DOI: 10.5281/zenodo.17108108
title: A Unified Sunspot Group Dataset (RGO/SOON/NOAA ) from 1874 to Present

description: The Royal Greenwich Observatory (RGO) compiled sunspot observations from a small network of observatories to produce a dataset of daily observations starting in May of 1874. The observatory concluded this dataset in 1976 after the US Air Force (USAF) started compiling data from its own Solar Optical Observing Network (SOON). This work was continued with the help of the US National Oceanic and Atmospheric Administration (NOAA) with much of the same information being compiled through to the present. Unfortunately, the more recent data is given in a different format from the original and there are definite changes in the reported parameters from the different sources. In an effort to append the RGO data with the more recent data David Hathaway has reformated the USAF and NOAA data to conform to the older RGO data format. The entire dataset is available below as ASCII text files containing records for individual years. Each file consists of records with information on individual sunspot groups for each day that spots were observed.Careful inspection of the data indicates that quantities such as sunspot area are not uniform across datasets or even within a given dataset. For example, the ratio of the umbral areas (the darker part of the sunspot) to total spot area (including the lighter penumbra) changes abruptly in 1941/1942 and the ratio of the total sunspot area to the sunspot number changes dramatically with the start of the USAF/NOAA data. In an effort to correct for these variations I have compared this data with the more uniform data compiled by Howard, Gilman, and Gilman (ApJ 283, 373, 1984) for the Mount Wilson photographic plate collection from 1917 to 1982. This comparison shows three epochs for the reported sunspot areas: for 1917-1941 Mt. Wilson Umbral Area = 0.35 RGO Umbral Area and Mt. Wilson Spot Area = 0.067 RGO Spot Area; for 1942-1968 Mt. Wilson Umbral Area = 0.41 RGO Umbral Area and Mt. Wilson Spot Area = 0.067 RGO Spot Area; for 1969-1981 Mt. Wilson Umbral Area = 0.59 RGO/USAF/NOAA Umbral Area and Mt. Wilson Spot Area = 0.094 RGO/USAF/NOAA Spot Area.In producing the butterfly diagram (86 kb png) (showing total sunspot area as a function of time and latitude) David Hathaway has retained the RGO Spot Areas prior to 1977 as reported but increased the USAF/NOAA Spot Areas by a factor of 1.4 after 1976. The data plotted in the Butterfly Diagram is contained in a (453KB ASCII text file) with a single record containing the Carrington rotation number followed by five records containing 10 values each of the total sunspot area (in units of millionths of a hemisphere) found in 50 latitude bins distributed uniformly in Sine(latitude).

Data Description

This Dataset contains the yearly RGO and USAF/NOAA data files. A list of corrections and updates to this database can be found  here.The data format is given in attached format.txt and can be also found here (data format). The series of data files from 1874-2024 are also available in a single 5 Mb ZIP file. Text files containing the monthly averages of the daily sunspot areas (again in units of millionths of a hemisphere) are also available for the full sun, the northern hemisphere, and the southern hemisphere. Another text file contains (daily sunspot areas) (1.51 Mb). These derived data include the correction factor of 1.4 for data after 1976. The missing days within the dataset are indicated by sunspot area values of -1 in the daily sunspot area file. The yearly RGO and USAF/NOAA data files are here.

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