arXiv subjects

F. J. Bailén

Publications and source records attributed to F. J. Bailén.

10 recordsLinked to original sources

arXiv2026

Origin of small-scale evaporation flows deep in the chromosphere during a solar flare

Flares are caused by an abrupt release of magnetic energy in the solar atmosphere. Plasma heated to well over 10 MK filling the post-flare corona originates from a rapid heating and ablation of the cooler chromospheric material. This chromospheric evaporation is thought to be facilitated primarily by nonthermal electrons impinging on to the lower atmosphere. Questions on when and where in the chromosphere these upflows originate, however, are not fully resolved. Here we report on unprecedented high-resolution observations of an M-class flare recorded by the Sunrise Ultraviolet Spectropolarimeter and Imager on board the balloon-borne SUNRISE observatory, that reveal highly structured upflows on spatial scales of ~100 km originating deep in the chromosphere. The flows even precede the onset of nonthermal electrons by about 10 minutes and last through the impulsive phase of the flare. Our observations shed new light on the lower atmospheric heating and mass circulation in flares that are challenging to reconcile with the standard solar flare model.

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arXiv2026

The Sunrise Chromospheric Infrared Spectro-Polarimeter SCIP: an instrument for SUNRISE III

The Sunrise balloon-borne solar observatory is equipped with a one-meter aperture optical telescope, offering a unique platform for uninterrupted seeing-free observations across ultraviolet, visible, and infrared wavelengths from altitudes higher than 33 km. For the third flight of the upgraded Sunrise observatory conducted in 2024, now called Sunrise III, a new spectro-polarimeter called the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) was developed for observing near-infrared wavelength ranges around 770 nm and 850 nm. These wavelength ranges contain many spectral lines, including two of the Ca II infrared triplet, K I D1 and D2 lines, and multiple Fe I lines, that are sensitive to solar magnetic fields and velocities in the photosphere and chromosphere. SCIP consists of a grating spectrograph in which polarimetric measurements are conducted using a rotating waveplate as a modulator and polarizing beam splitters placed in front of the cameras. The spatial and spectral resolutions are 0.21" and 1x10^5, respectively, and a polarimetric sensitivity of 0.03% (1sigma) of the continuum intensity is achieved with a 10 s integration time per a resolution element. To achieve high-precision detection of small polarization signals, we carefully designed the optical and mechanical systems, polarization components, control electronics, and onboard data processing. Together with the other post-focus instrumentation developed for Sunrise III, the Sunrise Ultraviolet Spectropolarimeter and Imager (SUSI) and the visible imaging spectro-polarimeter Tunable Magnetograph (TuMag), SCIP provides novel observations that help elucidate energy transfer and time-dependent phenomena across the solar photosphere and chromosphere.

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arXiv2026

Solar photospheric velocities measured in space: a comparison between SO/PHI-HRT and SDO/HMI

The Polarimetric and Helioseismic Imager (SO/PHI) onboard Solar Orbiter is a spectropolarimeter scanning the Fe I line at 617.3 nm, providing data of the solar photosphere. The same line is sampled by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and many other on-ground instruments. In this paper, we aim at assessing the consistency between line-of-sight (LoS) velocity measurements from the two instruments. Reliable measurements of up and down flows from SO/PHI are crucial and unique when Solar Orbiter is facing the far side of the Sun. Also, a combination of measurements from two vantage points to study horizontal flows must rely on consistent observations. For this purpose, we compare the LoS velocity measured by SO/PHI's High Resolution Telescope (SO/PHI-HRT) and SDO/HMI on 29 March 2023, when Solar Orbiter was crossing the Sun-Earth line at a distance of 0.39 au from the Sun. Because such co-alignments are rare, this configuration offered an almost unique opportunity to directly compare data products from both telescopes. The data are aligned and remapped to allow a pixel-by-pixel comparison of the whole time series of 4 hours length. Temporal and spatial variations are considered for a direct combination of the measurements. The LoS velocity distributions are evaluated and a clear linear relation is found between the two instruments with a slope of 0.96 and a correlation of 92%. We find that the signals form at similar heights, with a separation of 7$\pm$14 km, which is larger than previous estimates. A close-up look at the penumbra of a sunspot and its Evershed flow is presented. We conclude that the signals inferred by SO/PHI-HRT and SDO/HMI show very good agreement and high correlation when instrumental effects and large-scale velocities on the Sun are properly accounted for.

