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Sunspots appear as dark spots on the surface of the Sun. They typically
last for several days, although very large ones may live for several weeks. Sunspots are
magnetic regions on the Sun with magnetic field strengths thousands of times stronger than
the Earth's magnetic field. Sunspots usually come in groups with two sets of spots. One
set will have positive or north magnetic field while the other set will have negative or
south magnetic field. The field is strongest in the darker parts of the sunspots - the
umbra. The field is weaker and more horizontal in the lighter part - the penumbra.
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 I have 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. The data
format is given in a text file. The series of data files
from 1874-1997 are also available in a single 4.9 Mb ZIP
file.
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 my butterfly diagram (142 kb GIF image) (570 kb postscript file) (showing total sunspot area as a
function of time and latitude) I have retained the RGO Spot Areas prior to 1964 as
reported but increased the USAF/NOAA Spot Areas by a factor of 1.4 after 1976 and ramped
up from a factor of 1.0 in 1964 to 1.4 in 1976 over those intervening years. 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 found in 50
latitude bins distributed uniformly in Sine(latitude). Note that the data in this file does
not contain the correction factors given above. Text files containing the monthly
averages of the daily sunspot areas 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 data do
include the correction factors.
The yearly RGO and USAF/NOAA data files are:
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