Protective sphere around Mercury due to its magnetic field about the size of the Earth. It varies considerably depending on where Mercury is on its orbit and solar activity. This variance leads to variable protection of molecules in its exosphere and causes them to vary. Stronger on the northern hemisphere than the southern one, allowing more solar wind to strike the South pole than the north one. Its boundary is the Magnetopause. It is rather leaky, much so than the Earth on stormy days, magnetic tornadoes offer paths for solar wind to directly impact the surface.
Confirmed bursts of energetic particles in Mercury's magnetosphere are a continuing product of the interaction of Mercury's magnetic field with the solar wind. Magnetic field stronger in North than South, indicating something odd about its core. This leaves the south polar regions more exposed to solar particles.