It is often said that the plasma exiting the coronal hole (of the Sun, obviously) follows the open magnetic field line. E.g. a direct quote:
"Coronal holes are regions of low-density plasma on the Sun that have magnetic fields that open freely into interplanetary space".
So my question is: since the magnetic field is in its essence a solenoidal vector field, could its lines be truly open? That would make the coronal hole a certain magnetic monopole, wouldn't it? Or do they actually close somewhere at the heliospheric boundary, for example?
It's either that or maybe my understanding of electromagnetism doesn't quite fit into plasma behaviour (I've heard this story of "frozen in magnetic field lines", but fail to see how it answers my question), in which case I also ask for some correction/explanation.