Towards the understanding of coronal hole occurrence during the Schwabe cycle
Authors:
M. Storini, M.Y. Hofer, J. Sykora
Image & caption:
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Image caption::
Distributions of the isolated CHs considering their central latitude (left panels) and results from the linear regression analyses between the CH central latitude and time (right panels). See text of the paper and the panel legends for details.
Description:
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 the 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.
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