High-resolution spectropolarimetric observations of the temporal evolution of magnetic fields in photospheric bright points

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
e-mail: p.keys@qub.ac.uk
² School of Information Technology, Faculty of Science, Engineering and Built Environment, Deakin University, 221 Burwood Highway, Burwood, Melbourne, VIC 3125, Australia
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
⁵ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶ Department of Physics and Astronomy, California State University Northridge, Northridge, CA 91330, USA
⁷ INAF-OAR National Institute for Astrophysics, Via Frascati 33, 00078 Monte Porzio Catone, RM, Italy
⁸ ASI Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, Italy

Received: 21 August 2019
Accepted: 13 November 2019

Abstract

Context. Magnetic bright points (MBPs) are dynamic, small-scale magnetic elements often found with field strengths of the order of a kilogauss within intergranular lanes in the photosphere.

Aims. Here we study the evolution of various physical properties inferred from inverting high-resolution full Stokes spectropolarimetry data obtained from ground-based observations of the quiet Sun at disc centre.

Methods. Using automated feature-tracking algorithms, we studied 300 MBPs and analysed their temporal evolution as they evolved to kilogauss field strengths. These properties were inferred using both the NICOLE and SIR Stokes inversion codes. We employ similar techniques to study radiative magnetohydrodynamical simulations for comparison with our observations.

Results. Evidence was found for fast (∼30−100 s) amplification of magnetic field strength (by a factor of 2 on average) in MBPs during their evolution in our observations. Similar evidence for the amplification of fields is seen in our simulated data.

Conclusions. Several reasons for the amplifications were established, namely, strong downflows preceding the amplification (convective collapse), compression due to granular expansion and mergers with neighbouring MBPs. Similar amplification of the fields and interpretations were found in our simulations, as well as amplification due to vorticity. Such a fast amplification will have implications for a wide array of topics related to small-scale fields in the lower atmosphere, particularly with regard to propagating wave phenomena in MBPs.