Thomas Wiegelmann

Context. Three-minute (∼5 mHz) oscillations are a prominent signature of wave propagation in the solar chromosphere, but their measured power can vary substantially between full-disc observations and small, high-resolution fields of view. This variability suggests that the detectability of large-scale chromospheric oscillatory power may depend not only on wave excitation, but also on the magnetic topology through which the waves propagate. Aims. We investigate whether chromospheric magnetic canopy topology, in particular its lateral continuity and areal filling, modulates the field-of-view-averaged three-minute power measured in chromospheric diagnostics. Methods. We analysed high-resolution H α and Ca II 8542 Å spectral imaging from the Swedish 1-m Solar Telescope (SST) and compared canopy-poor quiet-Sun scenes with canopy-dominated active-region environments. These observations were complemented by radiative magnetohydrodynamic (MHD) Bifrost simulations representing canopy-poor and canopy-rich atmospheres. We computed field-of-view-averaged power spectra from pixel-wise fixed-wavelength intensities and bisector-derived line-of-sight velocities in the observations, and from synthetic intensities, vertical velocities, and temperature proxies in the simulations. We also quantified the relation between integrated 3–5 mHz excess power and canopy filling factor in both simulations and observations, using Helioseismic and Magnetic Imager (HMI)-based magnetic-field extrapolations for the latter. Results. In both observations and simulations, canopy-poor scenes show clear enhancements of field-of-view-averaged power in the 3–5 mHz band, whereas canopy-dominated scenes show strongly reduced measured power in the same frequency range. The trend is present in H α and Ca II 8542 Å intensity diagnostics and is supported by complementary velocity and temperature proxies. In both the simulations and SST observations, integrated 3–5 mHz excess power decreases with increasing canopy filling factor. Conclusions. Chromospheric field-of-view-averaged three-minute power is therefore a topology-weighted observable. Dense, laterally continuous magnetic canopies can strongly reduce the measured large-scale three-minute signature without requiring the absence of local wave activity. This effect provides a practical criterion for when chromospheric three-minute power should be recovered in spatially averaged diagnostics and should be considered when interpreting oscillation power as a proxy for atmospheric wave energy in the Sun and, more broadly, in magnetised cool-star atmospheres.

Context. Largely radial ray-like structures, termed plumes, are often observed in coronal holes. Plumes have been proposed to channel magnetohydrodynamic (MHD) waves and the solar wind into the heliosphere. High-speed propagating disturbances (PDs), though well detected in plumes, cannot yet be clearly assigned to MHD waves or to mass flows. Additionally, plume bases as observed in the extreme ultraviolet are riddled with small-scale transients, which could be related to PDs. Aims. We studied three plumes within an equatorial coronal hole observed by the EUV High Resolution Imager (HRI _EUV ) of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. The properties of small-scale brightenings at the plume bases were investigated to interpret their nature and the possible relation with PDs in plumes. Methods. A 30 Mm × 30 Mm sub-region is selected for each plume base. We processed the images with the difference of Gaussians (DoG) method to highlight the target brightenings, which were further identified with two different approaches. In the 30 minute observation, 50 brightenings were visually selected, which also helps to set thresholds for automatic detection, where we found 451 brightenings. Their properties, including apparent velocities on the plane of sky (PoS), were analyzed statistically. Potential field extrapolation based on the magnetic field data from the Polarimetric and Helioseismic Imager on board Solar Orbiter was used for correcting the PoS velocity to the real velocity along the magnetic field. Results. We observe that the majority of base brightenings are small scale (with an area of less than 1.3 Mm ² ), short lived (lasting less than five minutes), and slightly elongated at the plume bases. They display intricate movements, with most showing velocities less than 10 km s ⁻¹ on the PoS. The velocity distribution of the base brightenings selected via visual identification shows a peak at 6–7 km s ⁻¹ , while that of the automatically detected base brightenings peaks at a lower velocity of less than 5 km s ⁻¹ . Their 3D velocities are found to be substantially lower than (and difficult to reconcile with) the speeds detected at greater heights in the plumes. Conclusions. A direct link between base brightenings and PDs remains inconclusive due to the huge difference between their velocities. We propose two possibilities for base brightenings: they may be related to wave-driven Type I spicules or originate from interchange reconnections. Further investigation is required to validate these hypotheses.

