Secondary radiation
Primary radiation includes cosmic rays, radiation from the decay of local atoms, and radiation from the sun. Secondary radiation is radiation created by particles hitting atoms and generating other forms of radiation. Low energy radiation produces little or no secondary radiation, the majority of secondary radiation comes from cosmic rays.
For example, a cosmic ray slams into the atmosphere and creates a mass of secondary particles, all with lower energies than the original particle. These travel within 1% of the direction of the original particle. Some of these will have charges (and are thus be effected by electric and magnetic fields), others will be neutral particles which will ignore them. The high energy secondary particles may have enough energy to make further (lower energy) particles, etc. This can create a swarm of secondary particles which are more dangerous than the single original cosmic ray.
On Mars, the thin atmosphere allows more cosmic rays to reach ground level, so less secondary radiation is produced. However, that which is produced is less likely to be slowed by more air, and so will slam into the surface. Secondary radiation can also be produce by hitting the walls of the habitat, or the radiation shielding of the habitat.
Overall, the result is that the secondary radiation from cosmic rays is higher than on Earth. When planning on creating radiation shielding, both primary and secondary radiation should be considered.
