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Description
Acoustic waves propagate into the chromosphere and upper atmosphere, contributing significantly to energy transport and dynamics. Their upward propagation, however, is restricted to frequencies above the acoustic cutoff frequency, which depends on atmospheric conditions. In particular, the magnetic field configuration plays a key role, as the cutoff frequency is reduced in regions where the field is inclined with respect to gravity, forming so-called magnetic portals. Previous studies reported links between magnetic fields and oscillations in quiet regions, but these analyses were based on photospheric magnetic field information, leaving the 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 infrared instrument onboard the balloon-borne observatory SUNRISE-3/SCIP.
LOS magnetic fields reveal that photospheric magnetic fluxtubes expand into the chromosphere. In the chromospheric velocity field, these fluxtubes exhibit large-amplitude 5-minute oscillations, while the surrounding regions show small-amplitude 3-minute oscillations. The enhanced oscillations are pronounced in fluxtubes that not only expand but also suggest a tilt of their tube axes toward the chromosphere. In these regions, sawtooth temporal velocity variations are associated with strong intensity enhancement, suggesting steepened shocks. Flux tubes with low-frequency, large-amplitude oscillations are identified in network regions and also in internetwork regions, where they are barely detectable in SDO/HMI magnetograms. These results provide direct observational evidence that expanding fluxtubes, possibly with tilted axes, act as magnetic portals in both network and internetwork regions, allowing low-frequency waves to propagate into the chromosphere and driving dynamics via shock formation.