Temperatures in the flares can get as high as 100,000,000 degrees K. effect of flare . Solar activity changes in step with the solar cycle. The changing surface of the sun has a great effect on the Earth. 1 CME anim 2 Satellites get fried by Coronal Mass Ejections. Powerlines get overloaded.
Astronauts can be in big trouble.
As we rely more on space for telecommunications space weather will be more of an issue. In the full chain is pretty complicated with He being the end result. During the reactions particles called neutrinos are produced. Energy, in the form of gamma rays, is produced in each reaction. One major issue is we don't detect the number of neutrinos on Earth we expect from theory.