A brief mention in New Scientist (17 April 2004, page 8):

"Astronomers have been puzzled by the Sun's X-ray halo since it was first detected in the 1940s... The surface of the sun is not hot enough to produce such a bright X-ray glow"

An, although the article suggests the decay of axions, they still don't know what causes the X-rays. In my mind this means they also don't know whether the amount of X-rays is prone to fluctuations.... big enough to fry us or at least mutate us!