Further Evidence for Looplike Fine Structure inside "Unipolar" Active Region Plages

TL;DR

This study provides further evidence of looplike fine structures within unipolar active region plages, suggesting that small-scale magnetic reconnection and flux cancellation are more prevalent than magnetograms indicate, impacting coronal heating understanding.

Contribution

It presents new observational evidence of looplike structures and inverted-Y features in unipolar plages, highlighting the underestimation of minority-polarity flux in current magnetograms.

Findings

Presence of small looplike features within plages.

Bright coronal loops often show fine structure and footpoint brightenings.

Magnetograms likely underestimate minority-polarity flux.

Abstract

Earlier studies using extreme-ultraviolet images and line-of-sight magnetograms from the Solar Dynamics Observatory (SDO) have suggested that active region (AR) plages and strong network concentrations often have small, looplike features embedded within them, even though no minority-polarity flux is visible in the corresponding magnetograms. Because of the unexpected nature of these findings, we have searched the SDO database for examples of inverted-Y structures rooted inside "unipolar" plages, with such jetlike structures being interpreted as evidence for magnetic reconnection between small bipoles and the dominant-polarity field. Several illustrative cases are presented from the period 2013--2015, all of which are associated with transient outflows from AR "moss." The triangular or dome-shaped bases have horizontal dimensions of $\sim$2--4 Mm, corresponding to $\sim$1--3 granular diameters. We also note that the spongy-textured Fe IX 17.1 nm moss is not confined to plages, but may extend into regions where the photospheric field is relatively weak or even has mixed polarity. We again find a tendency for bright coronal loops seen in the 17.1, 19.3, and 21.1 nm passbands to show looplike fine structure and compact brightenings at their footpoints. These observations provide further confirmation that present-day magnetograms are significantly underrepresenting the amount of minority-polarity flux inside AR plages and again suggest that footpoint reconnection and small-scale flux cancellation may play a major role in coronal heating, both inside and outside ARs.