arXiv subjects

Johannes Hölken

Publications and source records attributed to Johannes Hölken.

10 recordsLinked to original sources

arXiv2026

Towards solar many-line inversions

The Sun's atmosphere is significantly influenced by events that often measure only tens of kilometres in the horizontal direction. They happen in both quiet and active regions and are determined by magneto-hydro-dynamic processes. Their origins often lie in the lowest layers of the solar atmosphere. To improve the understanding of chromospheric and coronal heating, the solar dynamo and its activity cycle, or to improve the prediction of space weather events we must therefore enhance our understanding of such small-scale events. In this work I first describe the process of calibrating spectropolarimetric observations with very high spatial resolution and put together the building blocks for the mathematical proof the MOMFBD image reconstruction algorithm. In the second part I describe the FISS-SP prototype which is designed to fully exploit the resolution power of a 1.6m telescope. This instrument is built to provide suitable data for image reconstruction of spectropolarimetric data. The resulting data can resolve small-scale features down to the diffraction limit. The simultaneous interpretation of a few lines is still the norm for spatially highly resolved solar spectra. Theoretical works have shown that the simultaneous interpretation of multiple solar lines can greatly improve the robustness against noise. Using observations from FISS-SP I advance the theoretical idea into a practical technique. With this I can retrieve a coherent model of the local solar atmosphere, resolving structures down to a horizontal size of about 49~km. Finally, I use those new techniques to interpret an observation of a solar pore. In this last part I describe and analyse small-scale structures, which could not be spectropolarimetrically resolved before. Typical pore-boundary features - namely, dark striations with bright grains moving into the pore - are identified as convective features.

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arXiv2026

Measuring the Expansion of Solar Magnetic Fields with Multiline Inversions from Sunrise III

Magnetic fields in the solar atmosphere are expected to expand with height, forming magnetic canopies that couple the photosphere and chromosphere. Except for sunspots, direct measurements of this expansion remain scarce because they require simultaneous magnetic diagnostics over a broad range of atmospheric heights. We analyze four consecutive high-resolution spectropolarimetric raster scans of an emerging flux region observed with the Sunrise III/SCIP instrument. Simultaneous inversions of five photospheric Fe I lines and two chromospheric Ca II lines provide a quasi-continuous reconstruction of the magnetic field from the deep photosphere to the lower chromosphere. We apply a constant-flux method that follows the same magnetic flux through different atmospheric layers to quantify magnetic expansion. All three pores analyzed exhibit an overall increase in magnetic area with height, approximately doubling their cross section between the lower photosphere and the highest atmospheric layers sampled by the inversions. The expansion remains broadly stable throughout the 48-minute observing sequence. We further characterize the expansion with a quadratic parameterization and investigate its relation to pore properties. The three pores exhibit similar core magnetic field strengths but different expansion behaviors, indicating that the magnetic core strength alone does not determine the expansion. However, the present sample is too small to establish whether these differences are related to the thermodynamic or magnetic properties of the pores and their surroundings. These results demonstrate the potential of simultaneous multiline inversions for investigating the three dimensional magnetic topology of solar magnetic structures and provide the first direct measurements of the height-dependent magnetic expansion of solar pores.

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arXiv2026

High-order Paschen emission from the quiet-Sun off-limb chromosphere

We report the detection of high-order hydrogen Paschen emission lines (Pa~15, Pa~16, and Pa~17) in the quiet-Sun chromosphere off the solar limb using the Chromospheric Infrared SpectroPolarimeter (SCIP) on board the {\sc Sunrise~iii} balloon telescope. These lines reveal thread-like structures resembling spicules and exhibit systematically smaller Doppler velocities than Ca~II~854.2~nm, suggesting that they are optically thinner and more affected by line-of-sight averaging, especially near the limb. Non-LTE radiative transfer synthesis using the spherically symmetric one-dimensional code \texttt{rhsphere} reproduces the overall spectral properties. The observed ratios among three Paschen lines show systematic deviations from synthetic and theoretical results, suggesting that additional physical effects may influence the formation of high-order Paschen lines. The study demonstrates the potential of high-order Paschen lines as a new diagnostic of optically thin plasma in the off-limb chromosphere.

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arXiv2026

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

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.

