If the magnetic field completely disappeared tomorrow the top layers of the atmosphere would start to be ionized and stripped away. There'd also be a slow chang

If the magnetic field completely disappeared tomorrow the top layers of the atmosphere would start to be ionized and stripped away. There'd also be a slow change in the composition of the atmosphere due to neutron bombardment. The process would only take a mere million years or so! In the meantime the Earths atmosphere alone also blocks cosmic radiation to the extent that there's no immediate harm to life.