It seems Swedish researchers at Lund and Uppsala universities are saying a rapid increase of radioactive carbon in tree rings at 774/5 and 993/4AD are due to some very big solar storms. They say that have found corresponding increases for the same periods in ice cores. As a result of this they say this means they can rule out other sources for the injections. However, they also go on to say, 'these solar storms by far exceeded any known events observed by instrumental measurements on Earth ….' which implies they now think massive solar storm are a reality instead of hypothetical.
At www.spaceweather.com (October 24th 2015) we are told that at the entrance to the stratosphere, at 55,000 feet up in the air, radiation levels are 80 times what they are at sea level. In addition, it has been estimated that a plane flying at 45,000 feet (below the stratosphere) is exposed to 2.77 uSv.hr and at that rate a passenger on that airliner would absorb as much radiation as you would in a dental x-ray (during a five hour flight). This shows that i) radiation is much lower below the stratosphere and ii) low levels of radiation are not harmful.