Upflows in funnel-like legs of coronal magnetic loops

TL;DR

This study investigates the nature of blue-shifted upflows in the quiet Sun's coronal magnetic loops, revealing their association with funnel-like magnetic structures, their role in mass supply, and their implications for coronal heating.

Contribution

It provides new insights into the magnetic structure of upflows, their relation to funnel-like loop legs, and the dominance of steady upflows over siphon flows in the quiet Sun.

Findings

Upflows are associated with funnel-like magnetic loop legs.

Mass flux in upflows is anti-correlated with funnel expansion.

A small-scale siphon flow was directly detected but likely unrelated to the main upflows.

Abstract

The prominent blue shifts of Ne viii associated with the junctions of the magnetic network in the quiet Sun are still not well understood. By comparing the coronal magnetic-field structures as obtained by a potential-field reconstruction with the conspicuous blue-shift patches on the dopplergram of Ne viii as observed in an equatorial quiet-Sun region, we find that most of the regions with significant upflow are associated with the funnel-like legs of magnetic loops and co-spatial with increments of the line width. These quasi-steady upflows can be regarded as the signatures of mass supply to coronal loops. By using the square-root of the line intensity as a proxy for the plasma density, the mass flux of the upflow in each funnel can be estimated. We find that the mass flux is anti-correlated with the funnel's expansion factor as determined from the extrapolated magnetic field. One of the loop systems is associated with a coronal bright point, which was observed by several instruments and exhibited various morphologies in different wavelengths and viewing directions. A remarkable agreement between its magnetic structure and the associated EUV emission pattern was found, suggesting an almost potential-field nature of the coronal magnetic field. We also report the direct detection of a small-scale siphon flow by both STEREO satellites. However, this transient siphon flow occurred in a weak mixed-polarity-field region, which was outside the adjacent magnetic funnel, and thus it is perhaps not related to plasma upflow in the funnel. Based on these observations, we suggest that at upper-TR temperatures the dominant flows in quiet-Sun coronal loops are long-lasting upflows rather than siphon flows. We also discuss the implications for coronal heating and unresolved magnetic structures.