Sentence:  Further , the corpuscular radiation , i.e. , the solar wind <WSD>protons</WSD> , must sputter away the surface atoms of the dust and cause a slow diminution in size , with a resultant increase in both the Poynting-Robertson effect and the ratio of the repulsive force to the gravitational force . 
Meaning of the word marked with <WSD> tags:  a stable particle with positive charge equal to the negative charge of an electron