N1RGR, you are talking about a slightly different thing I think, not HAARP.
The "drifters" (what I call them in my log) that you are describing have been around for decades. I remember hearing them in the 60's when I first started playing with radios. I have no real idea what they are, but from their actions/characteristics you can make some assumptions of what they are not. There are some known signals that sweep, various radars, sounders, and HAARP, as mentioned before. But that still leaves a lot that fall into “other” categories.
First, not all of them are the same, I have seen several different types, and they have different actions. Several may be natural, but the very repeatable ones (and the majority I have seen) probably are not. The natural ones are more random and hard to pin down, the probably not natural ones often have indicators below.
Many have very fixed and linear sweep rates across wide bands. This linear rate would argue against accidental, and clearly indicate against natural.
Many display very low levels of modulation on the carrier. In several cases I have seen 120 Hz hum on the signal. This would possibly indicate a source with 60 Hz mains power. On others I have seen 100 Hz hum, likewise possibly indicating 50 Hz mains power. These would argue against natural, but say nothing in regards to accidental.
Many show a very clear and low noise carrier, with the exception of the modulation I mention above. This would argue against natural.
Almost all of them I have seen drift down in frequency. No idea of what that might indicate, but if accidental I would assume a more random distribution of sweep directions.
Most have not displayed detectable levels of synthesizer step. This could be as simple as they are not synthesized, or might just be that the steps are below my ability to detect. One means analog, and the other means nothing at all
T!