Then you might find this interesting: https://www.jpl.nasa.gov/images/pia03480-estimated-radiation-dosage-on-mars

The main impact of a magnetosphere is that it protects atmospheric water from having its hydrogen split off by the solar wind, and then escaping, which hydrogen is far more prone to doing due to its lower molecular mass. Hence why Venus is bone dry but has nearly a hundred times as much atmosphere as Earth despite getting twice as much solar radiation. This is far too slow to be of significance to human activities, though. Terraforming will involve undoing billions of years of losses in just centuries, if you can terraform a planet then maintaining its environment is trivial.