Axial-Dipole and Equatorial-Dipole Coronal Holes: Congruent Magnetic Geometry, Divergent Evolution
Abstract
While the Sun's polar fields reverse, the axial dipole is weak and low-latitude coronal holes are common. Equatorial-dipole coronal holes (EDCHs) appear when the equatorial dipole is strong, forming at its magnetic poles, similar to the axial-dipole coronal holes (ADCHs) of minimum. Not every low-latitude hole qualifies: the 2024 maximum was quadrupole-led. Parker Solar Probe found fast wind from an EDCH deeply sub-Alfvénic at 9.9 R⊙, at only 60% of an asymptotic speed matching polar wind, which is usually thought to accelerate faster. We ask whether coronal structure predicts this, using potential-field models of synthetic and observed fields at three cycle phases. We show that pure axial and equatorial dipoles are exactly congruent, with the same open areas and core expansion factors, so any difference must arise from orientation relative to the differential-rotation pattern or from the surrounding multipoles. We find that ADCHs are stationary, sustained by converging meridional flow, whereas EDCHs depend on an equatorial dipole that active-region emergence builds and flux transport dismantles: over $18$-day sequences the axial-dipole strength at minimum is steady to 1%, while the equatorial dipole varies by 21% at maximum and 8% in the declining phase. Most open flux in EDCHs expands less than in ADCHs, but their boundaries are pseudostreamer-dominated, with non-monotonic expansion. Despite these differences, magnetic geometry alone does not predict slower acceleration in EDCHs; we argue that any real difference points to extended heating by reflection-driven Alfvén-wave turbulence, perhaps fed differently at those boundaries, which potential-field models cannot capture.
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BibTeX
@misc{panasenco2026axialdipole,
title = {{Axial-Dipole and Equatorial-Dipole Coronal Holes: Congruent Magnetic Geometry, Divergent Evolution}},
author = {Olga A. Panasenco and Yi-Ming Wang and Samuel T. Badman and Adam J. Finley and Yeimy J. Rivera},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07421},
eprint = {2610.07421},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07421},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Axial-Dipole and Equatorial-Dipole Coronal Holes: Congruent Magnetic Geometry, Divergent Evolution
AU - Olga A. Panasenco
AU - Yi-Ming Wang
AU - Samuel T. Badman
AU - Adam J. Finley
AU - Yeimy J. Rivera
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.07421
UR - https://arxiv.org/abs/2610.07421
AB - While the Sun's polar fields reverse, the axial dipole is weak and low-latitude coronal holes are common. Equatorial-dipole coronal holes (EDCHs) appear when the equatorial dipole is strong, forming at its magnetic poles, similar to the axial-dipole coronal holes (ADCHs) of minimum. Not every low-latitude hole qualifies: the 2024 maximum was quadrupole-led. Parker Solar Probe found fast wind from an EDCH deeply sub-Alfvénic at 9.9 R_⊙, at only 60% of an asymptotic speed matching polar wind, which is usually thought to accelerate faster. We ask whether coronal structure predicts this, using potential-field models of synthetic and observed fields at three cycle phases. We show that pure axial and equatorial dipoles are exactly congruent, with the same open areas and core expansion factors, so any difference must arise from orientation relative to the differential-rotation pattern or from the surrounding multipoles. We find that ADCHs are stationary, sustained by converging meridional flow, whereas EDCHs depend on an equatorial dipole that active-region emergence builds and flux transport dismantles: over $18$-day sequences the axial-dipole strength at minimum is steady to 1%, while the equatorial dipole varies by 21% at maximum and 8% in the declining phase. Most open flux in EDCHs expands less than in ADCHs, but their boundaries are pseudostreamer-dominated, with non-monotonic expansion. Despite these differences, magnetic geometry alone does not predict slower acceleration in EDCHs; we argue that any real difference points to extended heating by reflection-driven Alfvén-wave turbulence, perhaps fed differently at those boundaries, which potential-field models cannot capture.
ER -