re: The main field is generated by electrical currents in the core-mantle boundary
How was this determined or is it just an educated guess?
But you are partly correct: the total field strength of the magnetosphere varies by up to 20 percent due to solar wind, which distorts the field lines and deposits charged particles into the van Allen belts.
I assume that these effects have been detected with instruments, so there is some basis to them.
What I'm wondering about is that, if the field can be affected up to 20 percent, then why not 100 percent?
Johannes