As a rough guess, though, how many people have "imagined" theories that turned out to be complete and utter rubbish? Rather a lot more often than the reverse, anyway. And Birkeland's initial imaginations were based on the knowledge of how to conduct an experiment, of the recent discovery of the existence of cathode rays, etc. Again, this is not to say that knowledge is more important than imagination -- merely that they both play their part, and it seems to me that placing one ahead of the other risks creating a false impression: either that you only need to read books to become a brilliant scientist, or that you needn't waste your time with such rubbish so long as you let your imagination run wild. Neither is true; a balance should be found.
There is this guide to a PhD that exists, that serves just as well for illustrating how human knowledge advances:
http://matt.might.net/articles/phd-school-in-pictures/
I think you could argue that the "circle picture" misses a subtlety that sometimes the circle is incomplete, so there is always the case for going back over and checking what we've done. But, by and large, the overwhelming majority of knowledge at any particular point in (certainly recent) human history proceeds on to the next point largely unchanged. Major revisions become scarcer -- at least one reason for this being that the number of scientists has increased significantly over time, so you have even more people around these days to check the work of everyone else.
Again, I'm not saying that imagination is unimportant -- merely that knowledge isn't unimportant either. The exact weighting between the two is surely impossible to estimate because, when trying to track a chain of reasoning, the precise starting point is usually debatable. In Birkeland's case, you might take the starting point to be when he imagined the idea of beams of cathode rays travelling from the sun to the Earth; I think it began earlier, when he set up the experiment that would go on to provide the origin of his imagination.
TL;DR -- isn't knowledge just as important really?