To follow up with the measurements described in 47289, Craig added two MICH length lines, at 12.3 and 100.0 Hz. They were both on for the entire duration of a long lock stretch yesterday night. By demodulating both lines in H1:SUS-BS_M3_ISCINF_L_IN1_DQ and H1:LSC-DARM_IN1_DQ we can track the evolution of the MICH to DARM coupling over time. The reason to have two lines is that looking at the transfer functions plotted in 47289, one can see that the high and low frequency components change in a different way.
The plot attached shows the DARM / MICH transfer function at the two lines, as a function of time, in hours from 1235873679. The coupling changes over time both at 12.3 and 100.0 Hz.
At 12.3 Hz there is a monotonic change from 4.6e-10 to 4.5e-10, with a phase rotation of about 5 degrees. This trend settles after a hour or so.
At 100.0 Hz the evolution is more complex. There seems to be an initial trend similar to the 12.3 Hz trend (except that the phase seems to be increasing), but then there are large fluctuations in both amplitude and phase. It would be interesting to understand what was happening to the IFO during those excursions.
This test will probably need to be redone. The ASC loops were mistuned for this lock (alog 47321). The scattering arches reached up to 200 Hz, and the times of maximum scattering overlap the dips in your 100 Hz line. See attached plot.