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arXiv2026

Horizontal Magnetic Fields Dominate the Quiet Sun Internetwork in Sunrise III Observations. Evidence from Traditional and Transformer-Based Inversions

The relative prevalence of horizontal and vertical magnetic fields in the quiet-Sun internetwork remains debated, owing to the weak linear polarisation signals that make the inferred magnetic inclination distributions sensitive to observational quality and inversion methodology. We investigate the magnetic topology of the quiet-Sun internetwork in {\sc Sunrise~iii} observations by independently applying transformer-based and traditional inversion techniques to the same spectropolarimetric dataset. We analyse observations obtained with the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP). Transformer-based inversions (SINN), trained exclusively on synthetic Stokes profiles generated from a large radiative magnetohydrodynamic simulation, are compared with independent inversions performed using the \textsc{DeSIRe} code. Both inversion methods recover a quiet-Sun magnetic topology in which internetwork fields are strongly dominated by the horizontal component, whereas network fields exhibit a substantially more balanced topology. At $\logτ=-1.2$, the median horizontal and unsigned line-of-sight field components in the internetwork are 64 and 7G with \textsc{DeSIRe}, and 71 and 8G with SINN, respectively, corresponding to a horizontal-to-line-of-sight ratio of approximately nine for both methods. The agreement between the two fundamentally different inversion methodologies indicates that the quiet-Sun internetwork is characterised by a substantially stronger horizontal than vertical magnetic field component. These results demonstrate that transformer-based inversions can be successfully transferred from synthetic training data to real spectropolarimetric observations, while providing inference orders of magnitude faster than traditional inversion techniques.

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arXiv2026

Expanding magnetic canopy structure and parasitic polarities in a quiet-Sun network element observed by Sunrise III/SCIP

We present high-resolution multi-line spectropolarimetric observations of a quiet-Sun network element obtained with the {\sc Sunrise~iii} Chromospheric Infrared SpectroPolarimeter. The observations combine photospheric, upper-photospheric, and chromospheric diagnostics at a spatial resolution and polarimetric sensitivity that allow the transverse magnetic structure of the network boundary to be examined directly. We find that the strongest linear polarisation is concentrated in a narrow ridge around the edge of the magnetic element, co-spatial with enhanced transverse magnetic field inferred from multiline inversions. The magnetic azimuth exhibits a coherent, predominantly radial organisation around a more vertical core, consistent with an expanding magnetic canopy. An azimuth proxy derived directly from the observed Fe~\textsc{i} and K~\textsc{i} linear polarisation reproduces the same large-scale organisation, showing that this structure is encoded in the Stokes profiles rather than imposed by the inversion. Response functions and a MURaM-based forward-synthesis test indicate that the Fe~\textsc{i}~8468~Å linear polarisation is sensitive to magnetic azimuth in the upper photosphere, with the closest proxy agreement occurring near $\logτ\approx-3$. We find no evidence for strong azimuthal shear between the Fe- and K-sensitive diagnostics. At the network boundary, we also identify localised parasitic-polarity patches associated with complex, multi-lobed Stokes $V$ profiles, and one case in which the Stokes $V$ polarity reverses between photospheric Fe~\textsc{i} and chromospheric Ca~\textsc{ii} lines. These results demonstrate that quiet-Sun network boundaries contain organised upper-photospheric canopy fields together with small-scale mixed-polarity structure, providing new constraints on the three-dimensional magnetic structure of network elements.

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arXiv2026

Three-dimensional Magnetic Field Structure of a Quiet-Sun Region Revealed by Sunrise III/SCIP

The balloon-borne stratospheric solar observatory Sunrise III successfully completed 6.5 days of observations in July 2024. One of its focal-plane instruments, the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP), is a slit-scanning spectropolarimeter that simultaneously measures full Stokes profiles of multiple spectral lines in the 850 nm and 770 nm bands. SCIP obtained an unprecedented data set of a quiet-sun region near disk center, covering a $58'' \times 58''$ field of view. With an integration time of 10 s per slit position, the scan was completed in 107 minutes without interruption, achieving remarkably stable polarimetric precision of 0.03-0.04% (1$σ$) of the continuum level. The multi-wavelength SCIP observations reveal that the chromospheric line-of-sight (LOS) magnetic field exhibits thread-like, elongated structures over the internetwork regions, with no obvious photospheric counterpart directly below. These threads are typically narrower than $1''$ and are embedded within the canopy fields extending from the network regions. Their LOS field strengths derived from the weak-field approximation are typically 10-20 G weaker than the surrounding canopy. In particularly clear cases, the magnetic polarity of the threads is opposite to that of the adjacent canopy. These findings suggest that the canopy field is not simply an expanding structure originating from network regions, but instead has a complex three-dimensional configuration containing numerous localized substructures. These observations provide new constraints on the quiet-sun magnetic topology from the photosphere to the chromosphere.

