. Those who take special interest in knowing
space weather range from NASA planners (who would not want
astronauts to be exposed to intense radiation during spacewalks) and
satellite operators to radio station engineers and airline pilots
(the possibility of communication problems). NOAA (National Oceanic
& Atmospheric Administration) operates the Space Environment
Center (SEC) which gathers data in real-time and issues daily space
weather forecasts and alerts.
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