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

P H Keys,D Del Moro,R L Hewitt,S Shelyag,M Stangalini,M Mathioudakis,S Jafarzadeh,D B Jess,A Reid,V M J Henriques

doi:10.1051/0004-6361/201936545

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.

Highlights

Magnetic bright points (MBPs) are ubiquitous in the quiet solar photosphere (Solanki 1993)
Using automated feature-tracking algorithms, we studied 300 MBPs and analysed their temporal evolution as they evolved to kilogauss field strengths
We examine the temporal evolution of various MBPs parameters, with emphasis on what we define as the “strong” group of MBPs

Summary

Introduction

Magnetic bright points (MBPs) are ubiquitous in the quiet solar photosphere (Solanki 1993). Theory suggests that convection can lead to kilogauss fields in these small-scale features by a process termed “convective collapse” (Spruit 1979) The basis of this process is that flux within intergranular lanes is subject to strong downflows, which results in the flux tube reducing in size to balance external forces from surrounding material on the tube. Recent work on magnetohydrodynamic (MHD) simulations (Calvo et al 2016) indicates that it is possible to have localised intensity enhancements within the photosphere in the absence of magnetic fields (nonmagnetic BPs) These bright points are caused by a reduced mass density within a swirling downdraft funnel and are at a scale of 60−80 km. The complexity of possible intensity enhancements highlights the need for continued study of the formation processes of these features in the photosphere

Methods

Results

Conclusion