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4 mins read 01 Apr 2022

Scientists Solve a Mystery of the Sun’s ‘Solar Wind’ Problem

New Zealand scientists have linked two theories to explain a key reason why the Sun's temperature gets hotter further away from its surface. 

The sun and its corona. Credit: Scimex

For most sources of heat, the further you move away from them, the cooler it is, but not the Sun. Bizarrely the Sun gets remarkably warmer - reaching over a million degrees Celsius as altitude climbs to a few hundred kilometres from its surface. Now, scientists from The University of Otago, Princeton University, and Oxford University think they may know why this occurs.

“This is so hot that the gas escapes the Sun’s gravity as ‘solar wind’, and flies into space, smashing into Earth and other planets,” said study lead Dr Jonathan Squire from the Department of Physics at the University of Otago.

“We know from measurements and theory that the sudden temperature jump is related to magnetic fields which thread out of the Sun’s surface. But, exactly how these work to heat the gas is not well understood – this is known as the Coronal Heating Problem.”  

“Astrophysicists have several different ideas about how the magnetic-field energy could be converted into heat to explain the heating, but most have difficulty explaining some aspect of observations,” said Dr Squire. 

Dr Squire and co-author Dr Romain Meyrand and their collaborators found that two previous theories can be merged into one to solve the key piece of the problem. The two popular theories are that the heating could be caused by turbulence or that the heating could be caused by a type of magnetic wave called ion cyclotron waves.

“Both, however, have some problem – turbulence struggles to explain why hydrogen, helium and oxygen in the gas become as hot as they do, while electrons remain surprisingly cold; while the magnetic waves theory could explain this feature, there doesn’t seem to be enough of the waves coming off the Sun’s surface to heat up the gas,” said Dr Meyrand.

The researchers used six-dimensional supercomputer simulations of the Sun’s coronal gas to show that these two theories are actually part of the same process, linked together by an effect called the ‘helicity barrier’. The occurrence of the helicity barrier was discovered in an earlier study at the University of Otago, led by Dr Meyrand.

“If we imagine plasma heating as occurring a bit like water flowing down a hill, with electrons heated right at the bottom, then the helicity barrier acts like a dam, stopping the flow and diverting its energy into ion cyclotron waves. In this way, the helicity barrier links the two theories and resolves each of their individual problems,” explained Dr Meyrand.

In this most recent study, the researchers stirred the magnetic field lines in simulations and found that the turbulence created waves, which then caused the heating.

“As this happens, the structures and eddies that form end up looking extremely similar to cutting-edge measurements from NASA’s Parker Solar Probe spacecraft, which has recently become the first human-made object to actually fly into the corona,” said Dr Meyrand.

“This gives us confidence that we are accurately capturing key physics in the corona, which – coupled with the theoretical findings about the heating mechanisms – is a promising path to understanding the coronal heating problem” 

Understanding more about the Sun, its atmosphere, and the subsequent solar wind has profound impacts on Earth. The interaction between the solar wind and the Earth’s magnetic sphere is called ‘space weather’. Space weather can cause everything from auroras to satellite-destroying particles, and geomagnetic currents which damage the power grid.

“All of this is sourced, fundamentally, by the corona and its heating by magnetic fields, so as well as being interesting for our general understanding of the solar system, the solar corona's dynamics can have profound impacts on Earth,” said Dr Meyrand.

“Perhaps, with a better understanding of its basic physics, we will be able to build better models to predict space weather in the future, thus allowing the implementation of protection strategies that could head off – literally – billions of dollars of damage.”