So I have a large number of signals like the one in the first picture below and I would like to extract and compare the frequencies within them. I applied a Fourier transform which resulted in results like those shown in the second picture. The large power of very low frequencies suggests to me that the signals include something that is interpreted as being of such low frequency that one cycle does not fit into the time window (which is 1.2 seconds). I hence applied a High pass filter with different cut-offs and the pattern is gone after using a HPF with 2Hz (last picture).
I have two questions about this: 1) Does my reasoning make sense? 2) How is such an effect / pattern called?
I need to include this in a paper I am currently drafting and hence am especially unsure about question 2). My supervisor's first reaction was that these are edge-effects, but looking into them I feel like that is not really what is at work here.