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Direct remote-sensing observations of the solar poles have been hindered by the restricted view obtained from the ecliptic plane. For the first time ever, Solar Orbiter with its remote-sensing instruments observed the poles of the Sun from out of the ecliptic in the spring of 2025. Here, we report the first measurements of the magnetic field of the solar poles taken when Solar Orbiter was at heliographic latitudes ranging between 14 .° 9 and 16 .° 7. The data sets were collected by the High Resolution Telescope of the Polarimetric and Helioseismic Imager on board Solar Orbiter (SO/PHI-HRT). Two sets of observations, approximately one month apart, for the south and north pole are considered in this work. The magnetic flux and flux density measured during these campaigns are reported as a function of the heliographic latitude observed by SO/PHI-HRT. The net fluxes show a different latitudinal distribution for the two polar caps. We also discuss the observed dependence of the measured fluxes on the viewing angle. These first results highlight the importance of high-resolution direct measurements of the polar field, paving the way for the high-latitude observations planned for SO/PHI-HRT in the coming years.

Our knowledge about the solar chromosphere and transition region (TR) has increased in the last decade thanks to the huge scientific return of space-borne observatories like SDO, IRIS, and Hinode, and suborbital rocket experiments like CLASP1, CLASP2, and Hi-C. However, the magnetic nature of those solar regions remain barely explored. The chromosphere and TR of the Sun harbor weak fields and are in a low ionization stage both having critical effects on their thermodynamic behavior. Relatively cold gas structures, such as spicules and prominences, are located in these two regions and display a dynamic evolution in high-resolution observations that static and instantaneous 3D-magnetohydrodynamic (MHD) models are not able to reproduce. The role of the chromosphere and TR as the necessary path to a (largely unexplained) very hot corona calls for the generation of observationally based, time-dependent models of these two layers that include essential, up to now disregarded, ingredients in the modeling such as the vector magnetic field. We believe that the community is convinced that the origin of both the heat and kinetic energy observed in the upper layers of the solar atmosphere is of magnetic origin, but reliable magnetic field measurements are missing. The access to sensitive polarimetric measurements in the ultraviolet wavelengths has been elusive until recently due to limitations in the available technology. We propose a low-risk and high-Technology Readiness Level (TRL) mission to explore the magnetism and dynamics of the solar chromosphere and TR. The mission baseline is a low-Earth, Sun-synchronous orbit at an altitude between 600 and 800 km. The proposed scientific payload consists of a 30 cm aperture telescope with a spectropolarimeter covering the hydrogen Ly-alpha and the Mg II h&k ultraviolet lines. The instrument shall record high-cadence, full spectropolarimetric observations of the solar upper atmosphere. Besides the answers to a fundamental solar problem the mission has a broader scientific return. For example, the time-dependent modeling of the chromospheres of stars harboring exoplanets is fundamental for estimating the planetary radiation environment. The mission is based on technologies that are mature enough for space and will provide scientific measurements that are not available by other means.

Phase diversity techniques are widely employed in solar astronomy to evaluate and correct the aberrations stemming from atmospheric turbulences, the telescope, and its instruments. The method uses information provided by a pair of images. One of them is usually focused while the other one is defocused. The amount of defocus to be induced is somehow arbitrary, though. In this work we carry out a series of numerical experiments with artificial solar images to investigate the performance of phase diversity for different choices of the relative defocus among the two images. The experiments allow us to determine the amount of defocus that produces the best wave front restoration when changing: (1) the number of Zernike polynomials of the retrieved and/or incident wave fronts; (2) the signal-to-noise ratio of the images; (3) the amplitude of the incident aberrations; and (4) the observed scene. We find a correlation between the amount of defocus needed for optimal restorations and the number of Zernike polynomials employed in the optimization. Values larger than the typically accepted choice of 1λ (peak to peak) are obtained in most cases.

Phase diversity techniques are usually based on the comparison of synchronously acquired pairs of focused–defocused images. This way, differences between both images are avoided except from random pixel variations due to noise on the detector and the phase diversity itself. In some astronomical instruments, though, the two images are not taken simultaneously. This work studies the impact of carrying out phase diversity with pairs of asynchronously acquired images while observing an evolving solar scene. We evaluate the performance of this technique as a function of the time gap between the images through the use of a magnetohydrodynamical simulation of the solar scene as observed by an instrument. We describe the incident wave front with two numbers of Zernike polynomials (20 or 32) to explore their effect on the wave front sensing accuracy and we employ two levels of noise to study their impact in the object restoration. We find that a time gap among our simulation images smaller than ∼10 s has a negligible impact on the performance of the method. The rms error of the Zernike coefficients fitting worsens exponentially from there on, but the evolution is similar no matter the number of polynomials used in the fitting. Meanwhile, the quality of the object restoration benefits from lower noise levels, but it decreases linearly with the time gap independently of the amount of noise.

