The sun’s outer atmosphere is far more complex than previously thought
NEW ORLEANS — Despite its smooth appearance, the sun’s wispy outer atmosphere is surprisingly full of
knots, whorls and blobs.
Newly analyzed observations from NASA’s STEREO spacecraft show that the sun’s outer corona is just as
complicated as the highly structured inner corona, solar physicists reported December 12 at the fall meeting of
the American Geophysical Union. That previously unseen structure could help solve some of the sun’s biggest
puzzles, including how the solar wind is born and why the corona is so much hotter than the solar surface.
The corona is made up of charged plasma, which roils in famous loops and fans that follow magnetic field lines
emerging from the surface of the sun (SN Online: 8/17/17). At a certain distance from the sun, though, that
plasma escapes the corona and streams through the solar system as the solar wind, a constant flow of charged
particles that pummels the planets, including Earth (SN Online: 8/18/17).
But solar physicists don’t know where the plasma gets enough energy to accelerate away from the massive,
magnetic sun. And they don’t understand why the corona, which sizzles at several million degrees Celsius, has
such higher temperatures than the solar surface, which chills at a mere 5,500° C (SN Online: 8/20/17).
Both problems might be cleared up by better understanding an energetic process called reconnection, which
happens when magnetic field lines merge when they get too close to each other. Reconnection releases energy
and helps move plasma around, so the process could be important to heating the corona and driving solar wind.
But in the best observations until now, the outer corona appeared smooth and uniform. To explain that
smoothness, field lines would have to keep their distance from each other without a lot of reconnection. What’s
more, physicists couldn’t tell where the boundary between the corona and the solar wind began, which might
help to find that missing energy source.
“That’s changed,” solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo., said at
the AGU meeting. “Using STEREO, we’ve recently been able to drill in deeply enough to see the transition at
the outer edge of the corona, where the dynamics change from what we might call coronal plasma to what we
might call the young solar wind plasma.”
DeForest and colleagues collected data for three days with STEREO in 2014 to gain more detail about
small-scale changes in the outer corona than previously obtained. The researchers also processed the resulting
images in a new way to bring those changes into focus.
Surprisingly, the team found that the outer corona is full of moving blobs and fine streams of plasma that vary in
density by a factor of 10, suggesting that the magnetic field lines there are moving and merging more than
scientists thought. “It turns out the apparent smoothness is a reflection of our instruments, not the corona itself,”
DeForest says. “There’s almost certainly reconnection in the outer corona.”
The researchers also found that the corona probably fades into the solar wind between 14 million and 56 million
kilometers away from the sun — about 10 to 40 times the sun’s diameter. That’s still a big range, but NASA’s
Parker Solar Probe spacecraft, scheduled to launch in 2018, will fly right through that boundary. The probe will
swoop within 6.4 million kilometers of the sun and take the first direct measurements of the corona — and
perhaps figure out more precisely where the corona becomes the solar wind.
For now, the STEREO observations “are just tantalizing hints at an entire new set of phenomena,” DeForest says.
Understanding the details of those processes “is going to require both careful analysis from Parker Solar Probe
and also new, better imaging instruments.”
Solar physicist Steven Cranmer of the University of Colorado Boulder, who has made simulations of magnetic
reconnection in the outer corona, finds the results exciting. Questions about the sun’s hot corona and the
acceleration of the solar wind are still unsolved “not because of a lack of ideas, but because there are too many
ideas,” he says. “I think we're getting close to having the data that will let us rule out a good swath of these
proposed ideas.”
NEW ORLEANS — Despite its smooth appearance, the sun’s wispy outer atmosphere is surprisingly full of
knots, whorls and blobs.
Newly analyzed observations from NASA’s STEREO spacecraft show that the sun’s outer corona is just as
complicated as the highly structured inner corona, solar physicists reported December 12 at the fall meeting of
the American Geophysical Union. That previously unseen structure could help solve some of the sun’s biggest
puzzles, including how the solar wind is born and why the corona is so much hotter than the solar surface.
The corona is made up of charged plasma, which roils in famous loops and fans that follow magnetic field lines
emerging from the surface of the sun (SN Online: 8/17/17). At a certain distance from the sun, though, that
plasma escapes the corona and streams through the solar system as the solar wind, a constant flow of charged
particles that pummels the planets, including Earth (SN Online: 8/18/17).
But solar physicists don’t know where the plasma gets enough energy to accelerate away from the massive,
magnetic sun. And they don’t understand why the corona, which sizzles at several million degrees Celsius, has
such higher temperatures than the solar surface, which chills at a mere 5,500° C (SN Online: 8/20/17).
Both problems might be cleared up by better understanding an energetic process called reconnection, which
happens when magnetic field lines merge when they get too close to each other. Reconnection releases energy
and helps move plasma around, so the process could be important to heating the corona and driving solar wind.
But in the best observations until now, the outer corona appeared smooth and uniform. To explain that
smoothness, field lines would have to keep their distance from each other without a lot of reconnection. What’s
more, physicists couldn’t tell where the boundary between the corona and the solar wind began, which might
help to find that missing energy source.
“That’s changed,” solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo., said at
the AGU meeting. “Using STEREO, we’ve recently been able to drill in deeply enough to see the transition at
the outer edge of the corona, where the dynamics change from what we might call coronal plasma to what we
might call the young solar wind plasma.”
DeForest and colleagues collected data for three days with STEREO in 2014 to gain more detail about
small-scale changes in the outer corona than previously obtained. The researchers also processed the resulting
images in a new way to bring those changes into focus.
Surprisingly, the team found that the outer corona is full of moving blobs and fine streams of plasma that vary in
density by a factor of 10, suggesting that the magnetic field lines there are moving and merging more than
scientists thought. “It turns out the apparent smoothness is a reflection of our instruments, not the corona itself,”
DeForest says. “There’s almost certainly reconnection in the outer corona.”
The researchers also found that the corona probably fades into the solar wind between 14 million and 56 million
kilometers away from the sun — about 10 to 40 times the sun’s diameter. That’s still a big range, but NASA’s
Parker Solar Probe spacecraft, scheduled to launch in 2018, will fly right through that boundary. The probe will
swoop within 6.4 million kilometers of the sun and take the first direct measurements of the corona — and
perhaps figure out more precisely where the corona becomes the solar wind.
For now, the STEREO observations “are just tantalizing hints at an entire new set of phenomena,” DeForest says.
Understanding the details of those processes “is going to require both careful analysis from Parker Solar Probe
and also new, better imaging instruments.”
Solar physicist Steven Cranmer of the University of Colorado Boulder, who has made simulations of magnetic
reconnection in the outer corona, finds the results exciting. Questions about the sun’s hot corona and the
acceleration of the solar wind are still unsolved “not because of a lack of ideas, but because there are too many
ideas,” he says. “I think we're getting close to having the data that will let us rule out a good swath of these
proposed ideas.”
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