On Anisotropy in Expansion of Magnetic Flux Tubes in the Solar Corona

TL;DR

This paper challenges the common assumption of circular cross-sections in magnetic flux tubes in the solar corona, showing that non-circular shapes better explain observed loop expansions and potential observational biases.

Contribution

It demonstrates that flux tubes often have non-circular cross-sections, affecting interpretations of their expansion and observational biases in the solar corona.

Findings

Flux tubes in the corona are generally non-circular in cross-section.

Non-circular flux tubes can explain observed expansion patterns.

Viewing angle biases influence the apparent properties of coronal loops.

Abstract

Most 1d hydrodynamic models of plasma confined to magnetic flux tubes assume circular cross-section of these tubes. We use potential field models to show that flux tubes in circumstances relevant to the solar corona do not in general maintain the same cross-sectional shape through their length and therefore the assumption of a circular cross-section is rarely true. We support our hypothesis with mathematical reasoning and numeric experiments. We demonstrate that lifting this assumption in realistic non-circular loops make apparent expansion of magnetic flux tubes consistent with that of observed coronal loops. We propose that in a bundle of ribbon-like loops those that are viewed along the wide direction would stand out against those that are viewed across the wide direction, due to the difference in their column depths. That would impose a bias towards selecting loops that appear not to be expanding seen projected in the plane of sky. An implication of this selection bias is that the preferentially selected non-circular loops would appear to have increased pressure scale height even if they are resolved by current instruments.