Could a solar flare cause the Earth's magnetic poles to reverse? With all the hype of the impending "2012 Mayan doomsday" I was thinking it might be interesting to see what principles of physics prevent the theories of doomsday from occurring. One overarching theory is that on December 21, 2012, the two magnetic poles will reverse as a result of a solar flare equivalent to 100 billion nuclear bombs, enough to reverse the geomagnetic poles.
Is there any way a solar flare could lead to reversal of the magnetic poles? If not, what prevents it from happening?
 A: 
Theory one
a solar flare equivalent to 100 billion nuclear bombs

This is about how strong they already are.

reverse the geomagnetic poles

The Earth's magnetic field is created by the spinning of the core as well as the flows within the mantle. Reversal of the field requires a series of strong, persistent changes over hundreds to thousands of years, and no run-of-the-mill astrophysical event can just "flip" it like a switch. And even if it did, the result would probably not be classified as "catastrophic".

Theory two
the black hole Saggitarius A

Yeah, no. Not unless it suddenly warped to within a few dozen parsecs of us. Which probably won't happen. And without that, the oceans just keep going up and down like they always do.
A: It has been suggested that external events could influence the magnetism of the core. See http://en.wikipedia.org/wiki/Geomagnetic_reversal#External_triggers for some examples, but it seems unlikely that solar flares could do this. In principle if the core were in a critical state, i.e. right on the cusp of changing, it's conceivable that a sufficiently strong external magnetic field could have some effect. However the mantle is believed to screen out fast changes to the magnetic field (because it's semi-conducting) and a flare wouldn't last long enough to have much effect.
A: On December 27th, 2004 the Magnatar SGR 1806-20 quaked Earth with a gamma ray blast. It caused free electrons in the upper atmosphere to reach 100,000 times the normal density. Alledgedly this charge disappeared within an hour:
http://news.stanford.edu/news/2006/march1/ainansr-030106.html
Since the start of this century, the magnetic north pole of the Earth has been on an accelerating path of change, from the 10 miles per decade witnessed last century, up to 100 miles per decade recently observed by the SWARM satellites. Whether or not this has anything to do with the magnatar, we are currently experiencing at least a temporary magnetic pole shift:
https://upload.wikimedia.org/wikipedia/commons/7/74/Magnetic_North_Pole_Positions_2015.svg