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arXiv2025

The first out-of-ecliptic observations of the polar magnetic field of the Sun

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$^\circ$ and 16.7$^\circ$. The data-sets were collected by the High Resolution Telescope of the Polarimetric and Helioseismic Imager (SO/PHI-HRT) on board Solar Orbiter. 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 to the high-latitude observations planned for SO/PHI-HRT in the coming years.

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arXiv2025

Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun

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 eight days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60\textdegree), 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.

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arXiv2025

TuMag: the tunable magnetograph for the Sunrise III mission

One of the instruments aboard the Sunrise III mission, the Tunable Magnetograph (TuMag), is a tunable imaging spectropolarimeter in visible wavelengths. It is designed to probe the vector magnetic field and the line-of-sight velocity of the photosphere and the lower chromosphere. The quasi-simultaneous observation of two spectral lines provides excellent diagnostic measurements of the magnetic and dynamic coupling in these layers. The key technologies employed for TuMag are an LCVR-based polarimeter and a solid, LiNbO3 Fabry-Pérot etalon as a spectrometer. However, it also incorporates several innovative features, such as home-made high-sensitivity scientific cameras and a double filter wheel. TuMag can sequentially observe any two out of the three spectral lines of Fe I at 525.02 and 525.06 nm and of Mg I at 517.3 nm. Laboratory measurements have demonstrated outstanding performance, including a wavefront root-mean-square error better than λ/13 for image quality, a full-width-at-half-maximum of 8.7 pm for the filtergraph transmission profile, and polarimetric efficiencies > 0.54. Here we report on the concept, design, calibration, and integration phases of the instrument, as well as on the data reduction pipeline.

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arXiv2022

On Fabry-Pérot etalon-based instruments. IV. Analytical formulation of telecentric etalons

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.

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arXiv2020

On Fabry-Pérot Etalon-based Instruments. III. Instrument applications

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.

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arXiv2019

On Fabry Pérot Etalon based Instruments. II. The Anisotropic (Birefringent) Case

Crystalline etalons present several advantages with respect to other types of filtergraphs when employed in magnetographs. Specially that they can be tuned by only applying electric fields. However, anisotropic crystalline etalons can also introduce undesired birefringent effects that corrupt the polarization of the incoming light. In particular, uniaxial Fabry-Pérots, such as LiNbO3 etalons, are birefringent when illuminated with an oblique beam. The farther the incidence from the normal, the larger the induced retardance between the two orthogonal polarization states. The application of high-voltages, as well as fabrication defects, can also change the direction of the optical axis of the crystal, introducing birefringence even at normal illumination. Here we obtain analytical expressions for the induced retardance and for the Mueller matrix of uniaxial etalons located in both collimated and telecentric configurations. We also evaluate the polarimetric behavior of Z-cut crystalline etalons with the incident angle, with the orientation of the optical axis, and with the f-number of the incident beam for the telecentric case. We study artificial signals produced in the output Stokes vector in the two configurations. Last, we discuss the polarimetric dependence of the imaging response of the etalon for both collimated and telecentric setups.

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arXiv2019

On Fabry Pérot Etalon based Instruments. I. The Isotropic Case

Here we assess the spectral and imaging properties of Fabry Pérot etalons when located in solar magnetographs. We discuss the chosen configuration (collimated or telecentric) for both ideal and real cases. For the real cases, we focus on the effects caused by the polychromatic illumination of the filter by the irregularities in the optical thickness of the etalon and by deviations from the ideal illumination in both setups. We first review the general properties of Fabry Pérots and we then address the different sources of degradation of the spectral transmission profile. We review and extend the general treatment of defects followed by different authors. We discuss the differences between the point spread functions (PSFs) of the collimated and telecentric configurations for both monochromatic and (real) quasi-monochromatic illumination of the etalon. The PSF corresponding to collimated mounts is shown to have a better performance, although it varies from point to point due to an apodization of the image inherent to this configuration. This is in contrast to the (perfect) telecentric case, where the PSF remains constant but produces artificial velocities and magnetic field signals because of its strong spectral dependence. We find that the unavoidable presence of imperfections in the telecentrism produces a decrease of flux of photons and a shift, a broadening and a loss of symmetrization of both the spectral and PSF profiles over the field of view, thus compromising their advantages over the collimated configuration. We evaluate these effects for different apertures of the incident beam.

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