The observations were made with the Visible Spectro-Polarimeter, or ViSP, an instrument attached to the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii, the largest solar telescope on Earth with its four-meter primary mirror. The team targeted an active-region plage within NOAA active region 14153 on July 25, 2025, during the telescope’s third observing cycle. Plage regions are fields of strong, nearly vertical, mostly unipolar magnetic flux rooted in the photosphere, and they are precisely the kind of environment where magnetic elements, jostled by convective currents below, can act as conduits for upward-traveling wave energy. What made these observations special was their cadence and their two-line strategy: the instrument recorded spectra in the neutral sodium line at 5896 angstroms (Na I D1) and the ionized calcium line at 8542 angstroms (Ca II) simultaneously, sampling two different layers of the low and middle chromosphere in a single scan.

Over roughly 36 minutes, the telescope swept a narrow slit across the target in a rapid three-step raster, revisiting the same locations every 19.6 seconds. That cadence is fast enough to capture oscillations in the 2.5 to 5.5 millihertz band, corresponding to periods of roughly three to seven minutes — the classic range where solar acoustic and magnetoacoustic oscillations live. Because the Sun was observed at an oblique angle rather than face-on, the measurements also carried sensitivity to transverse motions, a detail that would prove important when the team tested whether the waves could be of the Alfvénic, twisting variety.

Extracting clean measurements from these data was itself a technical tour de force. The Na I D1 line profiles were well behaved, showing a single clear absorption core that could be fitted with a simple quadratic curve to yield precise line-core intensities and Doppler velocities. The Ca II 8542 line, by contrast, is notoriously temperamental: its profiles frequently displayed asymmetries, multiple minima, emission shoulders, and partial self-reversals. Rather than forcing one method onto everything, the researchers deployed an adaptive, morphology-aware strategy, switching between quadratic fits, a center-of-gravity technique, and a proper orthogonal decomposition method that reconstructs the most reliable line-core position from a local ensemble of profiles. Meanwhile, line-of-sight magnetic fields were derived using the weak-field approximation, a technique that relates the circularly polarized Stokes V signal to the wavelength derivative of the intensity profile and is well suited to the weak-to-moderate fields of plage, where Zeeman splitting is not resolved.

The headline measurement concerns magnetic oscillations. Within plage elements, the line-of-sight magnetic field in the Na I D1 line was found to oscillate with a root-mean-square amplitude of about 9 gauss on average, comfortably above the roughly 5 gauss statistical uncertainty estimated through a thousand Monte Carlo noise realizations. This is a genuinely small signal: a few gauss represents only about 5 percent of the typical 200-gauss field strength in the low-chromospheric plage. Detecting it at all is a testament to DKIST’s light-gathering power and polarimetric precision. Intriguingly, the corresponding magnetic oscillations in the Ca II line, which samples a higher layer where the density is roughly an order of magnitude lower, were only marginally above the noise floor, and the team urges caution in interpreting them.

Non-local thermodynamic equilibrium (non-LTE) inversions, performed with the Stockholm Inversion Code, supplied the missing structural context: mass densities at the two formation heights and the geometrical distance between them. The inversions put the average separation between the heights sampled by the Na I D1 and Ca II line cores at about 325 kilometers — a remarkably short stretch of atmosphere over which the wave character, as we shall see, appears to change. The density at the calcium height came out roughly a factor of ten lower than at the sodium height, consistent with the steep density drop through the low chromosphere.

With intensities, velocities, and magnetic fields in hand, the team turned to cross-spectral analysis, comparing the phase relationships between the different signals at each spatial pixel and testing statistical significance with a rigorous Fourier-surrogate method that randomizes relative phases a thousand times per pixel. Within each line, the velocity and intensity perturbations were found to be locked at a phase difference of about minus 90 degrees — a quarter-cycle offset that is the hallmark of compressive oscillations, where the gas is being squeezed and rarefied in step with its motion. Crucially, the same quarter-cycle relation appeared when the team compared velocities to inversion-derived temperature perturbations, showing that the result was not an artifact of how line-core intensity responds to radiation.

The inter-line analysis told an even richer story. Comparing the Na I D1 and Ca II velocity time series revealed that the higher, calcium-sampling layer systematically lags the lower, sodium-sampling layer — the signature of upward propagation. The mean travel time between the layers was about 70 seconds in statistically significant plage pixels, translating to an apparent phase speed that clusters near the expected chromospheric sound speed. When the team compared these phase speeds against a conservative lower-limit Alfvén speed computed from the measured fields and inverted densities, the vast majority of the upward-propagating pixels came in below that limit. That combination — transonic phase speeds, sub-Alfvénic propagation, and quarter-cycle compressive phase relations — is precisely what theory predicts for slow magnetoacoustic waves channeled along the plage’s near-vertical field lines.

Perhaps the most thought-provoking result concerns how the magnetic phase relationships evolve with height. At the calcium height, the phase relations among magnetic field, velocity, and intensity were broadly consistent with slow standing or partially standing sausage-like solutions — oscillations in which the magnetic flux tube alternately narrows and widens. But at the sodium height, the magnetic phases instead overlapped qualitatively with fast propagating surface sausage-mode solutions. In other words, within a few hundred kilometers of atmosphere, the observable character of the wave field appears to transform, suggesting a superposition of upward-propagating disturbances with partially reflected components bouncing back from greater heights, possibly near the transition region. The team is careful to note that the slit geometry prevents a definitive identification of the tube eigenmode, and effects such as opacity variations and line-of-sight projection could muddy the picture, but the height-dependent behavior opens a new observational window on how waves couple, reflect, and convert in the chromosphere’s layered structure.

The sobering punchline comes from the energy accounting. Integrating the velocity power spectra over a broad 2.5 to 20 millihertz band and using inversion-derived densities and group speeds, the team computed the kinetic wave-energy flux deposited between the lower and upper layers. The median deposited flux was around 180 watts per square meter in plage and 450 watts per square meter in surrounding non-plage regions, with an estimated uncertainty of about 40 percent dominated by density uncertainties. Those numbers fall substantially short of the canonical energy requirements for the low chromosphere above active regions, which are roughly an order of magnitude larger and, according to some high-resolution studies, even higher with strong spatial structure. The mismatch does not rule out waves as heating contributors — the measurement captures only the kinetic component, ignores the magnetic Poynting flux that would require full vector measurements, and may underestimate power due to limited resolution and viewing geometry — but it makes clear that slow magnetoacoustic waves alone cannot keep the plage chromosphere warm. As DKIST continues its campaigns, these multi-height, magnetic-and-kinematic wave portraits are set to become a standard tool for reading the Sun’s hidden energy budget, one carefully timed oscillation at a time.

Subject of Research: Magnetoacoustic wave propagation and energy transport in solar plage observed at two chromospheric heights

Article Title: Multi-height spectropolarimetric signatures of magnetoacoustic waves in solar plage

Article References: Sitterson, T., da Silva Santos, J. M., Bentley, J., Schad, T., Molnar, M., De Pontieu, B., Martinez-Sykora, J., & Pietrow, A. G. M. (2026). Multi-height spectropolarimetric signatures of magnetoacoustic waves in solar plage. Frontiers in Astronomy and Space Sciences, 13, Article 1954064. https://doi.org/10.3389/fspas.2026.1954064

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1954064

Keywords: DKIST, solar plage, magnetoacoustic waves, chromosphere, spectropolarimetry, Na I D1, Ca II 8542, magnetic oscillations, wave energy flux, sausage modes, solar physics, non-LTE inversions