First observation of a focused plasma wave in the Sun

For the first time, scientists have observed plasma waves from a solar flare focused by a coronal hole, similar 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 Communicationsit can be used to diagnose plasma properties, including the “solar tsunami” created by solar flares, and in the investigation of focused plasma waves from other astronomical systems.

The solar corona is the outermost part of the sun’s atmosphere, a region composed of magnetic loops of plasma and solar flares. Composed mainly of charged ions and electrons, it stretches millions of kilometers into space and has a temperature of over a million Kelvin, and is particularly visible during a total solar eclipse, when it is called the “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 that leave the corona and travel into space.

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

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

The eruption excited high-intensity, quasi-periodic disturbances that moved along the solar surface. A form of magnetohydrodynamic waves, the data revealed a series of bow-shaped wavefronts with the center of the flare at the center.

This wave train spread towards the center of the solar disk and moved through a coronal hole – a region of relatively cold plasma – at a low latitude compared to the sun’s equator, at a speed of about 350 kilometers per second.

A coronal hole is a temporary region of cold, less dense plasma in the solar corona; here the sun’s magnetic field extends into space beyond the corona. Often the enhanced magnetic field returns to the corona in a region of opposite magnetic polarity, but sometimes the magnetic field allows a solar wind to escape into space much faster than the surface wave speed.

In this observation, as the wavefronts moved through the far edge of the coronal hole, the original bow-shaped wavefronts changed to an anti-bow shape, with the curvature reversed by 180 degrees, from outwardly curved to outwardly saddle-shaped. They then converged to a point centered 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 wave, corona, and coronal hole properties confirmed that convergence was the expected result.

The group was able to determine the change in the amplitude of the intensity of the waves only 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 center point between two and six times, and the energy flux density increased by a factor of almost seven from the pre-focusing region to the region near the center point, showing that the coronal hole also focused energy, much like a convex telescopic lens.

The focal point was about 300,000 km from the edge of the coronal hole, but the focus is not perfect because the shape of the coronal hole is not precise. Therefore, this type of magnetohydrodynamic lensing can be expected to occur with planetary, stellar, and galactic formations, much like the gravitational lensing of (multi-wavelength) light that has been observed around some stars.

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

With these in mind, numerical simulations explained the lensing effect through the methods of classical geometrical acoustics, used to explain the behavior of sound waves, similar to the geometrical optics of light waves.

“The coronal hole acts as a natural structure for focusing magnetohydrodynamic wave energy, similar to the scientific 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 Space Storm Numerical Prediction at the Harbin Institute of Technology in Guangdong, China.

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