Solar activity over time. (a) Butterfly diagram for the Sun. Vertical colour bands highlight periods of maximum (1988–1992, 1999–2002) and minimum activity (2006–2009). (b) Averaged spot area for the highlighted periods. Blue and purple marks in (b) are the weighted mean for the activity latitude. (c) Filters F(θ|X) used for this study superimposed onto solar data (1986–2016). The parameters θ₀ and δ are the latitude and the extension of the active region, respectively.