Undetected Minority-polarity Flux as the Missing Link in Coronal Heating

TL;DR

This paper suggests that undetected minority-polarity magnetic flux plays a crucial role in coronal heating, challenging previous assumptions and providing estimates that support reconnection-driven energy deposition at low heights.

Contribution

It reveals that magnetograms underestimate minority-polarity flux in plages and proposes small-scale flux reconnection as a significant coronal heating mechanism.

Findings

Magnetograms underrepresent minority-polarity flux in plages.

Small loops are a major component of Fe IX 17.1 nm moss.

Reconnection energy flux is sufficient to heat the corona.

Abstract

During the last few decades, the most widely favored models for coronal heating have involved the in situ dissipation of energy, with footpoint shuffling giving rise to multiple current sheets (the "nanoflare" model) or to Alfv{\'e}n waves that leak into the corona and undergo dissipative interactions (the wave heating scenario). As has been recognized earlier, observations suggest instead that the energy deposition is concentrated at very low heights, with the coronal loops being filled with hot, dense material from below, which accounts for their overdensities and flat temperature profiles. While an obvious mechanism for footpoint heating would be reconnection with small-scale fields, this possibility seems to have been widely ignored because magnetograms show almost no minority-polarity flux inside active region (AR) plages. Here, we present further examples to support our earlier conclusions (1) that magnetograms greatly underrepresent the amount of minority-polarity flux inside plages and "unipolar" network, and (2) that small loops are a major constituent of \ion{Fe}{9} 17.1 nm moss. On the assumption that the emergence or churning rate of small-scale flux is the same inside plages as in mixed-polarity regions of the quiet Sun, we estimate the energy flux density associated with reconnection with the plage fields to be on the order of 10$^7$ erg cm$^{-2}$ s$^{-1}$, sufficient to heat the AR corona.