Monster Waves Behind Sun's Coronal Heating Mystery?
One of the most enduring space mysteries is closer to home than you might think.
Why the sun's atmosphere is so hot has foxed solar physicists for decades, and while it may not appear to have the glamour of the mysteries of dark matter, dark energy and the hunt for the Higgs boson, this solar mystery is at the very core of space weather prediction, impacting our everyday lives.
And today, it has been announced that we may be one step closer to understanding why the sun's atmosphere - a.k.a. the corona - is over 20 times hotter than the solar surface and how the solar wind is accelerated to hundreds of kilometers per second.
"Because It's... Hot?"
The first question solar physicists are usually faced with when talking about the coronal heating problem is: "The sun's hot! Why are you surprised that its atmosphere is hot too?" It's not that the corona is hot that surprises solar astronomers, it's the fact that whereas the solar "surface" (the photosphere) is known to be a few thousand degrees Kelvin (or Celsius), the tenuous atmosphere just above has been measured in the millions of degrees Kelvin.
An analogy of this would be the air surrounding a light bulb being hotter than the light bulb's surface. It would actually get hotter as you moved your hand away from the bulb! That wouldn't make much sense, and the fact that the sun's atmosphere is hotter than the solar surface suggests something very weird is going on.
Countless theories have been proposed in the hope of explaining this weirdness. But a solid answer has yet to be found as the solar environment is so extreme, we can't simply send a spacecraft deep into the corona to take a look (although it is hoped that NASA's Solar Probe Plus will get close). So, solar physicists need to depend on observations taken by powerful telescopes on the ground and in space to zoom deep into the solar corona to see how it is energized.
Researchers from the National Center for Atmospheric Research (NCAR) in Boulder, Colo., Lockheed Martin Solar and Astrophysics Lab, University of Oslo, Norway, and Belgium's Catholic University of Leuven have now found strong evidence to suggest waves may be behind the coronal heating (and energization) problem.
"We now understand how hot mass can shoot upward from the solar interior, providing enough energy to maintain the corona at a million degrees and fire off particles into the high-speed solar wind," said Scott McIntosh, the study's lead author and a scientist at NCAR's High Altitude Observatory, in today's press release. "This new research will help us solve essential mysteries about how energy gets out of the Sun and into the solar system."
The study will be published this week in the journal Nature.
Plasma, Magnetism and Wavy Things
Full disclosure time: In 2006, I finished my solar physics Ph.D. thesis that focused on wave heating in the lower solar corona.
Specifically, I constructed computer models of coronal loops - those beautiful, brightly glowing arcs of solar plasma trapped in twisted magnetic field lines protruding from the sun's interior (as pictured, top) - and simulated the propagation of Alfvén waves along their lengths. (See: "Coronal loops heated by turbulence-driven Alfvén waves: A two fluid model," O'Neill and Li, A&A, 2005. DOI:10.1051/0004-6361:20041596)
Put simply, Alfvén waves are generally known to propagate along the magnetic field lines that project from the solar surface much like the vibrations along a guitar string. As solar magnetic field lines are bathed in solar plasma, these waves are an attractive mechanism to deliver huge quantities of energy from the solar interior to the corona.
In my coronal loop research, the resulting acceleration and heating of the plasma was then compared with observations. It was a very groovy project that used some of the mechanisms described in McIntosh's research, so I've been familiar with his work for some time.
The great thing is that, back in 2006, Alfvén waves in the corona were little more than theory. Although we've known for some time that they must exist deep inside the corona, there were no direct observations of their presence. However, in 2007, a year after I left physics research, they were positively observed by a team of researchers - including McIntosh - at the National Solar Observatory, New Mexico. (See: "Alfvén Waves in the Solar Corona," Tomczyk et al., Science, 2007. DOI:10.1126/science.1143304)
At the time, however, the identified Alfvén waves were measured to have an amplitude of 0.5 kilometers per second - this was too low for Alfvén waves to be the key carrier of energy from the solar interior and injecting it into the corona.
But now, using the awesome power of NASA's Solar Dynamics Observatory (SDO), high definition images of the solar corona were captured and the true extent of Alfvén wave activity in the corona has been revealed.
A Solar Microscope
"It's like getting a microscope to study the sun's corona, giving us the spatial and temperature coverage to focus in on the way mass and energy circulate," said McIntosh.
The SDO revealed the motion of Alfvén waves as they caused high-speed jets of material, known as spicules, to sway. Measuring the amplitude of this swaying - a feat only possible through the high-definition eyes of SDO - McIntosh's team was able to deduce the nature of these Alfvén waves.
As reported by Discovery News in January 2011, McIntosh discovered a previously unseen type of spicule ("Type II") that gets heated very rapidly to coronal temperatures shortly after leaving the solar surface. Combined with this most recent finding, it appears the Alfvén waves are intrinsically involved - they "ride" the spicules, delivering heating energy into the spicule plasma to coronal temperatures.
By analyizing the SDO images of swaying spicules, the team has detected Alfvén waves a hundred times more powerful than the 2007 observations, with amplitudes of 20 kilometers per second. This key observation reveals Alfvén waves that are more than capable of injecting sufficient quantities of energy, and driving mass, into the corona from the sun's interior, thus heating the sun's atmosphere and accelerating the solar wind.
From the Sun to Earth
But where do these waves come from?
"They are generated by the stirring and boiling of the photosphere dragging and plucking the magnetic field," McIntosh told me via email. This basically means the motion from the solar interior passes energy though the photosphere, generating the waves that pump energy along magnetic fields, high into the sun's atmosphere.
Although the wave generation process may sound fairly straightforward, how the energy passes from wave to plasma is less well understood. "We don't know how the waves lose their energy to the plasma - there are many theories hypothesized," McIntosh continued. "Up until now the waves have not been observed to be energetic enough in the corona so I guess that wasn't on the radar. I expect to see a flurry of activity in this area, hopefully we'll learn a lot."
All this talk of Alfvén waves, spicules and magnetic fields may sound alien to us. After all, what does that have to do with life on Earth?
"The sun's corona - and the mass cycle between the chromosphere and corona - creates the solar wind that creates some of the auroral activity on Earth and produces the [ultraviolet and extreme-ultraviolet emissions] that can impact upper atmospheric chemistry," McIntosh said.
Therefore, what happens deep in the corona, doesn't stay deep in the corona.
The energetic mechanisms that blast matter and energy into space alter the interplanetary environment our planet lives in, so the quicker we can resolve the coronal heating problem, the better we can forecast the impact of solar storms that can affect our everyday lives.
Image (top): The million degree plasma of the sun's corona as seen by NASA's Solar Dynamics Observatory (NASA/SDO/AIA)