Sunspot Unit Area: A New Parameter to Describe Long-Term Solar Variability

In this Letter we introduce a new parameter, the sunspot unit area (SUA), which describes the daily average size of sunspots produced by the dynamo in a solar cycle. The monthly average of the SUA is studied to show its variation during the solar cycle and is compared with the two commonly used parameters, sunspot numbers and sunspot areas. Our results show that (1) the new SUA parameter waxes and wanes during sunspot cycles in the same way as the other two parameters, but with a relatively small magnitude of fluctuations, (2) the Waldmeier effect and the Gnevyshev-Ohl rule, which appear in the variations of both sunspot numbers and sunspot areas, are not shown in the variations of SUA during solar cycles, and (3) the most pronounced period of SUA variation is about 10.13 yr, which is statistically significant at any time of the considered time span.