On the Penumbra-to-Umbra Ratio from 1660 to 1676

TL;DR

This study analyzes historical sunspot engravings from the Maunder minimum to assess the penumbra-to-umbra ratio, finding it consistent with modern observations and suggesting stable near-surface convection over centuries.

Contribution

It provides the first quantitative evaluation of the penumbra-to-umbra ratio from 17th-century sunspot engravings, confirming its stability across centuries.

Findings

P/U ratio during 1660-1676 was approximately 4.2 and 3.8.

Results align with modern sunspot observations across different solar cycles.

Schematic drawings tend to underestimate the P/U ratio.

Abstract

Sunspot engravings made in the Maunder minimum are used to evaluate the fine structure of sunspots. Based on 78 images of the full solar disk and 77 images of individual sunspots, we have evaluated the ratio of penumbral-to-umbral area (P/U) to be 4.2 $\pm$ 2.5 and 3.8 $\pm$ 2.3, respectively. These results are consistent with previous estimates before, during, and after the Maunder minimum, as well as with the P/U ratio observed in the largest sunspot groups in solar cycle 24. This suggests that the near-surface convection mode has most likely remained unchanged since the early seventeenth century. We also found that schematic sunspot drawings tend to underestimate the P/U ratio.