First observation of a focused plasma wave on 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 finding, which appears in Nature Communicationsit could be used to diagnose plasma properties, including “solar tsunamis” generated by solar flares, and to investigate 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 loops of magnetic plasma and solar flares. Composed mainly of charged ions and electrons, it stretches millions of kilometers into space and has a temperature of over one million Kelvin, and is especially pronounced during a total solar eclipse, when it is called a “ring of fire”.

Magnetohydrodynamic waves in the corona are oscillations in electrically charged fluids under the influence of the Sun’s magnetic fields. They play a fundamental role in the corona, heating the coronal plasma, accelerating the solar wind and creating 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 the 2011 solar flare.

The flare caused high-intensity, almost periodic perturbations that moved along the sun’s surface. As a form of magnetohydrodynamic waves, the data revealed a series of bow-shaped wavefronts with the center of the flare at their center.

That train of waves propagated toward the center of the solar disk and moved through the coronal hole – a region of relatively cool 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 cold, 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 in 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 arc-shaped wavefronts changed to an anti-arc shape, with the curvature reversed by 180 degrees, from curved outward to saddle outward. They then converged to a point directed at the far side of the coronal hole, resembling a light wave passing through a converging lens, and the shape of the coronal hole acts as a magnetohydrodynamic lens.

Numerical simulations using wave, corona, and coronal hole properties confirmed that convergence is the expected result.

The group was able to determine the variation in the amplitude of the waves’ intensity only after a series of waves – a 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 nearly seven from the prefocusing region to the near-focal region, indicating that the coronal hole also focused the point energy, just like a convex telescope 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 correct. Therefore, this type of magnetohydrodynamic lensing can be expected to occur with planetary, stellar, and galactic formations, similar to the gravitational lensing of (multi-wavelength) light observed around some stars.

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

With this in mind, numerical simulations explained the effect of the lens 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 for focusing magnetohydrodynamic wave energy, much like a science textbook on friction [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 for Numerical Space Storm Prediction at the Harbin Institute of Technology in Guangdong, China.

© 2024 Science X Network