Evolution of sunspot properties during solar cycle 23

TL;DR

This study analyzes sunspot properties during solar cycle 23 using SOHO data, revealing their distribution, magnetic field variations, and challenging previous reports of a secular decrease in sunspot magnetic fields.

Contribution

It provides a comprehensive analysis of sunspot properties over cycle 23 with a uniform dataset, offering new insights into magnetic field evolution and distribution patterns.

Findings

Total umbral area is 20-40% of sunspot area.

Sunspot number correlates with solar activity indices.

Magnetic field strength varies with the solar cycle, with a less pronounced decrease than previously reported.

Abstract

The long term study of the Sun is necessary if we are to determine the evolution of sunspot properties and thereby inform modeling of the solar dynamo, particularly on scales of a solar cycle. We aim to determine a number of sunspot properties over cycle 23 using the uniform database provided by the SOHO Michelson Doppler Imager data. We focus in particular on their distribution on the solar disk, maximum magnetic field and umbral/penumbral areas. We investigate whether the secular decrease in sunspot maximum magnetic field reported in Kitt Peak data is present also in MDI data. We have used the Sunspot Tracking And Recognition Algorithm (STARA) to detect all sunspots present in the SOHO Michelson Doppler Imager continuum data giving us 30 084 separate detections. We record information on the sunspot locations, area and magnetic field properties and corresponding information for the umbral areas detected within the sunspots, and track them through their evolution. We find the total visible umbral area is 20-40% of the total visible sunspot area at all stages of the solar cycle. We find that the number of sunspots observed follows the Solar Influences Data Centre International Sunspot Number with some interesting deviations. Finally, we use the magnetic information in our catalogue to study the long term variation of magnetic field strength within sunspot umbrae and find that it increases and decreases along with the sunspot number. However, if we were to assume a secular decreaseas was reported in the Kitt Peak data and take into account sunspots throughout the whole solar cycle we would find the maximum umbral magnetic fields to be decreasing by 23.6 \pm 3.9 Gauss per year, which is far less than has previously been observed by other studies. If we only look at the declining phase of cycle 23 we find the decrease in sunspot magnetic fields to be 70 Gauss per year.