Long-term Evolution of Sunspot Magnetic Fields

TL;DR

This study tracks long-term changes in sunspot magnetic fields, revealing a decline independent of the solar cycle, which could forecast fewer sunspots in upcoming cycles and highlights the importance of infrared spectral lines for direct magnetic field measurement.

Contribution

It provides the first long-term analysis of sunspot magnetic field evolution using infrared spectral lines, showing a persistent decline and discussing measurement methods.

Findings

Sunspot magnetic fields have decreased over time, independent of the solar cycle.

Infrared spectral lines offer a direct measurement of sunspot magnetic fields.

The relationship between magnetic fields, brightness, and size remains constant during the cycle.

Abstract

Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25. We examined synoptic observations from the NSO Kitt Peak Vacuum Telescope and initially (with 4000 spots) found a change in sunspot brightness which roughly agreed with the infrared observations. A more detailed examination (with 13,000 spots) of both spot brightness and line-of-sight magnetic flux reveals that the relationship of the sunspot magnetic fields with spot brightness and size remain constant during the solar cycle. There are only small temporal variations in the spot brightness, size, and line-of-sight flux seen in this larger sample. Because of the apparent disagreement between the two data sets, we discuss how the infrared spectral line provides a uniquely direct measurement of the magnetic fields in sunspots.