Scientists have been trying to figure out why the Sun's outer atmosphere, or corona, gets so hot while its surface remains relatively cool for decades. Now, a new study, published in Nature Astronomy, has provided a significant hint.
The first convincing evidence of small-scale torsional Alfvén waves throughout the corona has been reported by an international team of scientists. These waves travel through magnetic fields, twisting as they go and carrying plasma upward.
Up until now, scientists had only found isolated, larger Alfvén waves that accompanied solar flares. Although not directly observed, the existence of smaller Alfvén waves in the corona had been conjectured.
These waves aid in the explanation of how extremely hot plasma moves from the Sun's surface, where temperatures hover around 5,500 °C (10,000 °F), to the corona, where temperatures reach millions of degrees Celsius, before releasing its energy.
"This discovery ends a protracted search for these waves that has its origins in the 1940s," said 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 US National Science Foundation Daniel K. Inouye Solar Telescope in Hawaii, the most potent solar telescope in the world, provided high-resolution imagery that enabled the discovery.
The telescope's instruments allow it to make incredibly accurate motion detections of solar plasma or charged particles. By searching for the movement of superheated iron, which emits redder light signatures as material moves away from Earth and bluer light signatures as material approaches, the plasma was tracked.
The data showed the travel of plasma and the twisting motion that the researchers were searching for after they were able to eliminate the interference of other plasma wave motions swaying back and forth.
"The movement of plasma in the Sun's corona is dominated by swaying motions," Morton said, adding, "These mask the torsional motions, so I had to develop a way of removing the swaying to find the twisting."
These discoveries improve our understanding of how the Sun's enormous furnace functions and contribute to studies on the solar winds that emerge from the Sun and make their way all the way to Earth, where they have the potential to disrupt power and satellite networks.
It is possible that small-scale torsional Alfvén waves are helping the corona reach its absurdly hot temperatures and providing the forces required to push these winds past the Sun's gravitational pull.
Space weather forecasts can be enhanced by seeing the processes in action and accurately modeling them, which could provide us with more warning of geomagnetic storms that could affect Earth.
Future research can examine the mechanisms and distributions of these tiny Alfvén waves in greater detail and over a larger area of the corona now that we have detected them. More thorough testing and research can be done on other hypotheses regarding the Sun's operation.
"This research provides essential validation for the range of theoretical models that describe how Alfvén wave turbulence powers the solar atmosphere," concluded Morton.
"Having direct observations finally allows us to test these models against reality."
