What causes the vertical bands of light in an aurora? 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?

 A: Individual rays in an aurora do not correspond to individual particles; they correspond to separated streams of particles.  Here is a good article that explains aurora borealis.
Edit 1: Take note: in the article it's implied that the rays correspond to magnetic field lines.  That's a bit misleading. Magnetic "lines" don't really exist. They are just lines that we draw to point "downhill" in a magnetic field. But "downhill" lines can be drawn everywhere; they're not confined to specific places. The imaginary things we call magnetic field lines simply indicate the local direction of the field. 
An incoming mostly uniform swarm of particles from the Sun can become separate particle streams via a very complicated interaction between the incoming mostly uniform swarm of particles from the sun, the Earth's magnetic field, and the Earth's upper atmosphere. The streams, once formed, follow the local direction of the geomagnetic field.
Edit 2:  This is an intriguing question.  A bit of further searching turned this up, about Birkeland currents, which addresses filamentation (breaking into separate streams) of currents in the aurora.
