There are scattered light arches in h(t) during bad weather that are correlated with the ground motion in h(t).

Figure 1 shows an example from May 21, the black line shows the speed of the ground motion in WEB (as measured by local controls that measure the position of the WE mirror compared to the ground), and that trace explains well the scattered light glitches seen in h(t)

Figure 2 shows the same time on the B8 photodiode, with superimposed the speed of the difference in position between SWEB and WE. The scattered light arches between 5Hz and 10Hz are well explained by the motion of SWEB compared to the suspended mirror.

Figure 3 shows again the B8 photodiode but with superimposed the double of the ground speed. This explains well the scattered light arches around 50Hz. Note that these are different from the scattered light arches seen in h(t).

Figure 4 shows the spectrum of B8 during that time. The scattered light arches below 10Hz have an amplitude that is ~100 higher than the arches around 50Hz, and most likely the scattered light seen at ~50Hz in B8 is to faint to be also visible in h(t).

Figure 5 and 6 are another example one minute earlier in the same night for h(t) and for B8.

Conclusions

• The SWEB control is working well to supress scattered light from h(t)
• h(t) sees scattered light from first order scattering from  ground connected object in WE building
• B8 sees mostly scattered light from a second order scattering from a ground object, so at double the frequency of what is in h(t)
• The scattering seen in h(t) doesn't look visible in B8, this could imply that scattering is happening on the HR side of WE. So B8 doesn't see it as it is attenuated by the HR transmission.

/users/mwas/SBE/scatter_20240521/arches.m