Context. Solar jets, characterized by small-scale plasma ejections along open magnetic field lines or the legs of large-scale coronal loops, play a crucial role in the dynamics of the solar atmosphere. They are often associated with other solar active phenomena, including campfires, filament eruptions, coronal bright points, flares, and coronal mass ejections. Aims. Although spectral and extreme-ultraviolet images have been widely used to analyze the formation and evolution of jets, the detailed 3D structure at the base of the jet has not been studied in detail, due to the limitations in the spatial resolution of observations. Methods. Solar Orbiter enables us to investigate the structure of solar jets with much higher spatial and temporal resolutions and from a different angle than from Earth. By combining observations made by instruments on board Solar Orbiter with data from the Solar Dynamics Observatory, we analyzed recurrent solar jets originating in a mixed-polarity region near an active region. Additionally, we employed potential field and magneto-hydrostatic extrapolation techniques to determine the magnetic field topology associated with the jets. Results. The jets display dynamic, multistrand outflows emanating from compact bright kernels above the magnetic inversion line, with apparent speeds exceeding 100 km s ⁻¹ . Magnetic field evolution reveals continuous flux cancellation at the jet footpoints. Throughout the sequence, base flows are confined within quasi-separatrix layers, with the highest velocities and temperatures located near coronal null points. Over four eruptions, the magnetic topology evolves from a simple fan–spine configuration with a single null to a more complex dome-shaped base containing multiple nulls with a separatrix curtain, accompanied by a morphological transition from a narrow, well-collimated spire to broader, fragmented outflows. Conclusions. These results provide the first direct observational evidence that dynamic changes in null-point geometry modulate jet morphology and energetics via successive reconnection episodes.

We report on the γ-ray emission above 100 MeV from the GOES M3.3 flare SOL2012-06-03. The hard X-ray (HXR) and microwave emissions have typical time profiles with a fast rise to a well-defined peak followed by a slower decay. The >100 MeV emission during the prompt phase displayed a double-peaked temporal structure with the first peak following the HXR and microwaves, and the second one, about 3 times stronger, occurring 17 ± 2 s later. The time profiles seem to indicate two separate acceleration mechanisms at work, where the second γ-ray peak reveals a potentially pure or at least largely dominant ion acceleration. The Atmospheric Imaging Assembly imaging shows a bright elliptical ribbon and a transient brightening in the northwestern (NW) region. Nonlinear force-free extrapolations at the time of the impulsive peaks show closed field lines connecting the NW region to the southeastern part of the ribbon, and the magnetic topology revealed clusters of nulls. These observations suggest a spine-and-fan geometry, and based on these observations, we interpret the second γ-ray peak as being due to the predominant acceleration of ions in a region with multiple null points. The >100 MeV emission from this flare also exhibits a delayed phase with an exponential decay of roughly 350 s. We find that the delayed emission is consistent with ions being trapped in a closed flux tube with gradual escape via their loss cone to the chromosphere.

Context. The present study provides statistical information on the coronal magnetic field and intensity properties of small-scale bright and faint loops in the quiet Sun. Aims. We aim to quantitatively investigate the morphological and topological properties of the coronal magnetic field in bright and faint small-scale loops, with the former known as coronal bright points (CBPs). Methods. We analyse 126 small-scale loops of all sizes using quasi-temporal imaging and line-of-sight magnetic field observations. These observations are taken by the Atmospheric Imaging Assembly (AIA) in the Fe XII 193 Å channel and the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We employ a recently developed automatic tool that uses a linear magneto-hydro-static (LMHS) model to compute the magnetic field in the solar atmosphere and automatically match individual magnetic field lines with small-scale loops. Results. For most of the loops, we automatically obtain an excellent agreement of the magnetic field lines from the LMHS model and the loops seen in the AIA 193 Å channel. One stand-out result is that the magnetic field is non-potential. We obtain the typical ranges of loop heights, lengths, intensities, mean magnetic field strength along the loops and at loop tops, and magnetic field strength at loop footpoints. We investigate the relationship between all those parameters. We find that loops below the classic chromospheric height of 1.5 Mm are flatter, suggesting that non-magnetic forces (one of which is the plasma pressure) play an important role below this height. We find a strong correlation (Pearson coefficient of 0.9) between loop heights and lengths. An anti-correlation is found between the magnetic field strength at loop tops and loop heights and lengths. The average intensity along the loops correlates stronger with the average magnetic field along the loops than with the field strength at loop tops. Conclusions. The latter correlation indicates that the energy release in the loops is more likely linked to the average magnetic field along the loops than the field strength on the loop tops. In other words, the energy is probably released all along the loops, but not just at the loop top. This result is consistent with a recent benchmarking radiative 3D MHD model.

