I was lucky enough to witness a medium-strength aurora recently in Iceland (KP ~3). We saw the classic rippling arc of green light, which at times seemed to consist of smaller flickering vertical columns of light with sharp boundaries. They were similar to the photo attached below. I believe they're described as 'Rayed Band (RB)' aurora.
I understand the basics of aurora formation (charged solar particles exciting atmospheric particles, which emit light). However I'm struggling to find any information about these vertical bands - I'd like to know specifically what causes them? Is it an optical phenomena or something astro-physical? Are the atmospheric particles streaking downwards as they emit light?