NASA’s Solar Dynamics Observatory Catches “Surfer” Waves on the Sun
06.07.11
Cue the surfing music. Scientists have spotted the iconic surfer's wave
rolling through the atmosphere of the sun. This makes for more than just
a nice photo-op: the waves hold clues as to how energy moves through
that atmosphere, known as the corona.
’ View larger
Surfer waves -- initiated in the sun, as they are in the water, by a
process called a Kelvin-Helmholtz instability -- have been found in the
sun's atmosphere. Credit: NASA/SDO/Astrophysical Journal Letters
Since scientists know how these kinds of waves -- initiated by a
Kelvin-Helmholtz instability if you're being technical -- disperse
energy in the water, they can use this information to better understand
the corona. This in turn, may help solve an enduring mystery of why the
corona is thousands of times hotter than originally expected.
"One of the biggest questions about the solar corona is the heating
mechanism," says solar physicist Leon Ofman of NASA’s Goddard Space
Flight Center, Greenbelt, Md. and Catholic University, Washington. "The
corona is a thousand times hotter than the sun's visible surface, but
what heats it up is not well-understood. People have suggested that
waves like this might cause turbulence which cause heating, but now we
have direct evidence of Kelvin-Helmholtz waves."
› Download video
On April 8, 2010, SDO recorded images of gas erupting through the sun's
atmosphere that formed the froth covered, curly waves that look like
surfing waves. Look for them rolling around the edges of the expanding
dark spots. Credit: NASA/Goddard Space Flight Center
Ofman and his Goddard colleague, Barbara Thompson, spotted these waves
in images taken on April 8, 2010. These were some of the first images
caught on camera by the Solar Dynamics Observatory (SDO), a solar
telescope with outstanding resolution that launched on February 11, 2010
and began capturing data on March 24 of that year. The team's results
appeared online in Astrophysical Journal Letters on May 19, 2011 and
will be published in the journal on June 10.
› View larger
K-H waves in the clouds Credit: Danny Ratcliffe
› View larger
The iconic surfer wave appears in between the bands of Saturn.
Credit: NASA/Cassini
› Click here to view simulation
This computer simulation shows how the conditions of erupting gas
flowing by stationary gas in the sun's atmosphere could give rise to
Kelvin-Helmholtz instabilities.
Credit: Ofman/Thompson/Astrophysical Journal Letters
That these "surfer" waves exist in the sun at all is not necessarily a
surprise, since they do appear in so many places in nature including,
for example, clouds on Earth and between the bands of Saturn. But
observing the sun from almost 93 million miles away means it's not easy
to physically see details like this. That's why the resolution available
with SDO gets researchers excited.
"The waves we're seeing in these images are so small," says Thompson who
in addition to being a co-author on this paper is the deputy project
scientist for SDO. "They're only the size of the United States," she
laughs.
Kelvin-Helmholtz instabilities occur when two fluids of different
densities or different speeds flow by each other. In the case of ocean
waves, that's the dense water and the lighter air. As they flow past
each other, slight ripples can be quickly amplified into the giant waves
loved by surfers. In the case of the solar atmosphere, which is made of
a very hot and electrically charged gas called plasma, the two flows
come from an expanse of plasma erupting off the sun's surface as it
passes by plasma that is not erupting. The difference in flow speeds and
densities across this boundary sparks the instability that builds into
the waves.
In order to confirm this description, the team developed a computer
model to see what takes place in the region. Their model showed that
these conditions could indeed lead to giant surfing waves rolling
through the corona.
Ofman says that despite the fact that Kelvin-Helmholtz instabilities
have been spotted in other places, there was no guarantee they'd be
spotted in the sun's corona, which is permeated with magnetic fields. "I
wasn't sure that this instability could evolve on the sun, since
magnetic fields can have a stabilizing effect," he says. "Now we know
that this instability can appear even though the solar plasma is
magnetized."
Seeing the big waves suggests they can cascade down to smaller forms of
turbulence too. Scientists believe that the friction created by
turbulence – the simple rolling of material over and around itself –
could help add heating energy to the corona. The analogy is the way
froth at the top of a surfing wave provides friction that will heat up
the wave. (Surfers of course don't ever notice this, as any extra heat
quickly dissipates into the rest of the water.)
Hammering out the exact mechanism for heating the corona will continue
to intrigue researchers for some time but, says Thompson, SDO's ability
to capture images of the entire sun every 12 seconds with such precise
detail will be a great boon. "SDO is not the first solar observatory
with high enough visual resolution to be able to see something like
this," she says. "But for some reason Kelvin-Helmholtz features are
rare. The fact that we spotted something so interesting in some of the
first images really shows the strength of SDO."
Karen C. Fox
NASA's Goddard Space Flight Center
› Back To Top