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arXiv2026

Magnetic Field Stratification in Active-Region Plage from Ca II K and Ca II 8542 Å Spectropolarimetry

We investigate the height variation of the line-of-sight (LOS) magnetic field in solar active-region (AR) plage from the upper photosphere to the upper chromosphere using co-spatial ultraviolet (UV) and infrared (IR) spectropolarimetric observations from the Sunrise III stratospheric balloon flight. The Ca II K and Ca II 8542 Å lines provide complementary chromospheric diagnostics, while nearby Fe I lines sample photospheric layers. The LOS magnetic field is inferred from the intensity and circular polarization profiles, applying the weak field approximation for the Ca II lines and the center-of-gravity method for the Fe I lines. The photospheric Fe I lines reveal strong, finely structured magnetic fields of the order of a kG. In contrast, the chromospheric Ca II diagnostics yield systematically weaker fields, typically in the $\sim$ 100-400 G range, with a more diffuse spatial distribution. Magnetic field maps inferred from the Ca II K line, formed higher in the chromosphere, are smoother and more extended than those derived from the Ca II 8542 Å line, which samples lower heights. We find that the magnetized area increases by a factor of $\sim$ 2 from the photosphere to the chromosphere. These results provide direct quantitative evidence that magnetic fields in AR plage weaken and expand with height, evolving from compact kG photospheric concentrations into weaker and more spatially extended structures in the upper chromosphere.

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arXiv2026

Height Dependent Phase Shifts of Wave Pulses in the Lower Solar Atmosphere Measured with SUNRISE III

We report on the measurement of the height-dependent time shifts of wave pulses in the lower solar atmosphere from high-resolution spectro-polarimetric observations obtained with the SUSI instrument on board the SUNRISE III balloon-borne solar observatory during its successful science flight in July 2024. The line-of-sight velocities derived from the line-core positions of 19 spectral lines in a 2 nm-wide window around the Ca II H line were used to determine the time shifts of propagating pulses at their respective formation heights. Our analysis reveals that these shifts are roughly ordered according to the computed formation heights of the respective spectral lines. A statistical analysis of the time shifts using sit-and-stare observations with a total duration of one hour reveals that wave pulses propagating upwards from near the solar surface to heights of approximately 500-700 km are most common, with average time lags of 20 s to 30 s between these heights. Also present are pulses with close-to-zero phase shifts, predominantly above intergranular lanes and areas of enhanced magnetic activity. Additionally, downward propagating wave pulses with negative time lags of 10 s to 15 s are seen, mostly above areas of enhanced magnetic activity. A common feature of all the observed pulses is that in the lower 250 km they show small time lags of zero to a few seconds, and only at higher layers do the propagating pulses become more dominant. This study demonstrates the potential of the many-line approach for investigating the height dependence of the physical conditions in the solar atmosphere.

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arXiv2026

Vector Magnetic Field associated with an Active Region Filament Observed by SUNRISE III/SCIP in the Ca II 8542 Å Line

We report high-spatial-resolution spectropolarimetric observations spatially associated with a solar filament, obtained with the SUNRISE Chromospheric Infrared spectro-Polarimeter (SCIP) onboard the SUNRISE III balloon-borne solar observatory on 15 July 2024. The observed filament was located near the solar disk center, adjacent to an active region, and remained quiescent for at least two hours during the observing period. SCIP recorded full Stokes profiles in the Ca II 8542 Å line, revealing clear signatures of linear polarization produced by the transverse Zeeman effect. The detected linear polarization signals within the filament region exceeded the 2$σ$ noise level and exhibited a characteristic Zeeman double-lobe spectral shape that distinguishes them from polarization due to scattering. The magnetic field strength derived using the weak field approximation is approximately -80 G along the line of sight and 300-500 G in the transverse direction. These values likely reflect the magnetic properties of the filament and its supporting chromospheric environment. The orientation of the magnetic field vector is nearly parallel to the filament axis in its northeastern portion, while the southeastern part of the filament extends outside the field of view. To our knowledge, this is the first unambiguous detection of linear polarization associated with a solar filament with the Ca II 8542 Å line. Our results open a new diagnostic window on the vector magnetic structure of solar filaments in the lower chromosphere, complementing existing He I based diagnostics that probe the upper chromosphere.

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