Chromospheric observations and magnetic configuration of a supergranular structure

Abstract

Unipolar magnetic regions are often associated with supergranular cells. The chromosphere above these regions is regulated by the magnetic field, but the field structure is poorly known. In unipolar regions, the fibrillar arrangement does not always coincide with magnetic field lines, and polarimetric observations are needed to establish the chromospheric magnetic topology. In an active region close to the limb, we observed a unipolar annular network of supergranular size. This supergranular structure harbours a radial distribution of the fibrils converging towards its centre. We observed this structure at different heights by taking data in the FeI 6301-6302 {\AA}, H-\alpha, CaII 8542 \AA\ and the CaII H&K spectral lines with the CRISP and CHROMIS instruments at the Swedish 1-m Solar Telescope. We performed Milne-Eddington inversions of the spectropolarimetric data of FeI and applied the weak field approximation to CaII 8542 \AA\ data to retrieve the magnetic field in the photosphere and chromosphere. We used magnetograms of CRISP, HINODE/SP and HMI to calculate the magnetic flux. We investigated the velocity using the line-of-sight velocities computed from the Milne-Eddington inversion and from Doppler shift of the K$_3$ feature in the CaII K spectral line. To describe the typical spectral profiles characterising the chromosphere above the supergranular structure, we performed a K-mean clustering of the spectra in CaIIK. The photospheric magnetic flux is not balanced. The magnetic field vector at chromospheric heights, retrieved by the weak field approximation, indicates that the field lines within the supegranular cell tend to point inwards, and might form a canopy above the unipolar region. In the centre of the supergranular cell hosting the unipolar region, we observe a persistent chromospheric brightening coinciding with a strong gradient in the line-of-sight velocity.