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

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.

24 pages, 10 figures, Accepted by Solar Physics

References in corpus (5)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• Penumbral structure and outflows in simulated sunspots
• Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase
• Hinode's SP and G-band co-alignment
• Sunspot umbra atmosphere from full Stokes inversion

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• Vector magnetic field measurements along a cooled stereo-imaged coronal loop
• No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
• Understanding the Fe I Line Measurements Returned by the Helioseismic and Magnetic Imager
• Superstrong magnetic fields in sunspot bipolar light bridges