Towards the understanding of coronal hole occurrence during the Schwabe cycle

The search for substructures in the latitude/time distribution of long-lived isolated coronal holes (age > 5 Carrington Rotations) during sunspot cycles 21 and 22 reveals that they are organized into two populations: (i) class-I, occurring around the maximum phase of the solar activity, made up by long-lived holes of both magnetic field polarities and lying on a medium-term (time extent: 2–4 years) diagonal area, from middle heliographic latitudes towards the equator and beyond; (ii) class-II, characterized by long-lived holes having a high probability to occur on a long-term (∼18 years, i.e. lasting for more than a solar activity cycle) diagonal region, one for each magnetic field polarity. Moreover, the slopes and time lengths of the diagonals for the class-I holes are different for the two cycles analysed, while the ones for the class-II are similar. While clues for class-I existence were found in the past, class-II identification based on the CH magnetic field polarity is made here for the first time. The discussion points out implications for the odd–even Schwabe cycle pairs.