The umbra-penumbra area ratio of sunspots during the Maunder Minimum

Abstract: The Maunder Minimum (MM) was a prolonged period of low solar activity that occurred between 1645 and 1715. The true level of solar activity corresponding to this epoch is still a matter of debate. In order to compare solar activity during the MM with that of other epochs, we have evaluated the umbra-penumbra area ratio (U/P hereafter) during the MM. Thus, we have analyzed 196 sunspot drawings including 48 different sunspots observed during the period 1660-1709. The mode value of the ratio obtained from the occurrence frequency distribution lies between 0.15 and 0.25. Furthermore, the median and mean values are equal to 0.24 \pm 0.07 and 0.27 \pm 0.08 with a sigma clipping, respectively. These results are consistent with recent research using more modern data. Higher U/P values mean faster sunspot decay rates. From our results, the almost absence of sunspots during the Maunder Minimum could not be explained by changes in the U/P since the values of the ratio obtained in this work are similar to values found for other epochs.