For decades, scientists have tried to understand how the Sun's corona (outer atmosphere) gets so blisteringly hot while its surface stays relatively balmy – and now a new study has delivered a big clue.
An international team of scientists reports the first clear evidence of small-scale torsional Alfvén waves all across the corona: These waves move through magnetic fields, twisting as they go, and transporting plasma upwards.
Until now, researchers had only detected larger, isolated Alfvén waves coinciding with solar flares. The presence of smaller Alfvén waves in the corona had been hypothesized but not directly observed.
These waves help explain how super-hot plasma travels from the Sun's surface where temperatures are around 5,500 °C (10,000 °F) to the corona – which hits millions of degrees Celsius – and then releases its energy.
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"This discovery ends a protracted search for these waves that has its origins in the 1940s," says physicist Richard Morton, from Northumbria University in the UK.
"We've finally been able to directly observe these torsional motions twisting the magnetic field lines back and forth in the corona."
The discovery was made possible by high-resolution imagery from the world's most powerful solar telescope, the US National Science Foundation Daniel K. Inouye Solar Telescope in Hawaii.
Instruments on the telescope enable it to detect the motion of solar plasma (charged particles) with exceptional precision. The plasma was tracked by looking for the movement of superheated iron – emitting bluer light signatures as material gets closer to Earth and redder light signatures as it moves away.
Once the researchers were able to remove the interference of other plasma wave motions swaying back and forth, the data revealed the travel of plasma and the twisting motion that the researchers were looking for.
"The movement of plasma in the Sun's corona is dominated by swaying motions," says Morton. "These mask the torsional motions, so I had to develop a way of removing the swaying to find the twisting."
These findings give us a much better idea of how the gigantic furnace of the Sun actually works, and feed into research about the solar winds that burst out of the Sun and travel all the way to Earth – potentially knocking out satellite networks and power systems.
Small-scale torsional Alfvén waves may be contributing to the forces needed to push these winds beyond the influence of the Sun's gravity, as well as helping the corona reach its ridiculously scorching temperatures.
Being able to see the processes in action and model them accurately means space weather forecasts can be improved, potentially giving us more warning of geomagnetic storms that could impact Earth.
Now that we've detected these small Alfvén waves, future work can look at their mechanisms and distributions in more detail, and across wider regions of the corona. Other theories about how the Sun works can also be more rigorously tested and investigated.
"This research provides essential validation for the range of theoretical models that describe how Alfvén wave turbulence powers the solar atmosphere," says Morton.
"Having direct observations finally allows us to test these models against reality."
The research has been published in Nature Astronomy.