Elusive waves observed in Sun's corona

Physicists claim to have solved a perplexing mystery as to why the Sun's atmosphere is much hotter than its surface. The answer may lie in a type of solar plasma wave that had been predicted to exist, but never observed until now.

The Sun's corona is a kind of superheated atmosphere of ionised gas, or plasma, that extends millions of kilometres into space. Researchers have long been puzzled that corona - at a temperature of millions of degrees Kelvin - is up to 200 times as hot as the surface.

Difficult to detect

Now, a new study in the U.S. journal Science reports that a solar weather phenomenon called Alfvén waves have been observed for the first time and may partially explain the temperature discrepancy.

Named after a Swedish physicist who postulated their existence in 1942, these plasma waves are like the vibrations that travel along a perturbed rope.

"Alfvén waves have long been postulated as a possible mechanism to transfer energy out into the corona, but until now they have not been observed." said Steve Tomczyk who headed the research U.S. team at the National Centre for Atmospheric Research (NCAR) in Boulder, Colorado.

Part of the reason the waves have been so difficult to detect is that, like sound waves, they cannot be compressed and they do not alter the heat and brightness of the solar material through which they pass, said Tomczyk. However these properties are also what may allow them to transport energy without dissipating easily, he said.

Tomczyk's team approached the problem using a new polarimeter at the NCAR's High Altitude Observatory to image the surface of the Sun as never before, in narrow bands and using polarised light. The approach has allowed them to detect the characteristic pattern of Alfvén waves travelling across the images.

They found that waves are ubiquitous in the corona and may therefore be partly responsible for transferring heat to it.

Solar storm detection

Tomczyk argues that it will now be possible to use Alfvén waves to measure the strength and direction of magnetic fields in the corona, valuable as they directly control the ejection of solar matter and solar storms that reach Earth.

"This research provides the first convincing observations of vigorous and ubiquitous magnetic wave activity in the solar corona," commented Tom Bogdan with the U.S. National Oceanic and Atmospheric Administration Space Environment Centre in Boulder, Colorado. "Before this critical breakthrough, most of our inferences about the Sun's magnetic field relied on theoretical models often plagued by uncertainties and ambiguities."

According to Bogdan, the observation of Alfvén waves not only increases our understanding of the complex behaviour of the Sun but will also allows us to more accurately predict space weather, that can knock out our power grids or damage orbiting satellites.

"Timely warnings of 'solar tsunamis' will enable high-technology sectors of our global economy including aviation, power grid operators, the satellite industry and commercial space endeavours to secure their assets and operations," he said.

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