Summary

Looking at one hour of data during the good lock of last night (47384) we can see that there is a lot of non-stationary noise at low frequency (below 30-40 Hz) most likely due to scatter light driven by the OMC ASC loops (47321)

Details

Here's a spectrogram of one hours of data from last night (using CAL-DELTAL_EXTERNAL and without compensating for the whitening, so that there's no spectral leakage from the low frequency motion).

A direct comparison of a "quiet" period and a "noisy" period is shown below (quiet is +500s from start, noisy is +3035s from start).

To better correlate the noise level with the OMC ASC motion, I computed band-limited RMS (BLRMS) of DARM in two regions: 10-14 Hz and 28-32 Hz. There is a correlation between the OMC ANG and POS signal RMS and the DARM noise. I compared the OMC ASC signals in the two "low" and "high" noise periods as above, and found that the main difference is in the amplitude of a 0.5 Hz peak:

Therefore I computed the BLRMS of the OMC ASC signals around 0.5 Hz (between 0.4 and 0.6 Hz), and compared it below with the DARM BLRMS. There is an obvious correlation between times when the OMC ASC signals move more at 0.5 Hz and higher scattered light noise in DARM.