Intensity contrast of solar network and faculae close to the solar limb, observed from two vantage points

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: albert@mps.mpg.de; solanki@mps.mpg.de
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos 3004, 18080 Granada, Spain
e-mail: jti@iaa.es
³ Univ. Paris-Sud, Institut d’Astrophysique Spatiale, UMR 8617, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁴ Instituto Nacional de Técnica Aeroespacial, Carretera de Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain
⁵ Universitat de València, Catedrático José Beltrán 2, 46980 Paterna-Valencia, Spain
⁶ Leibniz-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
⁷ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany
⁸ University of Barcelona, Department of Electronics, Carrer de Martí i Franquès, 1 – 11, 08028 Barcelona, Spain
⁹ Instituto Universitario “Ignacio da Riva”, Universidad Politécnica de Madrid, IDR/UPM, Plaza Cardenal Cisneros 3, 28040 Madrid, Spain
¹⁰ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹¹ Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut, EMI, Ernst-Zermelo-Str. 4, 79104 Freiburg, Germany

Received: 31 January 2023
Accepted: 3 September 2023

Abstract

Context. The brightness of faculae and network depends on the angle at which they are observed and the magnetic flux density. Close to the limb, the assessment of this relationship has until now been hindered by the increasingly lower signal in magnetograms.

Aims. This preliminary study aims to highlight the potential of using simultaneous observations from different vantage points to better determine the properties of faculae close to the limb.

Methods. We used data from the Solar Orbiter/Polarimetric and Helioseismic Imager (SO/PHI), and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI), recorded at an ∼60° angular separation of their lines of sight at the Sun. We used the continuum intensity observed close to the limb by SO/PHI and complemented it with the co-observed B_LOS from SDO/HMI, originating closer to disc centre (as seen by SDO/HMI), thus avoiding the degradation of the magnetic field signal near the limb.

Results. We derived the dependence of facular brightness in the continuum on disc position and magnetic flux density from the combined observations of SO/PHI and SDO/HMI. Compared with a single point of view, we were able to obtain contrast values reaching closer to the limb and to lower field strengths. We find the general dependence of the limb distance at which the contrast is maximum on the flux density to be at large in line with single viewpoint observations, in that the higher the flux density is, the closer the turning point lies to the limb. There is a tendency, however, for the maximum to be reached closer to the limb when determined from two vantage points. We note that due to the preliminary nature of this study, these results must be taken with caution.

Conclusions. Our analysis shows that studies involving two viewpoints can significantly improve the detection of faculae near the solar limb and the determination of their brightness contrast relative to the quiet Sun.