The magnetic geometry of the solar atmosphere, combined with projection effects, makes it difficult to accurately map the propagation of ubiquitous waves in fibrillar structures. These waves are of interest due to their ability to carry energy into the chromosphere and deposit it through damping and dissipation mechanisms. To this end, the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope was employed to capture high-resolution Hα spectral scans of a sunspot, with the transverse oscillations of a prominent superpenumbral fibril examined in depth. The oscillations are reprojected from the helioprojective Cartesian frame to a new frame of reference oriented along the average fibril axis through nonlinear force-free field extrapolations. The fibril was found to be carrying an elliptically polarized, propagating kink oscillation with a period of 430 s and a phase velocity of 69 ± 4 km s⁻¹. The oscillation is damped as it propagates away from the sunspot with a damping length of approximately 9.2 Mm, resulting in the energy flux decreasing at a rate on the order of 460 W m⁻²/Mm. The Hα line width is examined and found to increase with distance from the sunspot, a potential sign of a temperature increase. Different linear and nonlinear mechanisms are investigated for the damping of the wave energy flux, but a first-order approximation of their combined effects is insufficient to recreate the observed damping length by a factor of at least 3. It is anticipated that the reprojection methodology demonstrated in this study will aid with future studies of transverse waves within fibrillar structures.

Deriving the physical parameters of observed phenomena in the solar atmosphere has fundamental importance, as these parameters are then employed to constrain and validate models. Here we report the development of a new computational algorithm based on a magneto-hydro-static model that computes the magnetic field in the solar atmosphere and automatically matches individual magnetic-field lines with observed structures that appear with enhanced emission in extreme-ultraviolet (EUV) images. Presently, for the quiet-Sun regions, we can only measure the vertical photospheric magnetic field $B_{z}$ B z , as accurate horizontal magnetic field measurements are not available. Thus, vertical photospheric magnetic-field measurements are extrapolated into the upper atmosphere, from the photosphere to the corona, with a magneto-hydro-static model. Free model parameters are then optimized with a downhill-simplex method by comparing quantitatively magnetic-field lines with the enhanced emission of loop structures composing the so-called coronal bright points recorded in EUV images taken with the Atmospheric Imaging Assembly on-board the Solar Dynamics Observatory. The algorithm will be applicable to any solar image data where individual structures with an enhanced emission can be resolved. Most importantly, the algorithm can be employed to obtain the magnetic properties of these structures above the photosphere.

The solar magnetic field is measured routinely onlyin the solar photosphere. While reliable measurementsof the photospheric magnetic field vector are only availablein active regions, in the quiet Sun at present only the vertical component of the magnetic field can be obtained accurately. To derive magnetic field structures throughout the solar atmosphere, from the chromosphere to the corona, we extrapolate these photospheric measurementsinto the upper photosphere, chromosphere and corona with a magneto-hydro-staticmodel. We optimize free model parameters by comparing the modelled magneticfield lines with structures observed in solar images. The comparison isdone automatically with a number of quantitative measurements andthe optimal model parameters are found with the help ofa downhill simplex minimization. This newly developed modelling approach canprovide an accurate and deep understanding of the magnetic field structuresthat extend to any height in the solar atmosphere.