Friday, March 31, 2017

Small-Scale Turbulence May Help Power Solar Explosions

The same sun that shines on bright, cheery days is also responsible for the biggest explosions in the solar system. These explosions, called solar flares, can detonate with the energy of more than one billion megaton bombs and spew dangerous radiation and high-energy particles into space.

An X2.7 class solar flare flashes on the edge of the sun on May 5, 2015. This image was captured by NASA's Solar Dynamics Observatory and shows the sun in a blend of two high-energy ultraviolet wavelengths. The Earth is shown to scale for reference.
Image Credit: NASA/GSFC/SDO.

Our distance from the Sun and the Earth’s magnetic field offer protection, but if the Earth is in the path of a big enough solar explosion, it could wreak havoc on the power grid, air traffic control systems, long-range communications, and more. Satellites and spacecraft can be destroyed by these events.

The key to protecting our infrastructure and space-based operations from these potential dangers lies in understanding the mechanism behind solar flares. This is easier said than done, but in an upcoming paper in the American Physical Society’s journal Physical Review Letters, a team of scientists show that turbulence in the plasma that surrounds the Sun probably plays an unrecognized but important role in the process.

Scientists generally agree that solar flares are caused by magnetic energy building up in the corona, the halo of plasma that surrounds the Sun. If the magnetic energy in a region builds up high enough, boom! Like a rubber band snapping, the stored magnetic energy is converted into rapidly moving electrons and ions, heat, and sometimes giant clouds of super-heated plasma. The particles, accelerated to nearly the speed of light, collide with other particles in the corona and emit radiation—the signature flash of a solar flare.

The missing link is this: How is magnetic energy converted into the kinetic energy of the particles?

Eight researchers from around the world joined forces to address this question. Drawn together “by a fascinating set of simultaneous observations,” according to team member Gordon Emslie from Western Kentucky University, they represent six languages, five solar instruments, and various areas of expertise on the Sun’s activity.

On May 15, 2013, a moderately large solar flare was captured by several different instruments*, resulting in radio wave maps, ultraviolet images, high-energy X-ray images, and ultraviolet spectroscopic observations of the same area of the Sun. Although flares are extremely bright and energetic, most of the activity takes place outside of the visible part of the electromagnetic spectrum.

NASA's Solar Dynamics Observatory captured this image of the X1.2 class solar flare on May 14, 2013 (May 15 in coordinated universal time). The image shows the Sun in high-energy ultraviolet light.
Image Credit: NASA/SDO.

The team studied these observations to see whether turbulence plays a role in the energy conversion process. This approach was based on two relationships suggested by previous theoretical studies and simulations. First, that turbulence can dramatically affect magnetic field activity. Second, that the magnetic properties of a plasma may play a key role in accelerating particles during solar flares. Combined, these strongly suggest that turbulence in the corona affects the magnetic properties of the plasma and therefore plays a key role in particle acceleration.

To see whether this might be the case, "we spent three years analyzing and checking the data" says lead author Eduard Kontar from the University of Glasgow. As often happens with collaborative science, most of the work was done remotely, but significant progress was made face-to-face in a pub one night over pints and notes scribbled on napkins.

By integrating the different sets of data, the team reconstructed the location, structure, and evolution of the solar flare. From this they could infer the path and distribution of the electrons and estimate their total kinetic energy at different times and locations. The team also estimated the total energy in the magnetic field of the region prior to the flare, and from this the amount of energy that could be converted into heat and accelerating particles.

Two important results stood out. First, signals from the area of the corona where most of the flare’s energy came from showed characteristic signs of turbulence. Second, the turbulent kinetic energy at this location peaked before the most of the electrons were accelerated. Together, these findings suggest that the magnetic energy builds up in the corona, flows into small-scale turbulent motions within the plasma, and then flows into the fast-moving particles. The turbulence seems to be like a reservoir that doesn’t contain a lot of energy, but allows energy to quickly flow in and out.

“The situation is like running a powerful shower into a bathtub with an open drain,” says Emslie.  “The bathtub is essential for channeling the water from the shower head into the drain.  And, although all the water flows from the shower through the drain via the bathtub, the amount of water in the bathtub is, at any given time, quite small.  Thus, although at any given time the energy in the turbulence is quite small compared to the total energy released, energy flows very quickly in and out of the turbulence, and, like the bathtub and the water, it provides a crucial conduit for the energy.”

This is the first clear evidence that turbulent motion plays an important role in transferring magnetic energy to accelerated particles in solar flares. Moving forward, scientists can test models of particle acceleration by turbulence against these results to see what best matches reality. The better we understand this process, the more likely it is that we will eventually be able to predict these events and protect our infrastructure and spacecraft from potentially catastrophic events.

Kendra Redmond

*The researchers combined data taken by instruments on three spacecraft (Reuven Ramaty High Energy Solar Spectroscopic Imager, Solar Dynamic Observatory, and Hinode) and at the ground-based Nobeyama Radio Observatory.

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