Rearrangements of Open Magnetic Flux and Formation of Polar Coronal Holes in Cycle 24

A method of synoptic map assimilation has been developed to study global rearrangements of open magnetic flux and formation of polar coronal holes (PCHs) in the current cycle. The analysis reveals ensembles of coronal holes (ECHs) that appear within unipolar magnetic regions associated with decaying activity complexes (ACs). The cause-effect relations between them explain the asynchronous PCH formation observed at the northern and southern hemispheres of the Sun. Thus, the decay of large ACs that were observed in 2014 led to formation of an extensive ECH, which then became the southern PCH in mid-2015. The intricate structure of the magnetic fields in the northern polar zone has impeded the formation of the northern PCH, although the dominant polarity at the North Pole reversed two years earlier than at the South Pole. The northern PCH formed only by mid-2016 as the result of a gradual merger of high-latitudinal ECHs associated with several decaying ACs.