Magnetoacoustic Portals in Quiet-Sun Fluxtubes Revealed by Chromospheric Spectropolarimetry with SUNRISE III/SCIP

Abstract: Acoustic waves propagate into the chromosphere, contributing to energy transport and their dynamics. Their upward propagation is restricted to frequencies above the acoustic cutoff frequency. The magnetic field configuration plays a key role in determining whether acoustic waves can propagate upward because the cutoff frequency is reduced in regions where the field is inclined with respect to gravity, forming so-called magnetoacoustic portals. Previous studies linked magnetic fields and oscillations in quiet regions, but these analyses were based on photospheric magnetic field information, leaving chromospheric structure unconstrained. This study investigates the coupling between acoustic waves and magnetic topology using photospheric and, for the first time, chromospheric spectropolarimetry in a quiet region, obtained with the Sunrise Chromospheric Infrared SpectroPolarimeter (SCIP) aboard the Sunrise iii balloon-borne solar observatory launched in 2024. The SCIP sit-and-stare observations sampled magnetic features in which the line-of-sight field strength exhibits multiple sharp spatial peaks in the photosphere while becoming broader and weaker at two heights in the chromosphere, indicating expanding fluxtubes. The chromospheric velocity field in these fluxtubes exhibits strong 5-minute oscillations, while the surrounding regions show weak 3-minute oscillations. In these fluxtubes, sawtooth temporal velocity variations are associated with intensity enhancements, suggesting steepened shocks. Fluxtubes with low-frequency oscillations are identified not only in network regions but also in weak internetwork regions. These results provide observational evidence that fluxtubes expanding into the chromosphere act as magnetoacoustic portals, in both network and internetwork regions, allowing low-frequency waves to propagate upward and driving chromospheric dynamics via shocks.

• The paper uses simultaneous photospheric and chromospheric SCIP spectropolarimetry to show that expanding fluxtubes transmit sub-cutoff, approximately 5-minute waves into the quiet-Sun chromosphere.
• Portal regions reach velocity standard deviations of 2.0 km/s versus 0.90 km/s outside them, implying about five times greater acoustic energy flux under equal density and sound-speed assumptions.
• The observations link fluxtube evolution to wave transmission and reveal Ca II 8542 Å shock signatures, including sawtooth Doppler patterns and brightenings, but only four of five chromospheric-reaching structures showed portal behavior.

Observational context and motivation

Acoustic waves generated by near-surface convection carry substantial kinetic energy, but most of that power lies below the acoustic cutoff frequency of roughly 5.2 mHz (period ~3 min) set by atmospheric stratification, and is therefore trapped in the photosphere as evanescent oscillations. Estimates of the high-frequency acoustic energy flux have long been contested: some studies find it insufficient to balance chromospheric radiative losses, while high-spatial-resolution measurements report fluxes comparable to those losses. The concept of a magnetoacoustic portal—rooted in the theoretical result that in low-$\beta$ plasma the cutoff frequency is reduced by a factor $\cos\theta$, where $\theta$ is the field inclination to gravity—offers a channel through which sub-cutoff waves can reach the chromosphere. Travel-time analyses at supergranular scales previously supported this picture for network fields, suggesting that portal-mediated low-frequency energy flux could reach several tens of percent, up to half, of the chromospheric heating requirement.

A central limitation of prior work is that magnetic topology was inferred from photospheric magnetometry or extrapolations, even though the cutoff frequency itself varies with height and observational studies indicate it increases toward the chromosphere. This paper addresses that gap directly: using the Sunrise Chromospheric Infrared SpectroPolarimeter (SCIP) aboard the Sunrise III balloon-borne observatory, the authors present the first analysis of acoustic-wave propagation constrained simultaneously by photospheric and chromospheric magnetic field measurements.

Observations and methods

Sunrise III flew on 10 July 2024 with a 1 m telescope providing seeing-free stratospheric observations and exceptionally stable pointing via a new gondola and Correlating Wavefront Sensor. SCIP is a slit spectropolarimeter observing two near-infrared bands (850 nm and 770 nm) whose spectral lines span photosphere-to-chromosphere heights, complemented by a slit-jaw continuum camera. The analysis uses a 23 min 38 s sit-and-stare sequence acquired on 16 July 2024 at $\mu = 0.87$, with full Stokes polarimetry at 1.536 s cadence and 0.21″ spatial resolution.

The study focuses on three lines: the photospheric Fe I 8468 Å line ($g = 2.50$) and the chromospheric Ca II 8498 Å and 8542 Å lines, the latter forming higher than the former. Line-of-sight (LOS) fields are retrieved with the weak-field approximation (WFA), applied in wavelength windows centered on each pixel's Stokes $I$ line core to mitigate Doppler-shift errors; COG-based field strengths agree within 10% but are noisier in weak-field regions. Doppler velocities use the center-of-gravity method, and an acoustic filter retaining phase speeds above 7 km/s isolates propagating waves. Two-dimensional spatial context comes from co-aligned TuMag magnetograms and SDO/HMI data, which identify the sampled features as network patches plus one weak internetwork element.

