In alog 16707, commissioners reported that the noise in the 30-300 Hz region was "breathing". Thomas checked that this was not due to DAC glitches. Here I argue that the noise breathing is likely due to fluctuations in the IFO alignment that can be seen in the AS WFS signals.

The first plot attached here is a spectrogram of the DARM_IN1 signal, in the quiet time period reported in the above mentioned alog. The non stationarity is very clear, and it seems slower than normal glitches. To confirm my impression, I computed the band-limited RMS (BLRMS) in the region between 200 and 300 Hz. Then, I tried to correlate the time series of the BLRMS with the time variation of other IFO channels (this is very similar to what I did at Livingston in the past, see for example 13375 and 13354).

I first looked at the IMC alignment (IMC-DOF_*), but there was no clear correlation. Then I looked into the ASC_AS_?_RF signals. The second attached plot shows a scatter plot of the BLRMS vs the various AS signals. Some of the scatter plots shows some dependency which looks like quadratic or higher order.

To confirm, I used the ASC signals, their square and their power up to the fourth to find the best reconstruction of the BLRMS. It turns out to be quite good, at least for the slowest oscillations in the noise, see the last attached plot. The 3rd and 4th orders are usefult to improve a bit the reconstruction, but not crucial.

So, my guess is that most of the noise non-stationarity comes from angular motion of the IFO. I also did the same analysis with REFL WFS, with no significant correlation.

MATLAB code is attached.