Are the brightest coronal loops always rooted in mixed-polarity magnetic flux?

Abstract

A recent study demonstrated that freedom of convection and strength of magnetic field in the photospheric feet of active-region (AR) coronal loops, together, can engender or quench heating in them. Other studies stress that magnetic flux cancellation at the loop-feet potentially drives heating in loops. We follow 24-hour movies of a bipolar AR, using EUV images from SDO/AIA and line-of-sight (LOS) magnetograms from SDO/HMI, to examine magnetic polarities at the feet of 23 of the brightest coronal loops. We derived FeXVIII emission (hot-94) images (using the Warren et al. method) to select the hottest/brightest loops, and confirm their footpoint locations via non-force-free field extrapolations. From 6"$\times$6" boxes centered at each loop foot in LOS magnetograms we find that $\sim$40\% of the loops have both feet in unipolar flux, and $\sim$60\% of the loops have at least one foot in mixed-polarity flux. The loops with both feet unipolar are $\sim$15\% shorter lived on average than the loops having mixed-polarity foot-point flux, but their peak-intensity averages are equal. The presence of mixed-polarity magnetic flux in at least one foot of majority of the loops suggests that flux cancellation at the footpoints may drive most of the heating. But, the absence of mixed-polarity magnetic flux (to the detection limit of HMI) in $\sim$40\% of the loops suggests that flux cancellation may not be necessary to drive heating in coronal loops -- magnetoconvection and field strength at both loop feet possibly drive much of the heating, even in the cases where a loop foot presents mixed-polarity magnetic flux.

11 pages, 5 figures

References in corpus (15)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
• Evidence of Non-Thermal Particles in Coronal Loops Heated Impulsively by Nanoflares
• Fine-scale explosive energy release at sites of prospective magnetic flux cancellation in the core of the solar active region observed by Hi-C 2.1, IRIS and SDO
• Compact solar UV burst triggered in a magnetic field with a fan-spine topology
• Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lane
• IRIS observations of short-term variability in moss associated with transient hot coronal loops
• Trigger Mechanism of Solar Subflares in a Braided Coronal Magnetic Structure
• Modeling X-ray Loops and EUV "Moss" in an Active Region Core
• The temporal evolution of coronal loops observed by GOES-SXI
• A new method to quantify and reduce projection error in whole-solar-active-region parameters measured from vector magnetograms
• New Evidence that Magnetoconvection Drives Solar-Stellar Coronal Heating
• Network Jets as the Driver of Counter-Streaming Flows in a Solar Filament/Filament Channel
• The Magnetic Properties of Heating Events on High-Temperature Active Region Loops
• Magnetic filed dynamics and varying plasma emission in large coronal loops
• An Observational Test of Solar Plasma Heating by Magnetic Flux Cancellation

Cited by in corpus (1)

• SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation