First observation of a concentrated 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 find, appearing in The nature of communicationcould be used to diagnose plasma properties, including “solar tsunamis” generated by solar flares, and in investigating 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. Composed mostly of charged ions and electrons, it extends millions of kilometers into space and has a temperature of over one million Kelvin, and is particularly 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 influenced by the Sun’s magnetic fields. They play a vital 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 in typical wave phenomena such as refraction, transmission and reflection in the corona, but have not yet been observed in focusing.

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

The flash produced high-intensity, almost 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 propagated toward the center of the Sun’s disk, moving through the coronal hole—a region of relatively cool plasma—at a low latitude relative to the Sun’s equator at 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 the space behind the corona. The extended magnetic field often flows back into the corona into 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.

In this observation, as the wavefronts moved through the far edge of the coronal hole, the original arcuate wavefronts changed to an anti-arc shape, with the curvature reversed by 180 degrees, from curved outward to saddle-out. They then converged to a focused point 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, corona and coronal holes confirmed that convergence was the expected result.

The team was able to determine the variation in the amplitude of the wave intensity after the wave train – a series of moving wave fronts – passed through the coronal hole.

As expected, the intensity (amplitude) of the magnetohydrodynamic waves increased two to six times from the aperture to the focus, and the energy flux density increased almost seven times from the prefocus region to the near-focus region. point, indicating that the coronal hole also focused the energy, as did the convex telescope 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 precise. Thus, this kind of magnetohydrodynamic lensing can be expected to occur in planetary, stellar, and galactic bodies, similar to the gravitational lensing of light (many wavelengths) that has been observed in some stars.

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

Because of this, numerical simulations explained 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 in a science book on friction. [and movie] “A ‘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 the Harbin Institute of Technology in Guangdong, China.

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