Properties of sunspot umbrae of leading and trailing polarity in 1917–2013

Using the software developed by us, we produced a digitized (tabulated) database of sunspot umbrae and pores observed at Mount Wilson Observatory (MWO) in 1917–2013. The database includes the heliographic coordinates, areas and the polarity and strength of magnetic fields of umbrae and pores in the MWO sunspot drawings. Using this database we study here the properties and long-term variation of sunspot umbrae and pores, separately for leading and trailing polarity spots. We find that the leading sunspots have tendency for larger umbrae and stronger magnetic field strength than the trailing spots. The average field strength and area of sunspot umbrae vary with sunspot cycle. Furthermore, the mean magnetic field strength in sunspot umbrae exhibits a gradual increase from early 1960s to 1990s. The nature of this increase is discussed.