First observation of a plasma wave focused on the sun

For the first time, scientists have observed plasma waves from a solar flare focused through 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 a microscope.

The discovery, published in Natural communicationscould be used to diagnose plasma properties, including “solar tsunamis” generated by solar flares, and to study the focusing of plasma waves from other astronomical systems.

The solar corona is the outermost part of the solar atmosphere, a region made up of magnetic plasma loops and solar flares. Composed mainly of charged ions and electrons, it extends millions of kilometers into space and has a temperature of more than a million Kelvin. It 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 undergoing typical wave phenomena such as refraction, transmission and reflection in the corona, but until now they have not been observed in focus.

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

The flare caused almost periodic disturbances of high intensity that moved along the solar surface. A magnetohydrodynamic waveform, the data revealed a series of arc-shaped wave fronts with the center of the eruption at their center.

This wave train propagated toward the center of the solar disk and moved through a coronal hole (a region of relatively cold 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 returns to the corona toward a region of opposite magnetic polarity, but sometimes the magnetic field allows a solar wind to escape into space much faster than the surface speed of the wave.

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

Numerical simulations using the 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 variation in intensity and amplitude 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 and six times, and the energy flux density increased almost sevenfold from the pre-focusing region to the close to the focal point. point, showing that the coronal hole also concentrates energy, just 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 exact. This type 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) observed around some stars.

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

Considering this, numerical simulations explained the lensing effect through the methods of classical geometric acoustics, used to explain the behavior of sound waves, similar to the geometric optics of light waves.

“The coronal hole acts as a natural structure to concentrate the energy of magnetohydrodynamic waves, similar to the scientific friction book (and film) ‘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 Harbin Institute of Technology in Guangdong, China.

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