More than 1 million degrees! Why is it hotter the farther away from the sun?

The farther away from a heat source, the cooler the air. Strangely, this isn't the case with the Sun, and now scientists in New Zealand may have found the key reason why.

The surface temperature of the sun is about 6000 degrees Celsius, but within a few hundred kilometers from the surface of the sun, the temperature will suddenly rise to more than 1 million degrees Celsius, becoming the sun's atmosphere, that is, the corona.

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"The temperature is so high that the gas escapes the Sun's gravitational pull and becomes the 'solar wind', which flies out into space and hits the Earth and other planets," said Dr Jonathan Squire, from the University of Otago's Department of Physics, who led the study.

Researchers knew from measurements and theory that the sudden increase in temperature was linked to magnetic fields on the sun's surface. But how these magnetic fields heat the gas was unclear - a problem known as coronal heating.

"Astrophysicists have several different ideas about how magnetic field energy is converted to heat to explain this heating, but most have difficulty explaining some aspects of the observations," Squire said.

The prevailing theory is based on heating caused by turbulence and heating caused by a type of magnetic waves called ion cyclotron waves.

Squire and co-author Dr. Romain Meyrand, working with scientists at Princeton University in the United States and the University of Oxford in the United Kingdom, found that the two previous theories can be merged into one, solving a key part of the problem. The team's findings were published today in Nature Astronomy.

"However, there are some problems with both - turbulence has difficulty explaining why the hydrogen, helium and oxygen in the gas get as hot as they do, while the electrons are unexpectedly cold; and while the magnetic wave theory can explain this feature, it seems that there are not enough waves on the surface of the sun to heat the gas," said Meyrand.

Using six-dimensional supercomputer simulations of the coronal gas, the team was able to show that the two theories are actually part of the same process, linked together by a strange effect called the "spiral barrier."

This intriguing phenomenon was discovered in earlier research at Otago led by Meyrand.

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"If we imagine that plasma heating occurs like water flowing down a hill and the electrons are heated at the bottom, then the helical barrier acts like a dam that stops the flow of water and converts its energy into ion cyclotron waves. In this way, the helical barrier connects the two theories and resolves their respective problems," explains Meyrand.

In this latest study, the team stirred the magnetic field lines in simulations and found that the turbulence created waves, which then triggered heating.

When this happens, the structures and eddies that form end up looking remarkably similar to those measured by NASA's Parker Solar Probe, which recently became the first human-made object to actually fly into the solar corona.

"This gives us confidence that we have accurately captured key physics in the corona, which, combined with theoretical findings about the heating mechanism, provides an effective way to understand coronal heating."

Squire explained that learning more about the sun's atmosphere and subsequent solar wind is important because they have profound effects on Earth.

The effects of the solar wind interacting with Earth's magnetic field are known as "space weather," and they cause everything from auroras to satellite-damaging radiation and geomagnetic currents that knock out power grids. Fundamentally, these are all triggered by heating of the corona and its magnetic field.

“Perhaps, with a better understanding of the underlying physics, we’ll be able to build better models to predict future space weather and implement protection strategies that could avoid billions of dollars in damage,” Squire said.

Source: China Science Daily

Author: Li Muzi

Related paper information:

https://doi.org/10.1038/s41550-022-01624-z

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