3.2 Sunspot areas

Sunspot areas are thought to be more physical measures of solar activity. Sunspot areas and positions were diligently recorded by the Royal Observatory, Greenwich (RGO) from May of 1874 to the end of 1976 using measurements off of photographic plates obtained from RGO itself and its sister observatories in Cape Town, South Africa, Kodaikanal, India, and Mauritius. Both umbral areas and whole spot areas were measured and corrected for foreshortening on the visible disc. Sunspot areas were given in units of millionths of a solar hemisphere (µHem). Comparing the corrected whole spot areas to the International Sunspot Number (Figure 6View Image) shows that the two quantities are indeed highly correlated (r = 0.994, r2 = 0.988). Furthermore, there is no evidence for any lead or lag between the two quantities over each solar cycle. Both measures could almost be used interchangeably except for one aspect – the zero point. Since a single, solitary sunspot gives a sunspot number of 11 (6.6 for a correction factor k = 0.6) the zero point for the sunspot number is shifted slightly from zero. The best fit to the data shown in Figure 6View Image gives an offset of about 4 and a slope of 16.7.
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Figure 6: RGO Sunspot Area vs. the International Sunspot Number at monthly intervals from 1997 to 2010. The two quantities are correlated at the 99.4% level with a proportionality constant of about 16.7.

In 1977 NOAA began reporting much of the same sunspot area and position information in its Solar Region Summary reports. These reports are derived from measurements taken from sunspot drawings done at the USAF SOON sites. The sunspot areas were initially estimated by overlaying a grid and counting the number of cells that a sunspot covered. In late 1981 this procedure was changed to employ an overlay with a number of circles and ellipses with different areas. The sunspot areas reported by USAF/NOAA are significantly smaller than those from RGO (Fligge and Solanki, 1997Baranyi et al., 2001Hathaway et al., 2002Jump To The Next Citation PointBalmaceda et al., 2009). Figure 7View Image shows the relationship between the USAF/NOAA sunspot areas and the International Sunspot Number. The slope in the straight line fit through the data is 11.32, significantly less than that found for the RGO sunspot areas. This indicates that these later sunspot area measurements should be multiplied by 1.48 to be consistent with the earlier RGO sunspot areas.

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Figure 7: USAF/NOAA Sunspot Area vs. the International Sunspot Number at monthly intervals from 1977 to 2007. The two quantities are correlated at the 98.9% level with a proportionality constant of about 11.3. These sunspot areas have to be multiplied by a factor 1.48 to bring them into line with the RGO sunspot areas.

Sunspot areas are also available from a number of solar observatories including: Catania (1978 – 1999), Debrecen (1986 – 1998), Kodaikanal (1906 – 1987), Mt. Wilson (1917 – 1985), Rome (1958 – 2000), and Yunnan (1981 – 1992). While individual observatories have data gaps, their data are very useful for helping to maintain consistency over the full interval from 1874 to the present.

The combined RGO USAF/NOAA datasets are available online (RGO).

These datasets have additional information that is not reflected in sunspot numbers – positional information – both latitude and longitude. The distribution of sunspot area with latitude (Figure 8View Image) shows that sunspots appear in two bands on either side of the Sun’s equator. At the start of each cycle spots appear at latitudes above about 20 – 25°. As the cycle progresses the range of latitudes with sunspots broadens and the central latitude slowly drifts toward the equator, but with a zone of avoidance near the equator. This behavior is referred to as “Spörer’s Law of Zones” by Maunder (1903) and was famously illustrated by his “Butterfly Diagram” (Maunder, 1904Jump To The Next Citation Point).

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Figure 8: Sunspot area as a function of latitude and time. The average daily sunspot area for each solar rotation since May 1874 is plotted as a function of time in the lower panel. The relative area in equal area latitude strips is illustrated with a color code in the upper panel. Sunspots form in two bands, one in each hemisphere, that start at about 25° from the equator at the start of a cycle and migrate toward the equator as the cycle progresses.

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