First observation of a focused plasma wave 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 through a telescope or microscope.

The finding appears in Nature communicationcould be used to diagnose plasma properties, including “solar tsunamis” produced by solar flares, and to study the focusing of plasma waves from other astronomical systems.

The solar corona is the outermost part of the Sun’s atmosphere, a region made up of magnetic plasma loops and solar flares. It is composed mainly of charged ions and electrons, extends millions of kilometers into space, and has a temperature of over a million Kelvin. It is particularly conspicuous during a total solar eclipse, when it is called the “ring of fire.”

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

It has already been observed that they undergo typical wave phenomena such as refraction, transmission and reflection in the corona, but their focusing has not yet been observed.

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 solar flare in 2011.

The eruption triggered strong, almost periodic disturbances that moved along the Sun’s surface. The data showed a type of magnetohydrodynamic waves and a series of arc-shaped wave fronts with the center of the burst in the middle.

This wave train propagated toward the center of the Sun’s disk and moved at a speed of about 350 kilometers per second through a coronal hole – an area of ​​relatively cool plasma – at a low latitude relative to the Sun’s equator.

A coronal hole is a temporary area of ​​cool, less dense plasma in the sun’s corona. Here the Sun’s magnetic field extends into space beyond the corona. Often the extended magnetic field returns to the corona, into an area of ​​opposite magnetic polarity. However, sometimes the magnetic field also allows a solar wind to escape into space much faster than the waves can sail at surface speeds.

In this observation, as the wave fronts moved through the outermost edge of the coronal hole, the original arc-shaped wave fronts changed into an anti-arc shape, with the curvature reversed by 180 degrees, from curved outward to saddle-shaped outward. They then converged to a point focused 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 as a magnetohydrodynamic lens.

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

The group was only able to determine the intensity amplitude fluctuation 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 almost seven times from the pre-focusing area to the area near the focal point, showing that the Coronal hole also focuses energy, just like a convex telescope 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 type of magnetohydrodynamic lensing can therefore be expected in planetary, star and galaxy formations, similar to the gravitational lensing of light (of many wavelengths) observed in 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 directly observed. The lensing effect is believed to be due to large changes (gradients) in 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.

With this in mind, numerical simulations explained the lensing effect using the methods of classical geometric acoustics, which are 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 focusing the energy of magnetohydrodynamic waves, similar to the scientific friction book.” [and movie] “The three-body problem, which uses the sun 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.

© 2024 Science X Network