First observation of a focused plasma wave on the sun

For the first time, scientists have observed plasma waves from a solar flare focused by a coronal hole, akin to the focusing of sound waves responsible for the Rotunda effect in architecture or the focusing of light by a telescope or microscope.

The finding, appearing in Nature Communications, could be used to diagnose plasma properties, including "solar tsunamis" generated by solar flares, and in the investigation of plasma wave focusing from other astronomical systems.

The solar corona is the outermost part of the sun's atmosphere, a region consisting of magnetic plasma loops and solar flares. Made mostly of charged ions and electrons, it extends millions of kilometers into space and has a temperature of over one million Kelvin, and is especially prominent during a total solar eclipse, when it is called a "ring of fire."

Magnetohydrodynamic waves in the corona are oscillations in electrically charged fluids influenced by the sun's magnetic fields. They play a fundamental role in the corona, heating the coronal plasma, accelerating the solar wind and generating powerful solar flares which leave the corona and travel into space.

They have previously been observed undergoing typical wave phenomena such as refraction, transmission and reflection in the corona, but until now, have not been observed being focused.

Using high resolution observations from the Solar Dynamics Observatory, a NASA satellite that has been observing the sun since 2010, a research group comprised of scientists from several Chinese institutions and one from Belgium analyzed data from a 2011 solar flare.

The flare excited large intensity, almost-periodic perturbations that moved along the solar surface. A form of magnetohydrodynamic waves, the data revealed a series of arc-shaped wavefronts with the flare's center at their center.

This wave train propagated towards the center of the solar disk and moved through a coronal hole— a region of relatively cool plasma—at a low latitude relative to the sun's equator, at a speed of about 350 kilometers per second.

A coronal hole is a temporary region of cool, less dense plasma in the solar corona; here the sun's magnetic field extends into space beyond the corona. Often the extended magnetic field loops back to the corona to a region of opposite magnetic polarity, but sometimes the magnetic field allows a solar wind to escape into space much more rapidly than the wave's surface speed.

In this observation, as the wavefronts moved through the far edge of the coronal hole, the original arc-shaped wavefronts changed to an anti-arc shape, with the curvature flipped by 180 degrees, from curved outward to saddle-shaped outward. They then converged to a point focused on the far side of the coronal hole, resembling a light wave passing through a converging lens, with the shape of the coronal hole acting as a magnetohydrodynamic lens.

Numerical simulations using properties of the waves, the corona and the coronal hole confirmed that convergence was the expected result.

The group was only able to determine the intensity amplitude variation of the waves after the wave train—the series of moving wave fronts—passed through the coronal hole.

As expected, the intensity (amplitude) of the magnetohydrodynamic waves increased from the hole to the focal point between two to six times, and the energy flux density increased by a factor of almost seven from the pre-focusing region to the region near the focal point, showing that the coronal hole also focused energy, just like a convex telescopic lens.

The focal point was about 300,000 km from the edge of the coronal hole, but the focusing is not perfect because the shape of the coronal hole is not exact. This kind of magnetohydrodynamic lensing can therefore be expected to occur with planetary, stellar and galactic formations, much like the gravitational lensing of light (of many wavelengths) that has been observed around some stars.

Although solar magnetohydrodynamic wave phenomena such as refraction, transmission and reflection in the corona have previously been observed, this is the first lensing effect of such waves that has been directly observed. The lensing effect is thought to be due to sharp changes (gradients) of the corona temperature, the density of the plasma and the solar magnetic field strength at the boundary of the coronal hole, as well as the particular shape of the hole.

Given these, numerical simulations explained the lensing effect through the methods of classical geometric acoustics, used to explain the behavior of sound waves, akin to the geometrical optics of light waves.

"The coronal hole acts as a natural structure for focusing the energy of magnetohydrodynamic wave, similar to the science friction book [and movie] 'The Three-Body Problem,' in which the sun is used as a signal amplifier," said co-author Ding Yuan of the Shenzhen Key Laboratory of Numerical Prediction for Space Storm at the Harbin Institute of Technology in Guangdong, China.

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