I wonder if the "ribbon" is due to magnetic reconnection where the heliospheric neutral current sheet in the interplanetary magnetic field (IMF) interacts with the galactic magnetic field. That's one mechanism for producing a feature with this geometry that would be capable of accelerating particles. (There may be others that I haven't thought of, of course.)
If the ribbon lies roughly in the plane of the ecliptic, that would tend to confirm the hypothesis. If the ribbon were to disappear temporarily when the IMF switches polarity at the end of an 11-year solar cycle, that would also be indicative.
ETA: The theorists took less than two months to produce a viable explanation in terms of the galactic magnetic field acting as a kind of particle mirror:
I'd still like to know if the current sheet has any influence.