First observation of a focused plasma wave on the Sun

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Numerical simulation of the MHD lens process at T/T0= 0.185 based on the observed geometric shape of the CH. Credit: Nature communication (2024). DOI: 10.1038/s41467-024-46846-z

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Numerical simulation of the MHD lens process at T/T0= 0.185 based on the observed geometric shape of the CH. Credit: Nature communication (2024). DOI: 10.1038/s41467-024-46846-z

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 through a telescope or microscope.

The find, published in Nature communicationcould be used to diagnose plasma properties, including “solar tsunamis” generated by solar flares, and in the investigation of plasma wave focusing of other astronomical systems.

The solar corona is the outer part of the Sun’s atmosphere, a region composed of magnetic plasma loops and solar flares. Made up mainly of charged ions and electrons, it extends millions of kilometers into space and has a temperature of over a 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: they heat the coronal plasma, accelerate the solar wind and generate 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 they were not observed to be focused.

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 one from Belgium analyzed data from a 2011 solar flare.

The flame caused almost periodic disturbances of high intensity that moved along the solar surface. The data, a form of magnetohydrodynamic waves, revealed a series of arc-shaped wave fronts with the center of the flame at the center.

This wave train propagated toward the center of the Sun’s disk, moving 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 220 miles 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 flows back toward the corona to 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.

Bottom left: a time-lapse of converging magnetohydrodynamic wavefronts (white) focused through the circular coronal hole on the left. Credit: Creative Commons Attribution 4.0 International License

In this observation, as the wave fronts moved through the far edge of the coronal hole, the original arc-shaped wavefronts changed into an anti-arc shape, with the curvature being reversed 180 degrees, from arcing outward to saddle-shaped outward. They then converged on a point focused on the opposite side of the coronal hole that resembled 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 outcome.

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

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

The focus 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 is therefore expected to occur in planetary, stellar, and galactic formations, similar to the gravitational lensing of light (of many wavelengths) observed around some stars.

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

Given these factors, numerical simulations explained the lens 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 for concentrating the energy of magnetohydrodynamic waves, similar to the scientific friction book [and movie] ‘The Three-Body Problem,’ using the sun as a signal booster,” 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.

More information:
Xinping Zhou et al., Resolved magnetohydrodynamic wave lenses in the solar corona, Nature communication (2024). DOI: 10.1038/s41467-024-46846-z

Magazine information:
Nature communication

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