We have here a list of magnetic excursions going back over 100 thousand years. This is not a complete list, but a list of events they have found evidence of. One explanation of this is that spiralling from the core is a dust sheet along the galactic plane. I don't have an article to specifically back this up but it is something I will get around to one day. Our solar system intersects with this spiralling cloud every twelve thousand years as we obit perpendicular to the plane. The hypothesis would be that we would traverse the denser part of the cloud as we pass through every twelve thousand years and a less dense part of the cloud six thousand years later as the solar system completes its orbit.
This opens up several times that need to be researched.