Structural Invariance of Sunspot Umbrae Over the Solar Cycle: 1993-2004

Abstract

Measurements of maximum magnetic flux, minimum intensity, and size are presented for 12 967 sunspot umbrae detected on the NASA/NSO spectromagnetograms between 1993 and 2004 to study umbral structure and strength during the solar cycle. The umbrae are selected using an automated thresholding technique. Measured umbral intensities are first corrected for a confirming observation of umbral limb-darkening. Log-normal fits to the observed size distribution confirm that the size spectrum shape does not vary with time. The intensity-magnetic flux relationship is found to be steady over the solar cycle. The dependence of umbral size on the magnetic flux and minimum intensity are also independent of cycle phase and give linear and quadratic relations, respectively. While the large sample size does show a low amplitude oscillation in the mean minimum intensity and maximum magnetic flux correlated with the solar cycle, this can be explained in terms of variations in the mean umbral size. These size variations, however, are small and do not substantiate a meaningful change in the size spectrum of the umbrae generated by the Sun. Thus, in contrast to previous reports, the observations suggest the equilibrium structure, as testified by the invariant size-magnetic field relationship, as well as the mean size (i.e. strength) of sunspot umbrae do not significantly depend on solar cycle phase.

17 pages, 6 figures. Published in Solar Physics

References in corpus (1)

• Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase

Cited by in corpus (14)

• The Solar Cycle
• Small-Scale and Global Dynamos and the Area and Flux Distributions of Active Regions, Sunspot Groups, and Sunspots: A Multi-Database Study
• Properties of sunspot umbrae observed in Cycle 24
• Evolution of sunspot properties during solar cycle 23
• Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter
• Sunspot areas and tilt angles for solar cycles 7-10
• On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter
• The Minimum of Solar Cycle 23: As Deep as It Could Be?
• A Multi-Instrument Analysis of Sunspot Umbrae
• Long-term Evolution of Sunspot Magnetic Fields
• The 17 GHz active region number
• Automated sunspot detection and the evolution of sunspot magnetic fields during solar cycle 23
• Superstrong magnetic fields in sunspot bipolar light bridges
• The behavior of the spotless active regions during the solar minimum 23-24