Phase diversity techniques commonly employ a pair of focused–defocused images to retrieve the incident wave front and to restore the observed scene. However, the combination of more images, each one affected by a different amount of defocus, has been barely explored in solar astronomy. In this work we reformulate the “classic” two-images phase diversity approach to accommodate an arbitrary number of phase differences and we investigate its performance in synthetic magnetohydrodynamical simulations of the solar scene corrupted by noise and degraded by a certain set of aberrations. We employ different combinations of images defocused from ±0.5 λ up to ±2 λ (peak to peak) and compare both the retrieved wave front with the incident one and the restored images with the unaberrated noiseless scene. We investigate the effect of using a series of images defocused both symmetrically and asymmetrically with respect to the focused one. In these two cases the performance of the method is improved with the use of more than two images, although it benefits more from the use of symmetric defocuses. We find also that there is a qualitative best choice of the number of phase diversity images in terms of the goodness of the wave front retrieval and of the restored object. The presented method has a potential use either in instruments equipped with a refocusing mechanism or during the laboratory calibrations of the instrument provided that an optical target can be defocused manually by different amounts.

During the last decades, the use of Fabry-Pérot etalons as filtergraphs has become frequent in solar instruments. The main reason is that they stand out for offering quasi-monochromatic, two-dimensional fields of view much higher than those provided by conventional slit-based spectrographs in a given time interval. Unfortunately, they also present several drawbacks. The number of etalons employed, the chosen way to illuminate them and the material they are made of have a large influence on the performance of the instrument. In this work we review and discuss the main results obtained by some of the most relevant studies in the design of etalon-based instruments. We present the general properties of etalons and their particularities when employed in solar instruments. We examine the (common) use of systems of several etalons to increase the free spectral range and to narrow down the filter transmission width. We compare the advantages and drawbacks of the two most common configurations —collimated and telecentric— paying special attention to their limitations. Finally, we also inspect the properties of crystalline etalons and their use in solar astronomy.

Deciphering the physical properties of flares is crucial for understanding the nature of one of the most energetic events on the Sun. We analyze high spatial and spectral resolution observations of a C-class flare, focusing on a complex light bridge in the active region. The observations show Stokes profiles with opposite Doppler shifts and asymmetries between photospheric and chromospheric spectral lines, and Ca ii infrared spectral lines in emission, tracing areas of extreme atmospheric heating. Inversion results are consistent with these spectral features, showing regions of opposite-sign plasma flows between the photosphere and chromosphere, adjacent to areas with extreme chromospheric temperatures. The latter areas show relatively weak line-of-sight velocities despite the intense heating. The magnetic field configuration shows abrupt changes with height in the regions with opposite-sign flows, while remaining nearly constant with height in the areas of extreme heating. These findings point to a scenario of small-scale magnetic reconnection in the photosphere, suggesting a flare triggering mechanism based on an avalanche of many small reconnection events.

Magnetoconvection at the solar surface governs the dynamics in the upper solar atmosphere and sustains the heliosphere. Properties of this fundamental process are poorly described near the solar poles. Here we report the first out-of-ecliptic remote-sensing observations of the south pole of the Sun from a high-latitude campaign of the Solar Orbiter spacecraft, which reveal spatial and temporal evolution of supergranular convective cells. The supergranular cells have spatial scales of 20–40 Mm. From 8 days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60°), with speeds in the range of 10–20 m s−1, depending on the structures being tracked. These results shed light on the buildup of the polar magnetic field that is central to our understanding of the solar cycle and the heliospheric magnetic field.

Fabry-Pérot etalons illuminated with collimated beams have been characterized analytically in detail since their invention. Meanwhile, most of the features of etalons located in telecentric planes have been studied only numerically, despite the wide use of this configuration in astrophysical instrumentation over decades. In this work we present analytical expressions for the transmitted electric field and its derivatives that are valid for etalons placed in slow telecentric beams, like the ones commonly employed in solar instruments. We use the derivatives to infer the sensitivity of the electric field to variations in the optical thickness for different reflectivities and apertures of the incident beam and we compare them to the collimated case. This allows us to estimate the wavefront degradation produced by roughness errors on the surfaces of the Fabry-Pérot and to establish the maximum allowed RMS value of the cavity irregularities across the footprint of the incident beam on the etalon that ensures diffraction-limited performance. We also evaluate the wavefront degradation intrinsic to these mounts, which is produced only by the finite aperture of the beam and that must be added to the one produced by defects. Finally, we discuss the differences in performance of telecentric and collimated etalon-based instruments and we generalize our formulation to anisotropic etalons.

The spectral, imaging, and polarimetric behavior of Fabry-Pérot etalons have an influence on imaging vector magnetograph instruments based on these devices. The impact depends, among others, on the optical configuration (collimated or telecentric), on the relative position of the etalon with respect to the polarimeter, on the type of etalon (air-gapped or crystalline), and even on the polarimetric technique to be used (single-beam or dual-beam). In this paper we evaluate the artificial line-of-sight velocities and magnetic field strengths that arise in etalon-based instruments attending to the mentioned factors. We differentiate between signals that are implicit to telecentric mounts due to the wavelength dependence of the point-spread function and those emerging in both collimated and telecentric setups from the polarimetric response of birefringent etalons. For the anisotropic case we consider two possible locations of the etalon, between the modulator and the analyzer or after it, and we include the effect on different channels when dual-beam polarimetry is employed. We also evaluate the impact of the loss of symmetry produced in telecentric mounts due to imperfections in the illumination and/or to a tilt of the etalon relative to the incident beam.