Regina Aznar Cuadrado

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. The HRI _EUV telescope was calibrated on ground at the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, using the Metrology Light Source (MLS) synchrotron in April 2017 during the calibration campaign of the Extreme Ultraviolet Imager (EUI) instrument on board the Solar Orbiter mission. Aims. We used the pre-flight end-to-end calibration and component-level (mirror multilayer coatings, filters, detector) characterization results to establish the beginning-of-life performance of the HRI _EUV telescope, which will serve as a reference for radiometric analysis and monitoring of the telescope in-flight degradation. Methods. Calibration activities at component level and end-to-end calibration of the instrument were performed at PTB/MLS synchrotron light source (Berlin, Germany) and the SOLEIL synchrotron (Orsay, France). Each component optical property is measured and compared to its semi-empirical model. This pre-flight characterization is used to estimate the parameters of the semi-empirical models. The end-to-end response is measured and validated by comparison with calibration measurements, as well as with its main design performance requirements. Results. The telescope spectral response semi-empirical model is validated by the pre-flight end-to-end ground calibration of the instrument. It is found that HRI _EUV is a highly efficient solar EUV telescope with a peak efficiency superior to 1 e ⁻ .ph ⁻¹ , low detector noise (≈2.8 e rms), low dark current at operating temperature, and a pixel saturation above 120 ke- in low-gain or combined image mode. The ground calibration also confirms a well-modelled spectral selectivity and rejection, and low stray light. The EUI instrument achieves a state-of-the-art performance in terms of signal-to-noise and spatial resolution.

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

Solar wind, classified by its bulk speed and the Alfvénic nature of its fluctuations, generates the heliosphere. The elusive physical processes responsible for the generation of the different types of this wind are a topic of active debate. Recent observations reveal intermittent jets, with kinetic energy in the picoflare range, emerging from dark areas of a polar coronal hole threaded by open magnetic field lines. These could substantially contribute to solar wind. However, their ubiquity and direct links to solar wind have not been established. Here, we report a unique set of remote-sensing and in situ observations from the Solar Orbiter spacecraft that establish a unified picture of fast and Alfvénic slow wind, connected to the similar widespread picoflare jet activity in two coronal holes. Radial expansion of coronal holes ultimately regulates the speed of the emerging wind.

Context. The identification of large numbers of localised transient extreme ultraviolet (EUV) brightenings, on very small spatial scales, in the quiet Sun corona has been one of the key early results from Solar Orbiter. However, there are still a great deal of unknowns about these events. Aims. In this work, we aim to better understand EUV brightenings by investigating their spatial distributions. Specifically, we have investigated whether they occur co-spatially with specific line-of-sight (LoS) magnetic field topologies in the photospheric network. Methods. We detected the EUV brightenings in this work using an automated algorithm applied to a high-cadence (3 s) dataset sampled over ∼30 minutes on 8 March 2022 by the Extreme Ultraviolet Imager’s 17.4 nm EUV High Resolution Imager (HRI_EUV). Data from the Solar Dynamics Observatory’s Helioseismic and Magnetic Imager (SDO/HMI) and Atmospheric Imaging Assembly (SDO/AIA) were used to provide context on the LoS magnetic field and for alignment purposes, respectively. Results. We found a total of 5064 EUV brightenings within this dataset that are directly comparable to events reported previously in the literature. These events occurred within around 0.015–0.020% of pixels for any given frame. We compared eight different thresholds to split the EUV brightenings into four different categories related to the LoS magnetic field. Using our preferred threshold, we found that 627 EUV brightenings (12.4%) occurred co-spatially with strong bipolar configurations and 967 EUV brightenings (19.1%) occurred in weak field regions. Fewer than 10% of EUV brightenings occurred co-spatially with the unipolar LoS magnetic field, no matter what threshold was used. Of the 627 strong bipolar EUV Brightenings, 54 were found to occur co-spatially with cancellation, whilst 57 occurred co-spatially with emergence. Conclusions. EUV brightenings are primarily found to occur co-spatially with the strong LoS magnetic field in the photospheric network. However, they do not predominantly occur co-spatially with (cancelling) bi-poles.

