A new solar signal: Average maximum sunspot magnetic fields independent of activity cycle

TL;DR

This study shows that the average maximum magnetic field strength in sunspot umbrae remains constant over the solar activity cycle, based on extensive IR and HMI observations, challenging previous assumptions of cycle dependence.

Contribution

It provides the first comprehensive analysis demonstrating that maximum sunspot magnetic fields are independent of the solar activity cycle, using IR and HMI data over five years.

Findings

Average maximum sunspot magnetic fields are constant at about 2050 G.

No significant cycle dependence found in magnetic field strength.

Results are consistent across different observational instruments.

Abstract

Over the past five years, 2010-2015, we have observed, in the near infrared (IR), the maximum magnetic field strengths for 4145 sunspot umbrae. Herein we distinguish field strengths from field flux. (Most solar magnetographs measure flux). Maximum field strength in umbrae is co-spatial with the position of umbral minimum brightness (Norton and Gilman, 2004). We measure field strength by the Zeeman splitting of the Fe 15648.5 A spectral line. We show that in the IR no cycle dependence on average maximum field strength (2050 G) has been found +/- 20 Gauss. A similar analysis of 17,450 spots observed by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory reveal the same cycle independence +/- 0.18 G., or a variance of 0.01%. This is found not to change over the ongoing 2010-2015 minimum to maximum cycle. Conclude the average maximum umbral fields on the Sun are constant with time.