Cron,
Sporadic E propagation results in extreme long-distance communications on 50 and 144Mhz amateur bands, often for short periods of time only (5-10 minutes is common) and of an unpredictable nature.
The precise mechanism involved within the ionosphere is still poorly understood. However, I wouldn't call it a form of sub-refraction.
I found quite a number of websites discussing it using Yahoo! as a search engine. Most were associated with amateur radio clubs or organisations and a bit of web surfing might repay your curiosity amply. My propagation theory is a bit dated now so I can't add much more than this. Hope it helps.