Figure 1 the decrease in range is also related to a strong increase in ground motion

Figure 2 the data below 50Hz became also more glitchy. This had already happened a few days ago, and to a lesser extent the last time there was larger ground motion on Feb 11.

So the origin of the change in noise level is likely ground motion. It would be very interesting to understand what is the noise coupling path and if there is a way of mitigating it so sensitivity is not spoiled and with many glitches during bad weather.

The attached plots compare hrec with other signals (I picked them by heart and by memory) which also show an increased rate of low frequency glitches in the period of high microseism (Feb 27 morning).

In the first: MICH,PRCL and SRCL lenghts show the correlated glitches. I guess this is expected.

In the second: SNEB, SWEB and also IB_TX show and increased glitch rate, yet these do not look correlated.

For comparison,  the third shows the same signals in a period of low microseism (Feb 26 morning). Are there other signals to look at? A brute force approach here would be surely more effective.

These look like scattered light when looking at LSC SRCL, something moving with a period of ~2.8s (a frequency ~350mHz). Scattered light arches happen at the maximum of the speed of the motion, so twice per period of motion of the scattering light.

In the attached plot I used as velocity sensor the CEB guralp. Although the amplitude does not match, the timing and pahse of the arches seems to correspond to microseism. Possibly A.Longo's brute force tools can help to identify the location.

Following suggestions of Maddalena we have found that the SRCL arches are reproduced by the RFC signal (Figure 1).

Figure 2 shows that the SRLC arches are very well reproduced by the velocity measured by SBE_EIB_GEO_H2_200Hz (blue curve), as already found by A.Longo.

The gps used is the same used by A.Longo: 1393042683, 60s

We also tried as velocity sensors the ENV episensor placed on EIB (ENV_EIB_SEIS_X and ENV_EIB_SEIS_Z) and the episensor placed on SIB1 tower South flange (ENV_IB_SEIS_Z). This is shown in figures 3, 4 and 5, respectively.  Among these, the arches are best reproduced by the sensor ENV_EIB_SEIS_X (Figure 3).

Yet overall the sensor SBE_EIB_GEO_H2_200Hz seems the winner. We do not know were exactly this sensor is located and how is oriented.

It would be interesting to see if a similar problem can be reproduced in a controllable by moving EIB in X  and Z during a calm time, and see if the same issues of scattered light appear in that case.

In the past we used to have issues with scattered light going back and forth between EIB and the laser bench. Are we able to reconstruct the relative motion between EIB and the laser bench out of the EIB sensor and the laser bench seismometer? To check if that is an even better explanation for the observed scattered light arches.