This sounds more like an Ionizing form of radiation on the order of Kryptonite in order to cancel out Superman's invulnerability.
Granted, energy given off by the sun but...
Wouldn't that be "filtered Solar Radiation" as the Earth's layered atmosphere and the polar magnetic force field would screen out most of the harmful direct effects of pure solar radiation. The screening process allows just enough radiation for plant photosynthesis, as opposed to "unfiltered radiation" form the sun - which no biological / corporeal life forms could / would exist, as it would be irradiated....however....
Superman, who has been at (in) the "suns core" - which is about 27million degrees (F) at 100Billion atmospheres of pressure with gamma radiation that is off the charts, not to mention, withstanding a super-nova blast shock (gamma ray-burst) proving that he can survive such harsh conditions.