First observation of a focused plasma wave on the Sun

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Numerical simulation of the MHD lensing process at T/T0= 0.185 based on the observed CH geometry. credit: Nature Communications (2024). DOI: 10.1038/s41467-024-46846-z

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Numerical simulation of the MHD lensing process at T/T0= 0.185 based on the observed CH geometry. credit: Nature Communications (2024). DOI: 10.1038/s41467-024-46846-z

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 find appearing in Nature Communicationsit can be used to diagnose the properties of plasma, including “solar tsunamis” generated by solar flares, and in studying the focusing of plasma waves 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 stretches millions of kilometers into space and has a temperature of over one million Kelvin, and is especially noticeable during a total solar eclipse, when it is called a “ring of fire.”

Magnetohydrodynamic waves in the corona are oscillations in electrically charged fluids affected by the sun’s magnetic fields. They play a major role in the corona by 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 have not been observed to be focused until now.

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

The eruption produced high-intensity, near-periodic disturbances that moved across the solar surface. A form of magnetohydrodynamic waves, the data revealed a series of arcuate wavefronts with the center of the flare at their center.

This train of waves propagates toward the center of the Sun’s disk and moves 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 flows back to the corona to a region of opposite magnetic polarity, but sometimes the magnetic field allows the solar wind to escape into space much faster than the surface wave speed.

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

In this observation, as the wavefronts moved through the far edge of the coronal hole, the original arcuate wavefronts changed to an anti-arcform, with the curvature reversed 180 degrees, from curved outwards to saddle-shaped outwards. 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 wave properties, the corona, and the coronal hole confirmed that convergence was the expected result.

The group was only able to determine the variation in the amplitude of the intensity 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 prefocusing region to the region near the focal point, indicating that the coronal hole also focuses energy, just like a convex telescope lens.

The focus 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. Therefore, this type of magnetohydrodynamic lensing can be expected to occur in planetary, stellar, and galactic formations, similar to the gravitational lensing of light (of many wavelengths) that is 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 to be observed directly. The lensing effect is thought to be due to sharp changes (gradients) in coronal temperature, plasma density, and solar magnetic field strength at the coronal hole boundary, as well as the specific shape of the hole.

That being said, numerical simulations explain the lensing effect using 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 focus magnetohydrodynamic wave energy, much like a 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.

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

Log information:
Nature Communications

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