I have been following the development of this phenomena on a huge thread at GLP. Many users there have written to NASA and all have been sent the following generic response letter. Take it or leave it, I for one still do not fully trust the reasoning NASA is giving to this event.
----------------------------------Clip From GLP----------------------
Compression artifacts, highly magnified. The images you are looking at are "space weather beacon mode" images that are telemetered down nearly continuously:
[link to stereo-ssc.nascom.nasa.gov]
in near-realtime, and are both binned (undersampled spatially) and heavily, lossily compressed digitally onboard (analogous to the various JPEG compression setting son a digital camera, but much more severe). Usually, by now (that is, three days or more after the data were obtained), we'd have the full-resolution (2048 x 2048) images, which are much less heavily, but still lossily, compressed, and are played back to a Deep Space Network (DSN) ground station via the high-gain antenna on one of the STEREO spacecraft. Unfortunately, a piece of ground hardware at DSN failed, and we're only slowly catching up on data from January 18 onward --- except the lower-resolution (256 x 256 or 512 x 512) beacon mode data.
The compression artifacts are particularly obvious when a particle (cosmic ray or solar energetic, charged particle) hits the CCD detector on the spacecraft. The compression scheme has a hard time mathematically representing sharp, single or few-pixel features, and you get a characteristic pattern of a bright dot in the middle of a compression block (a subsection of the image) surrounded by a pattern of dark dots.
Best,
Joe Gurman
(Dr.) Joseph B. Gurman
STEREO Project Scientist
--------------------------END CLIP------------------------
-E