tokens: T.W. thanks Durham University Department of Mathematical Sciences for funding his Ph.D. studentship. A.R.Y. thanks STFC for financial support. A.R.Y. and G.J.D.P. thank ISSI, Bern for supporting their collaboration as part of an International Team on global magnetic modelling. A.M.J. is very grateful to George Fisher and Stuart Bale for their support at the University of California, Berkeley, and Phil Scherrer for his support at Stanford University. This research is partly funded by the NASA Grand Challenge grant NNH13ZDA001N and NASA LWS grant NNX16AB77G. Data were acquired by SOLIS instruments operated by NISP/NSO/AURA/NSF. We are grateful to David Hathaway for also supplying data for this study and to Ian Vernon for his invaluable statistical advice as well as to the anonymous referee for useful suggestions.
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