On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

TL;DR

This study analyzes a large dataset of sunspot umbrae to confirm the absence of a long-term decline in their magnetic field strength, exploring relationships between magnetic properties, size, and brightness over solar cycles.

Contribution

It provides a comprehensive analysis of sunspot magnetic fields using Hinode data, confirming no significant long-term trend in mean magnetic field strength and examining magnetic field organization.

Findings

No significant long-term trend in mean magnetic field strength.

Magnetic field strength correlates non-linearly with umbral temperature and size.

Umbral magnetic azimuths show weak influence from Joy's law.

Abstract

We study 7530 sunspot umbrae and pores measured by the Hinode Spectropolarimeter (SP) between November 2006 and November 2012. We primarily seek confirmation of the long term secular decrease in the mean magnetic field strength of sunspot umbrae found by Penn and Livingston (2011, IAU Symp. 273,126) between 1998 and 2011. The excellent SP photometric properties and full vector magnetic field determinations from full-Stokes Milne-Eddington inversions are used to address the interrelated properties of the magnetic field strength and brightness temperature for all umbral cores. We find non-linear relationships between magnetic field strength and umbral temperature (and continuum contrast), as well as between umbral radius and magnetic field strength. Using disambiguated vector data, we find that the azimuths measured in the umbral cores reflect an organization weakly influenced by Joy's law. The large selection of umbrae displays a log-normal size spectrum similar to earlier solar cycles. Influenced by the amplitude of the solar cycle and the nonlinear relationship between umbral size and core magnetic field strength, the distribution of core magnetic field strengths, fit most effectively with a skew-normal distribution, shows a weak solar cycle dependence. Yet, the mean magnetic field strength does not show a significant long term trend.