Magnetic topology: expanding fluxtubes

The SCIP slit intersects several negative-polarity network concentrations with photospheric field strengths of a few hundred gauss. At both chromospheric heights probed, these features become weaker and spatially broader—the signature of fluxtube expansion with height. The authors quantify expansion with a proxy $\Gamma$, the ratio of maximum photospheric to maximum chromospheric LOS field; values range from 2.0 to 3.1 for three network fluxtubes. The proxy is admittedly crude, since sit-and-stare sampling provides only a one-dimensional cut through each structure.

Notably, a positive-polarity internetwork feature with a measured photospheric field of only 43 G also extends into the chromosphere with $\Gamma = 2.4$. This feature would register only ~20 G in HMI data and could be nearly undetectable there. It fragments and disperses during the sequence, and its chromospheric behavior tracks that evolution—a point developed below.

Oscillatory signatures of portals

The chromospheric velocity field divides cleanly by magnetic environment. In the weakly magnetized atmosphere ($|B_{\mathrm{LOS}}| < 10$ G along the slit), the dominant period is approximately 3 min (~8 cycles over 24 min), consistent with cutoff-suppressed transmission of only high-frequency waves. Within the expanding fluxtube spanning $y = 15''$–$\cos\theta$0, the dominant period shifts to approximately 5 min (~5 cycles over 24 min), and the velocity standard deviation rises to 2.0 km/s versus 0.90 km/s in the weakly magnetized atmosphere—an amplitude enhancement implying roughly five times larger acoustic energy flux under equal density and sound-speed assumptions.

Two further results strengthen the causal link between fluxtubes and wave transmission. First, the low-frequency oscillations above the weak internetwork feature appear only while the feature exists; they vanish when it fragments, directly demonstrating that transmission changes track magnetic-structure evolution. Second, chromospheric oscillations show apparent lateral propagation from a localized origin within the expanding fluxtube, traceable over ~5 Mm, with no simple time-delayed relation to the photospheric velocity field. This supports a scenario in which slow-mode waves enter through the portal and are then guided laterally by the expanding chromospheric field geometry, as seen in numerical simulations.

Intensity behavior mirrors the velocity results: Ca II 8542 Å core brightenings are large within the expanding fluxtube and small elsewhere, frequently coinciding with redshift-to-blueshift transitions and sharing the diagonal $\cos\theta$1–$\cos\theta$2 tracks of the velocity signal.

Shock diagnostics from spectral shapes

Wavelength–time diagrams reveal a clear sawtooth pattern in the Ca II 8542 Å line within the expanding fluxtube—gradual redshift followed by abrupt blueshift—with line-core intensity enhancements coinciding with the abrupt transitions. The authors interpret these brightenings as shock-induced source-function enhancement, following the mechanism proposed for Ca II H&K: shock passage raises electron density enough to push the non-LTE source function toward LTE, with temperature excursions potentially exceeding 1 kK. No comparable sawtooth signatures appear in the weakly magnetized atmosphere or in the photospheric Fe I line. Since similar earlier quiet-Sun shock reports lacked spectropolarimetry, these observations establish that, at the 1.0–1.5 Mm heights sampled by Ca II 8542 Å, shocks are not ubiquitous but preferentially form inside expanding fluxtubes, where enhanced amplitudes approaching the sound speed promote nonlinear steepening. Photospheric driving amplitude also matters: the strongest chromospheric brightening coincides with enhanced photospheric oscillation.

Limitations and open questions

Several caveats bound the conclusions. Not every expanding fluxtube acts as a portal: the feature at $\cos\theta$3 shows expansion but neither strong nor low-frequency chromospheric oscillations, plausibly because its ~100 G photospheric field falls well below the ~400 G equipartition value needed for a low-$\cos\theta$4 environment. More generally, WFA retrievals assume unit filling factor and are inclination-sensitive, so measured 200–300 G fields may underestimate intrinsic strengths of unresolved sub-resolution elements—meaning some nominally sub-equipartition portals may in fact host adequate field strength locally. The centroid displacement between photospheric and chromospheric field at one location hints at fluxtube-axis tilt, but merging structures make the interpretation inconclusive. Most importantly, the sample is small: eight photospheric features, five extending into the chromosphere, four showing portal signatures. No statistical claim about portal occurrence or energy budget follows from this dataset alone, though the detection of a portal signature at 43 G implies previous flux estimates based on MDI/HMI sensitivity may have underestimated the contribution of internetwork elements.

Conclusion

Using simultaneous photospheric and chromospheric spectropolarimetry from SCIP/Sunrise III, this study provides the first observationally grounded connection between chromospheric magnetic topology and acoustic-wave propagation in the quiet Sun. Expanding fluxtubes—in both network and internetwork environments—act as magnetoacoustic portals admitting 5-minute waves into the chromosphere, amplifying velocity amplitudes, guiding lateral wave propagation over multi-megameter distances, and preferentially hosting shock formation visible as sawtooth Ca II 8542 Å signatures. The principal open tasks identified by the authors are statistical extension to larger samples of weak magnetic elements, spectropolarimetric inversions to measure chromospheric field inclination directly, and scanned observations enabling three-dimensional topology reconstruction—all needed to quantify how magnetic geometry modifies the cutoff frequency and to reassess the portal-mediated contribution to chromospheric heating.