Photospheric magnetic flux and coronal emission properties of small-scale bright and faint loops in the quiet Sun^⋆

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
² Space Research and Technology Institute, Bulgarian Academy of Sciences, Acad. Georgy Bonchev Str., Bl. 1, 1113 Sofia, Bulgaria
e-mail: mmadjarska@space.bas.bg
³ School of Mathematics and Statistics, University of St Andrews, North Haugh, St Andrews, KY16 9SS Scotland, UK

Received: 31 May 2023
Accepted: 13 July 2023

Abstract

Context. The study explores the photospheric magnetic properties of bright and faint small-scale loop systems in the solar atmosphere of the quiet Sun, also known as X-ray or coronal bright points.

Aims. To understand how plasma confined in small-scale loops is heated to million degrees, the loop-associated photospheric and coronal magnetic flux properties should be known because the magnetic field is generally assumed to be the main energy source or waveguide. This and follow-up studies aim to provide a qualitative and quantitative investigation of these magnetic properties and their impact on the heating of plasma to million degrees.

Methods. We used quasi-temporal imaging observations taken in the 193 Å channel of the Atmospheric Imaging Assembly (AIA) and line-of-sight magnetograms from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The observations cover 48 h of data at a 6 min cadence with a field of view of 400″ × 400″, from which 90 loop systems (of which 83 are CBPs) were extracted and analysed in full detail.

Results. We obtain the evolution properties of both faint and bright small-scale loop systems (SSLSs) related to either magnetic flux emergence or magnetic flux coalescence and a chance encounter of magnetic fluxes. We estimate the lifetimes of the two loop systems and the impact of the magnetic flux evolution on their life span. The photospheric magnetic flux associated with SSLSs confining plasma heated to coronal temperatures is found to cover at least two orders of magnitude from 3.0 × 10¹⁸ Mx to 1.8 × 10²⁰ Mx. The analysis of the maximum intensity of SSLSs during their lifetime shows numerous spikes of intensity that are identified as small (a few AIA pixels) compact brightenings associated with cancelling magnetic fluxes. Most of them are identified as microflares. The intensity flux range of these spikes is reported. The coronal intensity flux evolution of SSLSs is strongly correlated with the total unsigned photospheric magnetic flux evolution when there is little or no contamination in the selected field of view of the SSLSs by unrelated magnetic fluxes or intensity features. We report on the footpoint separation and change during the lifetime of the faint and bright SSLSs. The magnetic flux emergence and decay rates of some of the SSLSs are also provided in this study.

Conclusions. The power-law index α of the logarithm of the total unsigned magnetic flux and the total intensity for the full lifetime of SSLSs is 1.10 ± 0.02, compared with 1.14 ± 0.03 for a previous study of the whole disc in the same intensity range (Fe XII 193–195 Å). This indicates that the emission of the corona of the quiet Sun at ∼1.25 MK is mostly confined to small-scale loops (some brighter, others fainter). Therefore, it is imperative to understand the mechanism that heats the plasma in these loops.