An R squared of 0.3 is not really that strong of a trend and it's obviously strongly affected by the 0 days in 2003 and 2004. I think the somewhat
arbitrary cutoff point of 11 for the "Extreme" rating may be problematic for a real analysis as well. As you say, using raw data would be more
helpful for a number of reasons.
Solar irradiance varies very little across the solar cycle so it's unlikely that would appear in data which involves a number of stronger influences.
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