Mark Shwartz, News Service
Phil Scherrer, Department of Physics
March 13, 2006
New technique provides the first full view of the far side of the sun
By Mark Shwartz
The hidden face of the sun is fully visible for the first time, thanks
to a new technique developed at Stanford University.
Only half of the sun -- the near side -- is directly observable. The far
side always faces away from Earth and is therefore out of view. But the
new technology allows anyone with a computer to download images of the
entire solar surface -- an important advance with practical
applications, say researchers, because potentially damaging solar storms
that form on the far side now can be detected days, or even weeks,
before they wreak havoc on Earth.
"Sunspots, solar flares and other active regions on the surface of the
sun emit radiation that can interfere with orbiting satellites,
telecommunications and power transmission," says Philip Scherrer,
research professor in the Department of Physics. "This new method allows
more reliable warning of magnetic storms brewing on the far side that
could rotate with the sun and threaten the Earth."
It takes about 27 days for the sun to rotate on its axis, so an active
region that forms on the far side can remain hidden for up to 13 days
and surprise Earth-bound observers when it finally rotates into view.
That's what happened in October 2003, when active regions from the back
side suddenly appeared on the eastern edge of the sun, spewing X-rays,
ultraviolet radiation and high-energy particles into space. "We were not
able to make a public prediction about the intensity of that activity,
because at the time we could only image about a quarter to a third of
the far side," Scherrer says. "The new method allows us to see the
entire far side, including the poles."
Scherrer and his Stanford colleagues study the sun using data from the
Solar and Heliospheric Observatory (SOHO), a research satellite launched
in 1995 by NASA and the European Space Agency. On board SOHO is the
Michelson Doppler Imager (MDI), an electronic instrument that creates
images of the sun's interior by measuring the velocity of sound waves
produced by hot, bubbling gases that well up to the surface -- a
technique called acoustic helioseismology.
"Helioseismology works on the same principle as medical ultrasound,
which can create an image of a fetus inside a pregnant woman," Scherrer
explains. "In this case, we're looking through a star with sound waves."
Positioned about 1 million miles above Earth, the SOHO satellite always
faces the visible side of the sun. In 2000 and 2001, scientists Charles
Lindsey and Doug Braun -- now at NorthWest Research Associates Inc. --
developed two techniques that resulted in the first pictures of the
sun's back side. However, both techniques had limitations. One method
only produced images near the center of the far side, while the other
was restricted to views at the edges. To get a complete image,
researchers would have to combine both methods, but that proved to be a
major technical challenge.
The problem was finally overcome last summer when a new computer
algorithm was developed by the Stanford SOHO/MDI team in collaboration
with Kenneth Oslund, an undergraduate at the California Institute of
Technology. Their work resulted in the MDI Farside Graphics Viewer,
which displays the first full images of the far side of the sun. The
viewer is available online at
"This new method is a vast improvement," Scherrer says. "It should be
especially important during the next solar maximum, which should begin
in 2011, when solar activity will be at its peak."
He points out that during the last "solar max," which lasted from 2000
to 2003, solar storms temporarily knocked out power in the northern
parts of Sweden and Canada and destroyed a satellite that was used to
verify credit card payments at numerous gas stations in the United
States. Air transportation also can be disrupted when solar radiation
interferes with the operation of Global Positioning System satellites,
or when aircraft that take shortcuts over the North Pole have to take
longer routes to prevent passengers and crew from being exposed to
intense X-ray radiation.
"Our goal is to give pilots and air traffic controllers a day or two
notice of a possible solar event," Scherrer says, adding that missions
to Mars and other planets also can be affected when solar storms
interfere with satellite communications to Earth. Last week, researchers
at the National Center for Atmospheric Research in Colorado released new
computer models predicting that the next solar cycle will be 30 to 50
percent stronger than last time.
In 2008, SOHO is scheduled to be replaced by NASA's Solar Dynamics
Observatory (SDO), a more advanced satellite designed to provide new
data about the magnetic forces inside the sun that drive the 11-year
solar cycle. Stanford, the University of Colorado and the Lockheed
Martin Corp. will lead the SDO research effort.
"With cell phones and other devices, we've gotten more and more
dependent on the space environment, so there are real economic reasons
for missions like SOHO and SDO," Scherrer says.
Videos and high-resolution images of the far side of the sun are
Relevant Web URLs:
* Stanford Solar Center
* Solar and Heliospheric Observatory (SOHO)