Hinode Satellite Sheds New Light On Alfven Waves
Images from NASA-funded
telescopes aboard a Japanese satellite have shed new light about
the sun's magnetic field and the origins of solar wind, which
disrupts power grids, satellites and communications on Earth.
Data from the Hinode satellite shows that magnetic waves play a
critical role in driving the solar wind into space. The solar wind
is a stream of electrically charged gas that is propelled away from
the sun in all directions at speeds of almost 1 million miles per
hour. Better understanding of the solar wind may lead to more
accurate prediction of damaging radiation waves before they reach
satellites.
The findings of the American-led international research teams
appear in the December 7 issue of the journal Science.
How the solar wind is formed and powered has been the subject of
debate for decades. Powerful magnetic Alfven waves in the
electrically charged gas near the sun have always been a leading
candidate as a force in the formation of solar wind since Alfven
waves in principle can transfer energy from the sun's surface up
through its atmosphere, or corona, into the solar wind.
In the solar atmosphere, Alfven waves are created when
convective motions and sound waves push magnetic fields around, or
when dynamic processes create electrical currents that allow the
magnetic fields to change shape or reconnect.
"Until now, Alfven waves have been impossible to observe because
of limited resolution of available instruments," said Alexei
Pevtsov, Hinode program scientist, NASA Headquarters, Washington.
"With the help of Hinode, we are now able to see direct evidence of
Alfven waves, which will help us unravel the mystery of how the
solar wind is powered."
Using Hinode's high resolution X-ray telescope, a team led by
Jonathan Cirtain, a solar physicist at NASA's Marshall Space Flight
Center, Huntsville, AL was able to peer low into the corona at the
sun's poles and observe record numbers of X-ray jets. The jets are
fountains of rapidly moving hot plasma. Previous research detected
only a few jets daily.
With Hinode's higher sensitivity, Cirtain's team observed an
average of 240 jets per day. They conclude that magnetic
reconnection, a process where two oppositely charged magnetic
fields collide and release energy, is frequently occurring in the
low solar corona. This interaction forms both Alfven waves and the
burst of energized plasma in X-ray jets.
"These observations show a clear relationship between magnetic
reconnection and Alfven wave formation in the X-ray jets." said
Cirtain. "The large number of jets, coupled with the high speeds of
the outflowing plasma, lends further credence to the idea that
X-ray jets are a driving force in the creation of the fast solar
wind."
Another research team
led by Bart De Pontieu, a solar physicist at Lockheed Martin's
Solar and Astrophysics Laboratory, Palo Alto, CA, focused on the
sun's chromosphere, the region sandwiched between the solar surface
and its corona. Using extremely high-resolution images from
Hinode's Solar Optical Telescope, De Pontieu's team found that the
chromosphere is riddled with Alfven waves. When the waves leak into
the corona, they are strong enough to power the solar wind.
"We find that most of these Alfven waves have periods of several
minutes, much longer than many theoretical models have assumed in
the past," says De Pontieu.
Comparisons with advanced computer simulations from the
University of Oslo, Norway, indicate that reconnection is not the
only source of the Alfven waves. "The simulations imply that many
of the waves occur when the sun's magnetic field is jostled around
by convective motions and sound waves in the low atmosphere,"
continued De Pontieu.
Hinode was launched in September 2006 to study the sun's
magnetic field and how its explosive energy propagates through the
different layers of the solar atmosphere. It is a collaborative
mission with NASA and the space agencies of Japan, the United
Kingdom, Norway and Europe and Japan's National Astronomical
Observatory.