It is the Earth's magnetic field (called the magnetosphere) that protects us from the harmful energetic particles caused by flares and CMEs, so without the magnetosphere we would probably get radiation sickness. Without an atmosphere, we would get severely sunburned because the atmosphere protects us from most of the harmful UV (ultra violet) radiation that is continuously coming from the Sun by absorbing it. Of course, without an atmosphere, we wouldn't actually be here to begin with since the atmosphere is a result of water, which is vital for life.

Holly