These belts, located in rings around the North and South poles . Yohkoh doesn't pass through these belts, but other satellites, such as the Chandra X-Ray Observatory and the Transition Region and Coronal Explorer pass through them on a regular basis. Chandra initially had difficulties with the Van Allen belts as the instrument was typically bore-sighted right through these radiation zones, and the energetic protons were focussed directly onto the CCDs. Another source for potential CCD damage would be solar events , discussed in myriad detail in these science nuggets. Recent nuggets demonstrate significant enhancements of the particle flux at Earth.
Dark current is spurious charge that builds up in each pixel on the CCD during exposure.
This is a function that is dependent on the substrate, the type of CCD, the mode it's operated in, the temperature it's operated at, how badly it has degraded over time, and a host of other things. First, let's have a look at the data. Here's two images from the Yohkoh dark current database. If one places any creedence in a statistical sample of two images, things have indeed changed in the intervening 9 years since launch. For a real study, I'd use the entire database, but for a short study like this, two images serve our purposes just fine.