Well the only thing I can add is anecdotal...

I get 'solar flare' headaches. I had one today, as a matter of fact -- mentioned it to my husband at the time and said, I wonder if we just had a solar flare.

On the day of the dreaded M9, I got no headache at all. NONE and I expected a blistering one. The only explanation I can come up with as to why the M9 produced no headache, but any C class flare (or even the B flare we had today) gave me headaches, sometimes bad enough that I needed some ibuprofen to get rid of them, is that *maybe* when it happened, I was inside a concrete building with a metal roof. But I can't corroborate that for sure since I didn't know we'd had the M9 until two days later (because I was so busy last weekend that I didn't even get on GLP or any other internet at all).

Sorry its not more conclusive -- but I was puzzled why I hadn't gotten a whopper of a headache that day -- I should have gotten a migraine based on how strong a headache I get when there are higher C flares.

*edit to add: I have noticed on C flare days, that even being inside my building at work is not quite enough to keep me from having a headache at all -- just diminishes it. So even if I was inside during the M9 I still should have felt something, and I didn't at all :-/