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

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.

accepted in ApJ

References in corpus (3)

• Constant cross section of loops in the solar corona
• Are constant loop widths an artifact of the background and the spatial resolution?
• Coronal Loop Expansion Properties Explained Using Separators

Cited by in corpus (14)

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• MHD modeling of coronal loops: the transition region throat
• Static and dynamic solar coronal loops with cross-sectional area variations
• An ultra-long and quite thin coronal loop without significant expansion
• On the Area Expansion of Magnetic Flux-Tubes in Solar Active Regions
• Multi-spacecraft observations of coronal loops to verify a force-free field reconstruction and infer loop cross sections
• Stereoscopic observations reveal coherent morphology and evolution of solar coronal loops
• Measuring and modeling the rate of separator reconnection between an emerging and existing active region
• Long-lived coronal loops in solar active regions
• The cross-sectional shape and height expansion of coronal loops: High-resolution Coronal Imager (Hi-C) analysis of AR 12712