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Accurate calculations of starspot spectra are essential for multiple applications in astronomy. The current standard is to represent starspot spectra by spectra of stars that are cooler than the quiet star regions. This implies approximating a starspot as a nonmagnetic 1D structure in radiative–convective equilibrium, parametrizing convective energy transport by mixing-length theory. It is the inhibition of convection by the starspot magnetic field that is emulated by using a lower spot temperature relative to the quiet stellar regions. Here, we take a different approach, avoiding the approximate treatment of convection and instead self-consistently accounting for the interaction between matter, radiation, and the magnetic field. We simulate spots on G2V, K0V, and M0V stars with the 3D radiative magnetohydrodynamics code MURaM and calculate spectra (R ≈ 500 from 250 to 6000 nm) using ray-by-ray radiative transfer with the MPS-ATLAS code. We find that the 1D models fail to return accurate umbral and penumbral spectra on K0V and M0V stars, where convective and radiative transfer of energy is simultaneously important over a broad range of atmospheric heights, rendering mixing-length theory inaccurate. However, 1D models work well for G2V stars, where both radiation and convection significantly contribute to energy transfer only in a narrow region near the stellar surface. Quantitatively, the 1D approximation leads to errors longward of 500 nm of about 50% for both umbral and penumbral flux contrast relative to quiet star regions on M0V stars and less than 2% (for umbrae) and 10% (for penumbrae) for G2V stars.

Poynting flux generated by random shuffling of photospheric magnetic footpoints is transferred through the upper atmosphere of the Sun where the plasma is heated to over 1 MK in the corona. High spatiotemporal resolution observations of the lower atmosphere at the base of coronal magnetic loops are crucial to better understand the nature of the footpoint dynamics and the details of magnetic processes that eventually channel energy into the corona. Here, we report high spatial resolution (∼0.″1) and cadence (1.33 s) hyperspectral imaging of the solar Hα line, acquired by the Microlensed Hyperspectral Imager prototype installed at the Swedish 1-m Solar Telescope, that reveal photospheric hot spots at the base of solar coronal loops. These hot spots manifest themselves as Hα wing enhancements, occurring on small spatial scales of ∼0.″2, and timescales of less than 100 s. By assuming that the Hα wings and the continuum form under the local thermodynamic equilibrium condition, we inverted the Hα line profiles and found that the hot spots are compatible with a temperature increase of about 1000 K above the ambient quiet-Sun temperature. The Hα wing integrated Stokes V/I maps indicate that hot spots are related to magnetic patches with field strengths comparable to or even stronger that the surrounding network elements. But they do not show the presence of parasitic polarity magnetic field that would support the interpretation that these hot spots are reconnection-driven Ellerman bombs. Therefore, we interpret these features as proxies of locations where convection-driven magnetic field intensification in the photosphere can lead to energy transfer into higher layers. We suggest that such hot spots at coronal loop footpoints may be indicative of the specific locations and onset of energy flux injection into the upper atmosphere.

We present a multiline characterization of how oscillatory power is organized across distinct magnetic environments in an active region using seeing-free, stratospheric near-ultraviolet (near-UV) spectroscopy from the Sunrise iii UV Spectropolarimeter and Imager (SUSI). A 2 hr time series of short raster scans in the line-rich 327–329 nm window samples along a single transect that contains the following regions: weak magnetic field surroundings, a plage, a sunspot, and a pore. From a set of 30 selected, relatively unblended absorption lines, we extract line-core Doppler velocity time series and compute Morlet-wavelet refined global spectra from which we form band-integrated power maps for three frequency bands (2–4, 4–6, and 6–12 mHz). The stacked, line-resolved maps reveal a clear environment-dependent redistribution of power: 2–4 mHz power is strongest in the weak-field/plage segments but is commonly suppressed in the umbra and pore cores, while 4–6 mHz and 6–12 mHz power becomes relatively enhanced in the strongest-field regions, with line-dependent behavior in the penumbra and plage. Across the line ensemble, this broad frequency structuring is coherent, but the detailed spatial distribution and relative band ranking are not identical from line to line—even among spectral lines with comparable effective formation depths—demonstrating clear line dependence. This novel result implies that single-line measurements may miss secondary components of the local wave spectrum because different lines weight coexisting perturbations and modes differently; therefore, the SUSI near-UV window provides a uniquely diagnostic-rich mapping of oscillations, offering leverage that is difficult to obtain with traditional one- or two-line approaches.

