Your assumption is correct - it is indeed about infrared (wavelength between 750 nm and 10,000 nm) and ultraviolet (wavelength between 100 nm and 400 nm) radiation. The sensor detected two specific wavelengths, one from each range.
Every body has an electromagnetic radiation which is consistent with its temperature. This radiation has an intensity distribution which shifts to shorter wavelengths with temperature. By comparing the intensity of the IR and UV parts of this radiation, the sensor can much better distinguish between radiation sources of different temperature. Flares are normally hotter than engine exhausts, so by detecting an excess of UV radiation, the sensor can distinguish between flares and exhaust pipes.
"Wiens law" by 4C - Own work, based on JPG version. Licensed under CC BY-SA 3.0 via Commons.
Natural disturbances means reflected sunlight off a polished aircraft surface or canopy, or off lakes or windows on the ground. Even after filtering by the upper atmosphere, sunlight will contain much more UV radiation than your average exhaust pipe, and glares were a big distraction to early thermal seekers. Sunlight would correspond to the green line in the diagram above, while the exhaust would be even below the dark red line.
Note that the absolute intensity also goes up with temperature, but would not help in identifying the temperature of a target if only one wavelength is observed. A more distant, hotter target would cause the same intensity than a colder, closer one. By sensing two wavelengths, the sensor could much better tell an exhaust pipe from other heat sources.
To answer the question directly: The UV signature of the aircraft itself is practically zero except for the reflection of the UV part of sunlight, and the active UV emission of a jet engine is extremely low and concentrated around its exhaust when seen from behind. Lighting the afterburner will increase the UV radiation, but it will still be much less relative to the IR radiation that that of, say, burning magnesium.