Axial-Dipole and Equatorial-Dipole Coronal Holes: Congruent Magnetic Geometry, Divergent Evolution
Year: 2026
Type: preprint
Abstract: While the Sun's polar fields reverse, the axial dipole is weak and low-latitude coronal holes are common. Where the equatorial dipole is strong, these holes form at its magnetic poles, as the axial-dipole coronal holes (ADCHs) of solar minimum form at the poles of the axial dipole; we call them equatorial-dipole coronal holes (EDCHs). Not every low-latitude hole is an EDCH: in 2024 August, near maximum, the quadrupole and higher multipoles led the field. Parker Solar Probe found fast wind from an EDCH deeply sub-Alfvénic at $9.9\,R_\odot$, at only $60\%$ of an asymptotic speed equal to that of polar wind, which is thought to accelerate faster. We ask whether coronal structure predicts this, using potential-field models of synthetic and observed fields. Pure axial and equatorial dipoles are exactly congruent, so any difference between the two classes must come from their orientation relative to differential rotation or from the surrounding multipoles. ADCHs are stationary and sustained by converging meridional flow. EDCHs depend on an equatorial dipole that active-region emergence builds and flux transport removes: over $18$-day sequences the axial dipole at minimum is steady to $1\%$, whereas the equatorial dipole varies by $21\%$ at maximum and $8\%$ in the declining phase. Most open flux in EDCHs expands less than in ADCHs, and their boundaries are dominated by pseudostreamers with non-monotonic expansion. Magnetic geometry alone does not predict slower acceleration from EDCHs; a real difference would point to extended heating by Alfvén-wave turbulence, which potential-field models cannot capture.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220953626