This Letter reports the first observation of the onset of fully developed turbulence in the solar corona. Long time series of white-light coronal images, acquired by Metis aboard Solar Orbiter at 2 minutes cadence and spanning about 10 hr, were studied to gain insight into the statistical properties of fluctuations in the density of the coronal plasma in the time domain. From pixel-by-pixel spectral frequency analysis in the whole Metis field of view, the scaling exponents of plasma fluctuations were derived. The results show that, over timescales ranging from 1 to 10 hr and corresponding to the photospheric mesogranulation-driven dynamics, the density spectra become shallower moving away from the Sun, resembling a Kolmogorov-like spectrum at 3 R _⊙. According to the latest observation and interpretive work, the observed 5/3 scaling law for density fluctuations is indicative of the onset of fully developed turbulence in the corona. Metis observation-based evidence for a Kolmogorov turbulent form of the fluctuating density spectrum casts light on the evolution of 2D turbulence in the early stages of its upward transport from the low corona.

Aims. We present an analysis of the first connection mosaic made by the SPICE instrument on board the ESA/NASA Solar Orbiter mission on March 2, 2022. The data will be used to map coronal composition that will be compared with in-situ measurements taken by SWA/HIS to establish the coronal origin of the solar wind plasma observed at Solar Orbiter. The SPICE spectral lines were chosen to have varying sensitivity to the first ionization potential (FIP) effect, and therefore the radiances of the spectral lines will vary significantly depending on whether the elemental composition is coronal or photospheric. We investigate the link between the behavior of sulfur and the hypothesis that Alfvén waves drive FIP fractionation above the chromosphere. Methods. We performed temperature diagnostics using line ratios and emission measure (EM) loci, and computed relative FIP biases using three different approaches (two-line ratio (2LR), ratios of linear combinations of spectral lines (LCR), and differential emission measure (DEM) inversion) in order to perform composition diagnostics in the corona. We then compared the SPICE composition analysis and EUI data of the potential solar wind source regions to the SWA/HIS data products. Results. Radiance maps were extracted from SPICE spectral data cubes, with values matching previous observations. We find isothermal plasma of around log T = 5.8 for the AR loops targeted, and that higher FIP-bias values are present at the footpoints of the coronal loops associated with two ARs. Comparing the results with the SWA/HIS data products encourages us to think that Solar Orbiter was connected to a source of slow solar wind during this observation campaign. We demonstrate FIP fractionation in observations of the upper chromosphere and transition region, emphasized by the behavior of the intermediate-FIP element sulfur.

Coronal loops are the fundamental building blocks of the solar corona. Therefore, comprehending their properties is essential in unraveling the dynamics of the upper solar atmosphere. In this study, we conduct a comparative analysis of the morphology and dynamics of a coronal loop observed from two different spacecraft: the High Resolution Imager (HRI_EUV) of the Extreme Ultraviolet Imager on board the Solar Orbiter, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. These spacecraft were separated by 43° during this observation. The main findings of this study are that (1) the observed loop exhibits similar widths in both the HRI_EUV and AIA data, suggesting that the cross-sectional shape of the loop is circular; (2) the loop maintains a uniform width along its entire length, supporting the notion that coronal loops do not exhibit expansion; and (3) notably, the loop undergoes unconventional dynamics, including thread separation and abrupt downward movement. Intriguingly, these dynamic features also appear similar in data from both spacecraft. Although based on observation of a single loop, these results raise questions about the validity of the coronal-veil hypothesis and underscore the intricate and diverse nature of the complexity within coronal loops.

Coronal loops serve as the fundamental building blocks of the solar corona. Therefore, comprehending their properties is essential in unraveling the dynamics of the Sun's upper atmosphere. In this study, we conduct a comparative analysis of the morphology and dynamics of a coronal loop observed from two different spacecraft: the High Resolution Imager (HRI$_{EUV}$) of the Extreme Ultraviolet Imager aboard the Solar Orbiter and the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. These spacecraft were separated by 43$^{\circ}$ during this observation. The main findings of this study are: (1) The observed loop exhibits similar widths in both the HRI$_{EUV}$ and AIA data, suggesting that the cross-sectional shape of the loop is circular; (2) The loop maintains a uniform width along its entire length, supporting the notion that coronal loops do not exhibit expansion; (3) Notably, the loop undergoes unconventional dynamics, including thread separation and abrupt downward movement. Intriguingly, these dynamic features also appear similar in data from both spacecraft. Although based on observation of a single loop, these results raise questions about the validity of the coronal veil hypothesis and underscore the intricate and diverse nature of complexity within coronal loops.