Scientists from the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, and IIT Delhi have developed a new method to detect hidden turbulence in the Sun’s corona, potentially providing fresh clues to one of solar physics’ biggest puzzles — why the corona is much hotter than the Sun’s visible surface.
The study, published in The Astrophysical Journal, used three-dimensional magnetohydrodynamic simulations and forward modelling to show how transverse waves travelling through the corona’s magnetic structures can produce distinctive changes in spectral lines.
The corona is permeated by magnetic structures carrying Alfvénic, or kink, waves. As these waves move outward along magnetic field lines, they cause the structures to sway sideways. Scientists have traditionally associated such waves with alternating red and blue Doppler shifts, reflecting plasma motion towards and away from observers.
However, the new study shows that these waves can also produce asymmetric spectral signatures. This is significant because blueward asymmetries observed in solar spectral lines have often been associated with upward plasma flows, jets or other mass motions. Transverse waves, being nearly incompressible, were not generally expected to produce such effects.
Researchers Ambika Saxena and Vaibhav Pant simulated an open-field region of the corona containing density variations and introduced transverse waves at its lower boundary. As the waves propagated through the structured magnetic field, density differences led to phase mixing, generating increasingly fine-scale structures and turbulence.
The team then used forward modelling to examine the resulting emission in the Fe XIII 10749 Å spectral line. Because the corona is optically thin, light from different regions overlaps along the observer’s line of sight. Plasma moving at different velocities can therefore produce an asymmetric combined spectral signal.
The simulations produced alternating red and blue asymmetries whose pattern changed with height and time. The asymmetries reached up to 20% of the line’s peak intensity, while secondary plasma velocities ranged from 30 to 40 kilometres per second. The alternating red-blue patterns also travelled outward at speeds corresponding to the propagating waves.
The findings indicate that transverse MHD waves alone can generate systematic spectral asymmetries, without the need for large-scale plasma flows or jets. The researchers say the signatures could provide a new diagnostic for studying turbulence and wave-driven heating in the solar corona.