A Curious History of Sunspot Penumbrae: An Update
V.M.S. Carrasco, J.M. Vaquero, R.M. Trigo, M.C. Gallego

TL;DR
This study investigates the historical behavior of sunspot penumbra-umbra area ratios, comparing data from Coimbra and RGO observatories, and finds discrepancies likely due to measurement methods rather than true secular variations.
Contribution
It provides a comparative analysis of sunspot area ratios from two observatories, highlighting measurement differences and questioning previous claims of secular variation.
Findings
Discrepancies in sunspot area ratios between observatories.
Measurement methods significantly affect sunspot area data.
No clear secular variation confirmed in the ratio.
Abstract
The ratio of penumbral to umbral area of sunspots is an important topic for solar and geophysical studies. Hathaway (Solar Physics, 286, 347, 2013) found a curious behaviour in this parameter for small sunspot groups (areas smaller than 100 millionths of solar hemisphere, msh) using records from Royal Greenwich Observatory (RGO). Hathaway showed that penumbra-umbra ratio decreased smoothly from more than 7 in 1905 to lower than 3 by 1930 and then increased to almost 8 in 1961. Thus, Hathaway proposed the existence of a secular variation in the penumbra-umbra area ratio. In order to confirm that secular variation, we employ data of the sunspot catalogue published by the Coimbra Astronomical Observatory (COI) for the period 1929-1941. Our results disagree with the penumbra-umbra ratio found by Hathaway for that period. However, the behaviour of this ratio for large (areas greater or equal…
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