Are coronal hole magnetic field lines open? 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.
Thanks.
 A: Field lines are always closed. The following diagram shows what your book appears to be talking about (from http://soi.stanford.edu/results/SolPhys200/Poletto/uvcs_spiral.jpg):

Some field lines look obviously "closed" while others "go off into space". But then they turn around and come back again... something like this:

I found the caption that goes along with this image at 
From there I quote (emphasis added by me):

The white rays are lines of magnetic force from a theoretical model
  of the magnetic field of the Sun at the minimum of solar activity
  (see Banaszkiewicz et al. 1998, Astron. Astrophys., 337, 940).
  The polar coronal holes (dark regions on the solar disk and in the
  extended corona) have primarily open magnetic field lines along
  which the high-speed component of the solar wind accelerates away
  from the Sun.  In the extended corona viewed by UVCS, the density of
  particles is so low that individual ions very rarely collide with
  other particles, and thus they execute spiraling motions around the
  magnetic field lines (see green curve for an illustration of an
  example particle's motion).

A: I know this is older thread but was doing a bunch of research on this, the is the best answer I found:

Without violation of Maxwell's laws, all magnetic flux must ultimately
form closed loops, but it is nevertheless useful in many situations to
talk in terms of “open” flux, meaning field lines which reach some
boundary before they close. In the solar literature, the source
surface is often used as the open/closed boundary [e.g., Wang and
Sheeley, 1995; Arge and Pizzo, 2000]. In the heliosphere, where all
field lines are open using the source surface definition, there are
observational signatures of magnetic loops with both foot points in
the solar atmosphere (and hence termed “closed”) and so “open flux” is
often defined as flux which reaches the heliopause, where it
presumably reconnects with the interstellar magnetic field.


https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JA016039
