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 (1929-1941) 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 affected measurements of umbra and penumbra. These facts illustrate the importance of the careful recovery of past solar data.

arXiv comment10 pages, 6 figures, accepted for publication in "Solar Physics"

References in corpus (11)

• Revisiting the Sunspot Number
• Magnetic Structure of Sunspots
• On-line Tools for Solar Data Compiled at the Debrecen Observatory and their Extensions with the Greenwich Sunspot Data
• A homogeneous database of sunspot areas covering more than 130 years
• Properties of sunspot umbrae observed in Cycle 24
• Evolution of sunspot properties during solar cycle 23
• Kodaikanal Digitized White-light Data Archive (1921-2011): Analysis of various solar cycle features
• Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms
• A Curious History of Sunspot Penumbrae
• Sunspot Catalogue of the Valencia Observatory (1920-1928)
• The Sunspot Catalogues of Carrington, Peters, and de la Rue: Quality Control and Machine-readable Versions

Cited by in corpus (7)

• The umbra-penumbra area ratio of sunspots during the Maunder Minimum
• Extending the Sunspot Area Series from Kodaikanal Solar Observatory
• Study of Sunspot Penumbra to Umbra Area Ratio using Kodaikanal White-light Digitized Data
• Comparison of solar activity proxies: eigen vectors versus averaged sunspot numbers
• Variations of the Internal Asymmetries of Sunspot Groups During their Decay
• Temporal and Latitudinal Variation in Penumbra-Umbra Ratios of the Sunspots: Analyses of RGO, Kodaikanal and Debrecen Databases
• Testing the accuracy of Coimbra Astronomical Observatory solar filament historical series (1929-1941)