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Sunspots intensified plasma waves

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Astronomers have proven that the waves in the plasma of the sun’s chromosphere are amplified when moving from the surface of the star due to the formation of acoustic resonators over the spots. Suitable conditions for resonance are formed due to high-temperature gradients at the edges of the chromosphere. The results may help to understand the main mystery of the sun’s physics – the mechanism of warming up the corona.

The atmosphere of the Sun consists of three main parts. Below all is the photosphere, the area of visible radiation formation, which is perceived by the eye as the size of a luminary. Above it is the chromosphere, in which most of the active processes, such as spicules and protuberances, arise. Above all is the corona – the most sparse, but at the same time the hottest outer shell of the Sun.

The most important mystery of the sun’s physics, to which scientists can not find an answer for more than half a century, lies in the high temperature of the corona. At the moment, two main competing hypotheses have been proposed to explain this phenomenon: a large number of small flares and magnetohydrodynamic waves in plasma. To confirm the first idea, an experimental recording is required, which is only possible through observations with high temporal and spatial resolution, and for the second is to find out the mechanism of the appropriate wave gain.

One of the proposed ways of amplification is related to sunspots that occur in the photosphere relative to dark areas. The theory suggests that acoustic resonators may form above them. It is known that in the spots themselves the temperature of the substance is lower than the surrounding, which is due to a powerful magnetic field, suppressing convection, which allows hotter plasma rise from the bowels of the star. These fields affect the plasma high above the spots, up to the very edges of the corona.

A team of astronomers led by David Jess from the British University of Belfast has confirmed the formation of sunspots suitable for the occurrence resonances of conditions. Observations and numerical simulations have shown that sharp temperature gradients at the edges of the chromosphere create partially transparent surfaces for waves. As a result, fluctuations can be repeatedly reflected from them, increasing in the process.

Scientists used variations in the brightness of individual elements’ radiation lines to determine the speeds and frequencies of waves. In the photosphere for this observed radiation of silicon, and at the top of the chromosphere – helium. It turned out that over the shadow of spots (the darkest central areas) there are stable fluctuations in the intensity of lines.

The Fourier spectrum of helium lines made it possible to distinguish three frequency regions with different properties, and a jump in the spectral energy density was observed between the second and third, and the slope of the spectrum in the third changed. The existence and parameters of the third region coincide with theoretical predictions for the presence of acoustic resonances.

The simulation successfully reproduced these features, but only if there is a sharp temperature gradient in the transitional layer between the chromosphere and the corona, which confirms the presence of a resonator. The analysis showed that the tilt of the spectrum depends on the position relative to the center of the spot: closer to it the slope is less sharp and becomes steeper to the boundary of the shadow and penumbra of the spot. The numerical simulations also allowed to determine the size of resonant cavities – about 2,300 kilometres in the center of the shadow of the spot and about 1,300 kilometres at its border.

The authors note that proof of the existence of resonators in the Sun’s atmosphere is important for several reasons. First, the parameters of the substance above the spots were heterogeneous, which should be included in the model of solar activity. Secondly, it brings us closer to understanding the true cause of crown warming up. Thirdly, such plasma resonant cavities can exist not only on the Sun but also in the Earth’s magnetosphere and other astrophysical conditions.

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