High-resolution observations of two pores with the integral field unit (IFU) of the GREGOR Infrared Spectrograph (GRIS)

Context. Solar pores are associated with two significant transitions in magnetohydrodynamics: the magnetic field becomes sufficiently strong to inhibit convective energy transport, and a critical change causes pores to develop a penumbra and to transform into sunspots. Aims. The goal is to compare the intricate details of the magnetic and flow fields around two solar pores, where one is part of an active region and the other is an isolated pore, with a secondary goal of demonstrating the scientific capabilities of the GREGOR Infrared Spectrograph (GRIS) integral field unit (IFU). Methods. Two pores were observed with the High-resolution Fast Imager (HiFI) and the GRIS IFU at the 1.5-meter GREGOR solar telescope on 29 May and 6 June 2019. The GRIS IFU mosaics provide spectropolarimetric data for inversions of the Ca I 1083.9 nm and Si I 1082.7 nm spectral lines, covering the deep and upper photosphere. The t-distributed Stochastic Neighbor Embedding (t-SNE) machine learning algorithm is employed to identify different classes of Si I Stokes-V profiles. The local correlation tracking (LCT) technique derives horizontal proper motions around the pores using speckle-restored G-band time-series. Results. Both pores contain a thin light bridge, are stable during the observations, and never develop a penumbra. The isolated pore is three times smaller and significantly darker than the active-region pore, which is not predicted by simulations. The LCT maps show inflows around both pores, with lower velocities for the isolated pore. Both pores are embedded in the photospheric line-of-sight (LOS) velocity pattern of the granulation but filamentary structures are only visible in the chromospheric LOS maps of the active-region pore. The t-SNE identifies five clusters of Si I Stokes-V profiles, revealing an 'onion-peel' magnetic field structure, despite the small size of the pores. The core with strong vertical magnetic fields is surrounded by concentric layers with lower and more inclined magnetic fields. The GRIS IFU spectra allowed for the tracking of the temporal evolution of the physical parameters, but the variations for both pores were nominal. Conclusions. The active-region pore shows some signatures of increased interaction between plasma motions and magnetic fields, which can be considered as early signs of penumbra formation. However, similar physical properties prevail for smaller pores. Therefore, a statistically meaningful sample, covering the size range and different morphologies of pores, is needed to distinguish between the formation mechanisms of active-region and isolated pores.