We present the first quiet Sun spectropolarimetric observations obtained with the Visible SpectroPolarimeter at the 4 m Daniel K. Inouye Solar Telescope. We recorded observations in a wavelength range that includes the magnetically sensitive Fe i 6301.5/6302.5 Å doublet. With an estimated spatial resolution of 0.″08, this represents the highest spatial resolution full-vector spectropolarimetric observations ever obtained of the quiet Sun. We identified 53 small-scale magnetic elements, including 47 magnetic loops and four unipolar magnetic patches, with linear and circular polarization detected in all of them. Of particular interest is a magnetic element in which the polarity of the magnetic vector appears to change three times in only 400 km and which has linear polarization signals throughout. We find complex Stokes V profiles at the polarity inversion lines of magnetic loops and discover degenerate solutions, as we are unable to conclusively determine whether these arise due to gradients in the atmospheric parameters or smearing of opposite-polarity signals. We analyze a granule that notably has linear and circular polarization signals throughout, providing an opportunity to explore its magnetic properties. On this small scale, we see the magnetic field strength range from 25 G at the granular boundary to 2 kG in the intergranular lane (IGL) and sanity-check the values with the weak and strong field approximations. A value of 2 kG in the IGL is among the highest measurements ever recorded for the internetwork.

Magnetohydrodynamic waves redistribute energy in magnetic structures of the lower solar atmosphere, yet constraints on how wave power and dominant frequencies are organized above sunspots remain limited, because most studies use only a few well-separated diagnostics. Here, we present multiline wave signatures in a sunspot from near-ultraviolet (near-UV) spectroscopy with the Sunrise-iii UV Spectropolarimeter and Imager (SUSI). We analyse a 2 hr time series of repeated raster scans of a sunspot near disc center in the 327–329 nm spectral window (>100 lines). From these, we select 44 lines that radiative-transfer calculations suggest sample effective formation heights within the umbral core, from the deep photosphere toward the low chromosphere. For each line, we extract line-core intensity and line-of-sight velocity time series using a dedicated multiline fitting routine and compute Morlet wavelet power spectra. The refined global wavelet spectra show that most lines (in both intensity and velocity) are genuinely multifrequency, with a dominant peak and substantial statistically significant power up to 12 mHz. Unsupervised clustering of the normalized spectra groups lines into families with similar spectral shapes and reveals a progression of dominant frequencies from ∼2 to ∼10 mHz across the ensemble, for both intensity and velocity (not necessarily in the same lines). This behavior is not reproduced by a simple formation-height ranking, suggesting that uncertainties in the formation-height estimates and line-dependent diagnostic response together shape the ordering. These Sunrise-iii/SUSI observations open a new regime for near-UV multiline wave studies and provide the first systematic characterization of frequency-structured sunspot wave behaviour in this spectral region.

A part of the magnetic energy released during a flare is transported to the lower atmosphere. High-resolution observations show that flare ribbons, sites of energy deposition at the footpoints of flaring loops that appear bright in the chromosphere and transition region, are structured on small spatial scales on the order of 100 km. Based on idealized numerical models of flares it is suggested that the ribbon fine structures could originate from a tearing instability and the development of plasmoids in current sheets. Here we report on Fe i 5250.6 Å and Mg i b2 5173 Å spectral observations of a solar flare from the Tunable Magnetograph on board the Sunrise iii balloon-borne mission that reveal an intricate link between the flare ribbon structure and the ambient chromosphere. We identified uncombed chromospheric loops and nonflaring fine structures that are interspersed among brighter flare ribbon threads. These loops remain stable on timescales of minutes. Spectral lines from these regions show reduced emission or self-reversal in the line core compared with the immediately adjacent flare ribbons. We discuss the potential role of these structures in the onset of a flare. Furthermore, we suggest that irrespective of the complexities in the flaring current sheet, uncombed chromospheric loops and nonflaring fine structure might play a role in spatially modulating the flare energy deposition in the lower atmosphere.

