Scientists have finally found direct evidence of small-scale torsional Alfvén waves in the Sun’s corona, solving an 80-year-old solar mystery. These magnetic waves could be the energy source that powers the Sun’s multi-million-degree outer atmosphere
A groundbreaking discovery in solar physics has provided the first direct evidence of small-scale torsional Alfvén waves in the Sun’s corona, elusive magnetic waves that scientists have been searching for since the 1940s. These findings, made possible by the world’s most powerful solar telescope, could finally explain the longstanding mystery of why the Sun’s outer atmosphere is millions of degrees hotter than its surface.
The research, led by Professor Richard Morton of Northumbria University and published in Nature Astronomy, utilised the U.S. National Science Foundation (NSF) Daniel K. Inouye Solar Telescope in Hawaii. The discovery of these continually present, small twisting waves suggests they may be the essential ingredient in heating the solar corona to temperatures exceeding one million degrees Celsius, dramatically hotter than the Sun’s surface, which is only around 5500°C.
Tracking Elusive Torsional Waves
Alfvén waves, named after Nobel Prize winner Hannes Alfvén who predicted their existence in 1942, are magnetic disturbances that transmit energy through plasma. While larger, isolated versions—often linked to solar flares—have been spotted previously, the small, ever-present torsional type remained unconfirmed until now.
Professor Morton described the breakthrough: “This discovery ends a protracted search for these waves that has its origins in the 1940s. We’ve finally been able to directly observe these torsional motions twisting the magnetic field lines back and forth in the corona.”
The challenge in spotting these waves is that the movement of plasma in the corona is largely dominated by swaying motions, or ‘kink’ waves, which visually mask the smaller, twisting torsional motions. Kink waves cause entire magnetic structures to sway and are visible in captured film of the Sun. In contrast, torsional Alfvén waves cause a twisting motion that can only be detected through spectroscopic analysis—measuring how plasma moves toward and away from Earth, which creates characteristic red and blue shifts on opposite sides of magnetic structures.
The power of the Inouye Solar Telescope
The breakthrough was achieved using the unique capabilities of the Daniel K. Inouye Solar Telescope’s Cryogenic Near Infrared Spectropolarimeter (Cryo-NIRSP). This cutting-edge instrument, highly sensitive to changes in plasma movement, allowed Professor Morton to track the movement of iron, heated to $1.6$ million degrees Celsius, in the corona.
The key to the discovery was Professor Morton’s development of entirely new analytical techniques to isolate the different types of wave motion in the data. He explained: “The movement of plasma in the sun’s corona is dominated by swaying motions… I had to develop a way of removing the swaying to find the twisting.”
With its four-meter-wide mirror, the Inouye Solar Telescope represents a massive leap in solar observation capabilities. Northumbria University played a crucial role in its development as part of a UK consortium that designed cameras for the telescope’s Visible Broadband Imager.
Implications for space weather
This research has profound implications for understanding not only how the Sun works but also for practical applications like space weather prediction. The superheated corona is the source of the solar wind, which fills our entire solar system. This solar wind carries magnetic disturbances that can disrupt satellite communications, GPS systems, and power grids on Earth.
Alfvén waves may also be the origin of ‘magnetic switchbacks’—significant carriers of energy in the solar wind that have been observed by NASA’s Parker Solar Probe.
“This research provides essential validation for the range of theoretical models that describe how Alfvén wave turbulence powers the solar atmosphere,” Professor Morton added. The team anticipates that this discovery will spark further investigations into how these waves propagate and dissipate their energy, opening new possibilities for studying wave physics in the solar atmosphere.
