John of Worcester, an English monk, recorded the sighting, on December 8, 1128, of two unusually large sunspots.
Conclusion
The Korean sighting helps to confirm John of Worcester's sighting.
Evidence
John recorded two unusually large sunspots on Dec 8th, 1128.
Five days later there was an aurora borealis observed in southern Korea, and sunspot activity is typically followed about five days later by an aurora borealis.
Evaluate
The author already provides some plausibility for the connection between sunspots and aurora borealis, but it's like this ...
If .... there is sunspot Then ... aurora borealis activity, five days later.
This argument is kind of going backwards. It establishes that an aurora borealis was sighted five days later and then tries to reason backwards to, "I guess there was sunspot activity".
We would potentially object to this argument by saying something like, .
Goal
If we make it seem like aurora borealis is a rare thing, that strengthens the likelihood that the one observed in 1128 was linked to sunspots. We could also strengthen by establishing the plausibility of the causal story --- would a sunspot seen from England actually materialize as aurora borealis in Korea?
Those two places are so geographically removed from each other that maybe we would doubt the events are connected because of that.
Look for something to rule out an alternate explanation for the Korean aurora borealis, or an answer that increases the plausibility that the Korean aurora borealis was the five-day follow-up from the sunspots that John observed.