alt Hacker News> Radiation at ISS: 144 mSv per year
> Radiation on a trip to Mars: ~340 mSv per year
This seems to track with research that during a geomagnetic excursion[1], where the field strength dropped to about 10%, the cosmic radiation seems to have roughly doubled[2].
To steelman the argument, perhaps what the magnetosphere is doing is stopping the atmosphere from making too much carbon-14. In shielding the surface from energetic cosmic rays, neutrons are produced, and these transmute N-14 to C-14 by the (n,p) reaction.