Scientists have finally captured a solar wave phenomenon they have been looking for since the 1940s: small-scale torsional Alfvén waves in the solar corona. The so-called torsional Alfvén waves refer to rotating magnetic waves that propagate along magnetic field lines and twist back and forth like a twisted spring. They are believed to be able to transport large amounts of energy in the solar atmosphere.
This type of fluctuation was first proposed by Swedish physicist Hannes Alvin in 1942. The latest results were confirmed with the use of the world's most powerful solar telescope - the Daniel K. Inouye Solar Telescope in Hawaii - and published in "Nature Astronomy". It is expected to provide key clues to solve the long-standing problem of why the corona is much hotter than the surface of the sun.
The solar corona is the sun's outermost atmosphere, which extends millions of kilometers and is composed of extremely hot ionized plasma. Plasma is known as the "fourth state" of matter. In this state, after atoms acquire a huge amount of energy, electrons are stripped from around the nucleus to form charged particles that are highly dominated by magnetic fields. In comparison, the sun's visible light surface temperature is about 5,500 degrees Celsius, while the corona temperature is astonishingly more than one million degrees Celsius. This huge contrast gave rise to the famous "coronal heating problem." The plasma in the corona continues to flow outward, forming a supersonic flow of charged particles - the solar wind, which fills the entire solar system and shapes the heliosphere, and may disrupt artificial satellites, navigation systems and ground power grids. How the corona continues to gain enough energy to maintain such high temperatures has been hotly debated for decades.
Among many theories, Alfvén waves are regarded as one of the most important candidate mechanisms. In a plasma environment composed of a large number of "flux tubes" (narrow beam magnetic structures used to transport plasma and energy), the only "pure" Alfvén mode is the torsional mode, that is, twisting around the axis of the magnetic field lines, rather than simply swinging from side to side. These magnetic structures act like "rails" to constrain the movement of charged particles because plasma naturally flows along magnetic field lines. Richard Morton, a professor at Northumbria University in the United Kingdom and the leader of this study, said: "This discovery ends the long-term search that has continued since the 1940s. We can finally directly see the torsional motion of the magnetic field lines in the corona being twisted back and forth."
The breakthrough was made possible by the cryogenic near-infrared spectropolarimeter (Cryo-NIRSP) equipped with the Inoue telescope. The instrument is designed to observe the extremely fine magnetic and plasma structures in the corona. In this study, the team tracked the emission signals of iron elements heated to about 1.6 million degrees Celsius and developed new analysis methods to distinguish the twisting motion from the more common rocking motion. Morton explained: "The motion of plasma in the corona is dominated by rocking, which will mask the twisting motion, so I had to develop a method to 'peel' away the rocking component first before I could identify the true twisting."
Unlike "kink waves" that cause the entire magnetic structure to swing from side to side, torsional Alfvén waves mainly produce twisting motion around an axis and can only be identified through spectroscopy. Spectroscopy studies the interaction of matter and light, and in solar physics, scientists use the Doppler effect to accurately measure the tiny shifts in spectral wavelengths caused by the motion of plasma. Plasma moving toward the Earth causes a slight "blue shift" in the spectral lines, and a "red shift" away from the Earth. By analyzing red-shifted and blue-shifted signals in opposite directions on either side of the magnetic structure, the researchers were able to identify the torsional motion hidden in the corona. The data show that such twisting Alfvén waves persist even in relatively "quiet" regions of the corona.
The current measured wave amplitude is relatively small, but scientists believe that this underestimates the actual value to a certain extent due to the observation method. Even by conservative estimates, these twisting Alfvén waves are likely to carry a significant portion of the energy required to maintain high temperatures in the corona and drive the solar wind. "This study provides a key validation of a series of theoretical models of how Alfvén wave turbulence powers the solar atmosphere, and with direct observational data we can finally compare these models with reality," Morton said.
This discovery has implications not only for how we understand the sun itself, but also for space weather forecasting. The solar wind carries magnetic disturbances that affect satellite operations, global navigation systems, radio communications, and power transmission and distribution networks. The researchers noted that Alfvén waves may also explain the phenomenon of "magnetic switchbacks" observed by NASA's Parker Solar Probe - these are sudden magnetic field foldback structures thought to transport large amounts of energy in the solar wind. As the Inouye Solar Telescope continues to provide ultra-high-resolution coronal observation images, the scientific community is expected to further reveal in the coming years how these magnetic waves propagate, interact and release energy in the solar atmosphere, thereby deepening our understanding of the sun and the entire space environment.