In their paper, the team from Nagoya described their results in measuring the amount of carbon-14 in Japanese cedar tree rings that represented the years AD 750 to AD 820 with one and two year resolution. In so doing, they found a rapid, 12 percent increase in the amount of carbon-14, over the period AD 774-775, indicating that an extremely energetic event of unknown origin had occurred during that time period. They noted that the bump was approximately 20 times that seen from normal solar activity and for that reason ruled out a solar flare as a possible cause. They also ruled out a supernova as a likely source as it would have been seen and noted by people living at the time.
Carbon-14 is a variant of normal carbon-12 and tends to show up on planet Earth when cosmic particles strike the atmosphere producing showers of neutrons, which in turn strike hydrogen nuclei causing a reaction that results in the creation of carbon-14. That carbon-14 then falls and in this case, abundant amounts landed on some cedar trees in Japan 1,238 years ago. The Japanese team suggest that if such a bump was due to a solar flare it would have had to have been thousands of times larger than any that has ever been recorded, making it an unlikely possibility.
Melott and Thomas disagree and write that it's possible a solar flare could have caused the bump if it shot out in blobs, rather than as a mass ejection that spewed cosmic particles in all directions. If that were they case they say, a solar flare just 10 or 20 times the size of the largest ever recorded (the Carrington event of 1859) could very easily explain the carbon-14 bump during that time period.(Underline added). This article from Mother Nature Network further explains:
The sun could have released a huge and powerful blast of plasma into space called a coronal mass ejection, which, when it hit Earth, could have sparked the creation of carbon-14, suggest astrophysicists Adrian Melott of the University of Kansas and Brian Thomas of Washburn University, also in Kansas, in a paper published the Nov. 29 issue of the journal Nature.
Carbon-14 is a variant of the normal form of carbon (carbon-12) that is common on Earth and throughout the universe. When cosmic ray particles hit Earth's atmosphere, they can produce showers of particles such as neutrons. Some of these neutrons, in turn, hit the nitrogen nuclei that are rife in the atmosphere, and a chemical reaction occurs that transforms the nitrogen into carbon-14.
This carbon variant is unstable and decays with a half-life of about 5,730 years (meaning half of any amount of carbon-14 will be gone in that time). For this reason, it's a useful date marker: A tree, for example, will stop absorbing carbon once it dies, so the amount of carbon-14 left in it is a reliable indicator of how old it is.
It had been widely known that a jump in carbon-14 occurred in the eighth century, but researchers first pinpointed this rise and fall on a year-to-year basis by looking at tree rings in a paper by Fusa Miyake of Japan's Nagoya University and colleagues, published in the June 14 issue of Nature.
"They found that whatever made that carbon-14 bump happened really fast, and took less than one year, which called out for some really major, powerful event," Melott told SPACE.com.
The Japanese researchers considered that it might be a solar flare, but calculated that it would have had to have been thousands of times more powerful than the greatest one ever known, which made such a scenario unlikely.
Now, in a new calculation, Melott and Thomas say a solar flare is a reasonable explanation.
"Their mistake was, they assumed that the energy shot out by the sun in one of these coronal mass ejections goes out in all directions, like the light from a light bulb, but in fact it's kind of shot out in blobs," Melott said.
That adjustment meant that a solar flare need have been only about 10 or 20 times more powerful than the greatest flare on record, the so-called Carrington event of 1859. ...
... Still, the scientists can't completely rule out other explanations, such as the possibility of a supernova star explosion, or a special type of supernova called a gamma-ray burst. Both could have created a strong wave of cosmic rays as well.
However, a nearby supernova would have been extremely bright, and likely noticed by the residents of Earth at the time, who largely noted nothing unusual. A gamma-ray burst, which condenses much of the radiation released from a supernova into two strong beams, could conceivably have packed the punch necessary for the carbon-14 spike, but Melott says this scenario is still less likely than a strong solar flare.
... while a strong sun flare would have had little effect on people in 774, a similar event could wreak significant havoc today. That's because our modern technology, including satellites, radio transmissions and power grids, could be seriously hampered by the particles sweeping in from a coronal mass ejection.