Calibrating 100 Years of Polar Faculae Measurements: Implications for the Evolution of the Heliospheric Magnetic Field

TL;DR

This study consolidates and calibrates a century's worth of polar faculae data to better understand the Sun's polar magnetic field evolution and its influence on the heliospheric magnetic field, revealing strong correlations during solar minima.

Contribution

It provides a validated, consolidated polar faculae dataset calibrated against magnetic measurements, improving proxies for the Sun's polar magnetic field over the last century.

Findings

Polar faculae measurements agree well with polar field and flux estimates.

Polar flux correlates strongly with heliospheric magnetic field at solar minimum.

Polar flux and sunspot area together better explain HMF evolution than sunspot area alone.

Abstract

Although the Sun's polar magnetic fields are thought to provide important clues for understanding the 11-year sunspot cycle, including the observed variations of its amplitude and period, the current database of high-quality polar-field measurements spans relatively few sunspot cycles. In this paper we address this deficiency by consolidating Mount Wilson Observatory polar faculae data from four data reduction campaigns, validating it through a comparison with facular data counted automatically from MDI intensitygrams, and calibrating it against polar field measurements taken by the Wilcox Solar Observatory and average polar field and total polar flux calculated using MDI line-of-sight magnetograms. Our results show that the consolidated polar facular measurements are in excellent agreement with both polar field and polar flux estimates, making them an ideal proxy to study the evolution of the polar magnetic field. Additionally, we combine this database with sunspot area measurements to study the role of the polar magnetic flux in the evolution of the heliospheric magnetic field (HMF). We find that there is a strong correlation between HMF and polar flux at solar minimum and that, taken together, polar flux and sunspot area are better at explaining the evolution of the HMF during the last century than sunspot area alone.