I didn't really understood in which way this is a "portal". Is this a clear, magnetized path, like a sudden "highway" from sun to Earth; or are we talking about portals like Portal, the game?

Neither this nor the NASA article [1] really explains what's happening. I think it's more like a "highway" or flux tube [2] but I'm not sure at all.

[1] http://www.nasa.gov/mission_pages/sunearth/news/mag-portals.... [2] http://en.wikipedia.org/wiki/Flux_tube