Magnetic pores are compact, strongly magnetized waveguides in the lower solar atmosphere and therefore provide favorable conditions for identifying magnetohydrodynamic (MHD) wave modes. Earlier seeing-free observations revealed concurrent sausage, kink, and fluting modes in photospheric pores, but only at a single sampled layer. In this letter, we exploit the dense spectral sampling of the near-ultraviolet 327–329 nm window observed by the Sunrise-iii UV Spectropolarimeter and Imager (SUSI) to investigate how pore wave modes behave across multiple photospheric and low-chromospheric heights spanning roughly 500 km. We analyze ∼75 min of a Sunrise-iii/SUSI time series containing a small solar pore. From eight selected spectral lines sampling different estimated formation heights, we identify the pore boundary at each line and time step and apply proper orthogonal decomposition (POD) to the boundary oscillations. In all eight lines, the first POD mode is consistently identified as an axisymmetric sausage mode, with dominant power at ∼1–2 mHz, and carries the dominant normalised eigenvalue fraction, typically about 66%–86%, while the second mode is a fluting mode with azimuthal wave number m = 2, dominant at ∼2–3.5 mHz, and contributes about 4%–10%. Cross-line wavelet phase analysis of the temporal coefficients shows that the sausage mode remains close to zero phase difference across the sampled heights, consistent with standing or near-standing behaviour, whereas the fluting mode displays a modest but systematic increase in phase with height, reaching about 50°, indicative of an upward-propagating component. These observations provide the first multiheight identification and phase characterisation of sausage and fluting modes inferred from pore-boundary oscillations.

Full-disk observations of the Sun in the Ca ii K line have been carried out since the late 19th century at various observatories worldwide. These long-term records of solar activity are crucial for reducing discrepancies among solar irradiance reconstructions and for advancing our understanding of the solar dynamo. To construct a consistent composite record, data from different observatories must be cross-calibrated to account for variations in spectral passband and spatial resolution, which are the primary sources of discrepancies between archives. In this study, we use high spectral and spatial resolution observations in the Ca ii K line from the state-of-the-art Sunrise iii mission to emulate different passbands and derive empirical contrast-contrast relationships between them. We find that these relationships are well described by a power law and provide coefficients for different combinations of passband widths in the range 0.1–9 Å and spatial resolutions between 1″ and 6″. Applying such a relationship to observations from two major Ca ii K archives demonstrates its potential to improve their cross-calibration. The results provide a foundation for the construction of a consistent, century-long time series of solar activity from historical and modern Ca ii K observations.

Sunrise iii is a balloon-borne solar observatory dedicated to investigating the physics governing the magnetism and dynamics in the lower solar atmosphere. The observatory is designed to operate in the stratosphere, at heights around 36 km (above 99% of Earth’s atmosphere), to avoid image degradation due to turbulence in the Earth’s lower atmosphere, to gain access to the NUV wavelengths down to 309 nm, and to enable (when flown during summer solstice) observing the Sun uninterruptedly 24 hours/day. It is composed of a balloon gondola (equivalent to a spacecraft bus) carrying a 1-m aperture telescope (the largest solar telescope to-date to fly in the stratosphere on a balloon) feeding an imaging vector magnetograph and two spectropolarimeters aiming at acquiring high spatial resolution high cadence time series maps of the solar vector magnetic fields, plasma flows, and temperature in the photosphere and chromosphere. In July 2024 Sunrise iii successfully completed a six and a half days long stratospheric flight from Kiruna (Sweden) to Northern Canada at an average altitude of 36 km. This was the third successful flight of the Sunrise observatory, which had previously flown in 2009 and 2013. For this flight it was upgraded substantially with a new and improved suite of three instruments carried by a completely new gondola with upgraded pointing control system. This article focuses on describing the design and flight performance of the Sunrise iii gondola and all its subsystems. It describes the gondola mechanical structure, its power system, its command and control system, and in particular its pointing control system which was key for achieving high spatial and spectral resolution images of the solar photosphere and chromosphere by the three instruments.

We report high-spatial-resolution spectropolarimetric observations spatially associated with a solar filament, obtained with the Sunrise Chromospheric Infrared Spectro-Polarimeter (SCIP) on board the Sunrise iii balloon-borne solar observatory on 2024 July 15. The observed filament was located near the solar disk center, adjacent to an active region, and remained quiescent for at least 2 hr 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 2σ 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 to the vector magnetic structure of solar filaments in the lower chromosphere, complementing existing He i based diagnostics that probe the upper chromosphere.