A Multi-Instrument Analysis of Sunspot Umbrae

TL;DR

This study analyzes 17 years of sunspot umbrae data from three instruments, revealing changes in magnetic fields and confirming the inverse relationship between magnetic field strength and umbral intensity, with implications for solar cycle understanding.

Contribution

It provides a multi-instrument, long-term analysis of sunspot umbrae, highlighting differences between solar cycles 23 and 24 and addressing previous data discrepancies.

Findings

Sunspot magnetic fields decreased by 375 Gauss over 17 years.

The magnetic field distribution in cycle 24 differs from cycle 23.

The relationship between magnetic field and umbral intensity remains consistent with past studies.

Abstract

The recent solar minimum and rise phase of solar cycle 24 have been unlike any period since the early 1900s. This article examines some of the properties of sunspot umbrae over the last 17 years with three different instruments on the ground and in space: MDI, HMI and BABO. The distribution of magnetic fields and their evolution over time is shown and reveals that the field distribution in cycle 24 is fundamentally different from that in cycle 23. The annual average umbral magnetic field is then examined for the 17 year observation period and shows a small decrease of 375 Gauss in sunspot magnetic fields over the period 1996 to 2013, but the mean intensity of sunspot umbrae does not vary significantly over this time. A possible issue with sample sizes in a previous study is then explored to explain disagreements in data from two of the source instruments. All three instruments show that the relationship between umbral magnetic fields and umbral intensity agrees with past studies in that the umbral intensity decreases as the field strength increases. This apparent contradiction can be explained by the range of magnetic field values measured for a given umbral intensity being larger than the measured 375 G change in umbral field strength over time.