The role of the chromospheric magnetic canopy in the formation of a sunspot penumbra

Abstract

Context. While it is conjectured that a chromospheric canopy plays a role in penumbra formation, it has been difficult to find observational evidence of the connectivity between the photosphere and the chromosphere. Aims. We investigate the existence of a chromospheric canopy as a necessary condition for the formation of a penumbra. Another aim is to find the origin of the inclined magnetic fields. Methods. Spectropolarimetric observations of NOAA AR 12776 from the GRIS spectrograph at the GREGOR telescope were analyzed. Atmospheric parameters were obtained from the deep photospheric Ca I 10 839 Å line (VFISV inversion code), the mostly photospheric Si I 10 827 Å line (SIR inversion code), and the chromospheric He I 10 830 Å triplet (HAZEL inversion code). We compared the photospheric and chromospheric magnetic topology of a sunspot sector with a fully-fledged penumbra to a sector where no penumbra formed. Additionally, imaging data from the BBI instrument, attached to the GREGOR telescope, in TiO-band and G-band were analyzed. Results. In the deepest atmospheric layers, we find that the magnetic properties (inclination and field strength distribution) measured on the sunspot sector with fully-fledged penumbra are similar to those measured on the sector without penumbra. However, in higher layers the magnetic properties are different. In the region showing no penumbra, almost vertical chromospheric magnetic fields are observed. Additionally, thin filamentary structures with a maximum width of 0.1″ are seen in photospheric high-resolution TiO-band images in this region. Conclusions. The existence of a penumbra is found to be determined by the conditions in the chromosphere. This indicates that a chromospheric canopy is a necessary condition for the formation of a penumbra. However, our results demonstrate that inclined fields in the chromospheric canopy are not needed for the development of inclined fields in the photosphere. We question the “fallen magnetic flux tubes” penumbra formation scenario and favor a scenario in which inclined fields emerge from below the surface and are blocked by the overlying chromospheric canopy.