What is a cosmic ray?

Possibly something that interacts with the earth's magnetic field and produces the spectacular "Northern Lights", or "Aurora Borealis" if you prefer latin. Charged particles moving at relativistically high speeds, some of which may be from the sun via solar flares. I think that mu mesons are cosmic rays. First discovered by a scientist in a hot air balloon (he 'd gone up with instruments to detect them), and later used to verify the time dilation result of special relativity by MIT and the "mount washington experiment".