Plasma flows and magnetic field interplay during the formation of a pore^⋆

¹ INAF Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
e-mail: ermolli@oaroma.inaf.it
² INAF Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ NSO National Solar Observatory, Sacramento Peak Box 62, Sunspot, NM 88349, USA
⁴ Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 20 March 2015
Accepted: 22 December 2016

Abstract

Aims. Recent simulations of solar magneto-convection have offered new levels of understanding of the interplay between plasma motions and magnetic fields in evolving active regions. We aim at verifying some aspects of the formation of magnetic regions derived from recent numerical studies in observational data.

Methods. We studied the formation of a pore in the active region (AR) NOAA 11462. We analysed data obtained with the Interferometric Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope on April 17, 2012, consisting of full Stokes measurements of the Fe I 617.3 nm lines. Furthermore, we analysed SDO/HMI observations in the continuum and vector magnetograms derived from the Fe I 617.3 nm line data taken from April 15 to 19, 2012. We estimated the magnetic field strength and vector components and the line-of-sight (LOS) and horizontal motions in the photospheric region hosting the pore formation. We discuss our results in light of other observational studies and recent advances of numerical simulations.

Results. The pore formation occurs in less than 1 h in the leading region of the AR. We observe that the evolution of the flux patch in the leading part of the AR is faster (<12 h) than the evolution (20–30 h) of the more diffuse and smaller scale flux patches in the trailing region. During the pore formation, the ratio between magnetic and dark area decreases from 5 to 2. We observe strong downflows at the forming pore boundary and diverging proper motions of plasma in the vicinity of the evolving feature that are directed towards the forming pore. The average values and trends of the various quantities estimated in the AR are in agreement with results of former observational studies of steady pores and with their modelled counterparts, as seen in recent numerical simulations of a rising-tube process. The agreement with the outcomes of the numerical studies holds for both the signatures of the flux emergence process (e.g. appearance of small-scale mixed polarity patterns and elongated granules) and the evolution of the region. The processes driving the formation of the pore are identified with the emergence of a magnetic flux concentration and the subsequent reorganization of the emerged flux, by the combined effect of velocity and magnetic field, in and around the evolving structure.