A Curious History of Sunspot Penumbrae: An Update

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 than 100 msh) and small groups registered in COI during 1929-1941 is similar to data available from RGO for the periods 1874-1914 and 1950-1976. Nevertheless, while the average values and time evolution of the ratio in large groups is similar to the ratio for small groups according to Coimbra data (1929)(1930)(1931)(1932)(1933)(1934)(1935)(1936)(1937)(1938)(1939)(1940)(1941) it is not analogous for RGO data for the same period. We also found that the behaviour of the penumbra-umbra area ratio for smaller groups in both observatories is significantly different. The main difference between the area measurements made in Coimbra and RGO is associated with the umbra measurements. We would like to stress that the two observatories used different methods of observation and while in COI both methodology and instruments did not change during the study period, some changes were carried out in RGO that could have V.M.S. Carrasco et al.2 affected measurements of umbra and penumbra. These facts illustrate the importance of the careful recovery of past solar data.

Variations of the Internal Asymmetries of Sunspot Groups During their Decay

“…The opposite trends of the two diagrams is conspicuous, larger umbrae decay faster than smaller ones and their areas with respect to their penumbrae are smaller than in small sunspots. This is in agreement with the theoretical result of Rempel & Cheung (2014) that the larger penumbra stabilizes the sunspot; but it contradicts to the results of Brandt et al (1990) and Carrasco et al (2018b) and Hathaway (2013). This dependence is also in contrast to Hoyt & Schatten (1997), who found a linear relationship between the U/P values and the decay rates.…”

“…Brandt et al (1990) studied 126 sunspots observed around 1980 and pointed out the U/P value is 0.24 for small and 0.32 for large spots. Carrasco et al (2018b) found different behavior in the variation of U/P ratio of the small and large groups by using the Royal Greenwich Observatory (RGO) series. The larger groups do not show significant changes from year to year while the smaller groups do.…”

The Umbra–Penumbra Area Ratio of Sunspots During the Maunder Minimum

“…Hathaway (2013) showed that the ratio values decreased smoothly from 7 in 1905 to lower than 3 around 1930 and then smoothly increased to 7 until 1961. However, the secular variation in the penumbra-umbra ratio for small sunspot groups was not confirmed by Carrasco et al (2018) using the Coimbra sunspot catalogue.…”

Testing the Accuracy of Coimbra Astronomical Observatory Solar Filament Historical Series (1929–1941)

“…Known examples of long series are those of the Royal Greenwich Observatory, UK (Willis et al, 2013), Paris Observatory, Meudon, France (Mein and Ribes, 1990), Kodaikanal, India (Mandal et al, 2017), Mount Wilson, USA (Lefebvre et al, 2005), Debrecen, Hungary (Baranyi, Győri, and Ludmány, 2016) and Arcetri, Italy (Ermolli et al, 2009). Other programs were also implemented in Ebro and San Fernando in Spain Curto et al, 2016) and Coimbra, Portugal (Mouradian and Garcia, 2007;, 2011Carrasco et al, 2018a and2018b). The Coimbra Astronomical Observatory (currently Geophysical and Astronomical Observatory of the University of Coimbra, "Observatório Geofísico e Astronómico da Universidade de Coimbra", hereafter OGAUC) started in 1925 a solar observation program that continues today, with essentially the same acquisition system, leading to a large and consistent database.…”

“…The urgent need for historical astronomical data digitization and preservation enabling scientific research is referred by Pevtsov et al (2019). Carrasco et al (2018a) provided a machine-readable version of sunspot data recorded in the Coimbra historical solar catalogues. Data on other solar features have not yet been addressed.…”