What causes geomagnetic activity during sunspot minimum?

@article{Kirov2014WhatCG,
  title={What causes geomagnetic activity during sunspot minimum?},
  author={Boian Kirov and Simeon Asenovski and Katya Georgieva and V. N. Obridko},
  journal={Geomagnetism and Aeronomy},
  year={2014},
  volume={55},
  pages={1033-1038}
}
It is well known that the main drivers of geomagnetic disturbances are coronal mass ejections whose number and intensity are maximum in sunspot maximum, and high speed solar wind streams from low latitude solar coronal holes which maximize during sunspot declining phase. But even during sunspot minimum periods when there are no coronal mass ejections and no low latitude solar coronal holes, there is some “floor” below which geomagnetic activity never falls. Moreover, this floor changes from… 

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