Recently, a research result completed by a multinational research team was published in Nature Astronomy, reporting an important breakthrough in the field of solar and heliospheric physics. The team used the Daniel K. Inouye Solar Telescope (DKIST) of the National Science Foundation of the United States to directly observe the ubiquitous torsional Alfvén waves in the outermost atmosphere of the Sun – the corona (see Figure1) for the first time [1]. This discovery provides new observational evidence for answering the core question of "why the corona can maintain a temperature of millions of degrees", and also demonstrates the unique advantages of near-infrared spectroscopy in the study of solar and heliospheric physics.
Fig. 1: Coronator torsional Alfvén wave observation. The left figure shows the observation area and instrument slit position of the DKIST telescope. The upper right figure shows the changes in radiation intensity and Doppler velocity along the slit over time. The middle right figure is a schematic diagram of the torsional Alfvén wave. The lower right figure shows the results of the numerical simulation, which reproduces the anti-phase Doppler signals on both sides of the magnetohydrodynamic tube during the observation (image courtesy of the National Solar Observatory of the United States, created by Tom Schad and John Williams based on relevant content in reference [1]) .
Since Hannes Alfvén, a Nobel laureate in physics, proposed the theory of magnetohydrodynamic waves in the1940s, Alfvén waves have been considered an important energy carrier driving coronal heating and solar wind acceleration. However, Alfvén waves exist in the corona as torsional oscillations, and their observational signals are extremely elusive – they are neither accompanied by obvious brightness changes nor cause any visible structural displacement. The only detectable clue is the Doppler velocity signals on both sides of the magnetohydrodynamic tubes (common columnar plasma structures in the corona) along the line of sight. To resolve these anti-phase velocity characteristics within the delicate magnetohydrodynamic tube structures of the corona requires extremely high spatial resolution, which is precisely what past solar observation equipment has struggled to achieve.
DKIST telescope, which officially began operation in 2022, has a4-meter aperture and is currently the world's largest and highest-resolution solar optical telescope. Its Cryo-NIRSP near-infrared coronal imaging spectrometer enables highly sensitive spectroscopic measurements at the Fe XIII 1074.7 nm spectral line, providing an unprecedented window into the existence of Alfvén waves in the corona.
During an observation in 2023, the research team used the DKIST telescope to conduct several hours of spectroscopic observations of the open magnetic field region of the solar corona. Through detailed data analysis, they detected periodic, out-of-phase Doppler oscillations on both sides of the coronal ring structure, perfectly consistent with the theoretical characteristics of torsional Alfvén waves. To verify the reliability of the observation results, the research team further conducted three-dimensional magnetohydrodynamic numerical simulations and synthesized observable near-infrared spectral features based on the simulation results. The simulation results successfully reproduced the main characteristics of velocity distribution and temporal evolution observed in the observations, confirming the reliability of the observational analysis results and laying the foundation for future research on the energy transport and dissipation mechanisms of Alfvén waves.
The study also shows that these torsional Alfvén wave signals are ubiquitous within the observation field and can persist for a considerable period. Based on preliminary estimates, the observed torsional Alfvén waves and their accompanying twisted waves (lateral oscillations of the magnetohydrodynamic tube) carry an energy flux density exceeding100 W·m, sufficient to meet the heating requirements of the corona in the quiet region, demonstrating the potential of these waves as channels for transporting energy in the solar atmosphere.
This achievement highlights the unique scientific value of conducting solar spectral observations in the near-infrared band. Traditional visible light observations primarily target the lower atmosphere of the Sun (photosphere and chromosphere) , while the extreme ultraviolet band, although suitable for corona observation, relies on space-based satellite platforms and is limited by instrument aperture and resolution. In contrast, the near-infrared band allows for high-resolution observations from theground and includes "forbidden" radiation formed in the high-temperature corona (such as the Fe XIII 1074.7 nm spectral line, corresponding to a temperature of approximately 1.6 MK) , thus becoming an ideal window for studying coronal dynamics and magnetic fields.
In addition, near-infrared observation also provides a breakthrough for measuring the coronal magnetic field – another long-standing problem in solar physics. In 2020, a collaborative team led by Chinese scholars used near-infrared spectral observations of the coronal multi-channel polarimeter (CoMP) to create the first global coronal magnetic map [2-3]. They diagnosed the coronal density by the intensity ratio of the two spectral lines of Fe XIII at 1074.7 nm and 1079.8 nm, and inverted the magnetic field intensity by combining the propagation velocity of the tortuous wave observed by CoMP. On this basis, the team further acquired more than 100 global coronal magnetic maps from February to October 2022, achieving a coronal magnetic field monitoring frequency of about once every two days, and initially completed the routine measurement of the coronal magnetic field [4]. This achievement extended the measurement range of the solar magnetic field from the photosphere to about 1.6 solar radii, laying the foundation for studying the evolution of the solar magnetic field and space weather prediction.
From the first detection of torsional Alfvén waves by the DKIST telescope to the measurement of the coronal magnetic field by CoMP, near-infrared spectroscopy is becoming a new frontier in coronal physics research. These findings collectively form an important piece of the puzzle in understanding the nature of coronal heating and solar activity. With the continuous development of ground-based and space-based observation instruments, humanity is getting closer and closer to obtain the final answer to the "coronal mystery." It is foreseeable that in the coming years, joint observations by high-resolution near-infrared spectrometers and other next-generation instruments will further advance our understanding of solar magnetic activity and its interactions with Earth/planetary systems.