Virgo Logbook

I made a study of the time evolution of the peaks in the range 5600 to 5900 Hz, using the data provided by the Line Monitor web tool (https://slvtf.virgo.infn.it/vtf/wwwDev/lmTest/stable/index.php). Many peaks are found to match, after a change of sign, the time evolution of the well known drum modes of the NE, WE, and NI. Thus they can be attributed to such mirrors. Figure 1 summarizes the identified peaks with color codes.
Here is the detailed list of frequencies:

- 5695, 5773, 5785, 5810 Hz peaks associates to the NI mirror. Figure 2 shows for example the 5785 Hz peak.

- 5615, 5640, 5708 Hz peaks associates to the NE mirror. Figure 3 shows for example the 5615 Hz peak.

- 5692, 5701, 5725, 5763, 5800 Hz associates to WE mirror. Figure 4 shows the 5701 Hz peak.

It is INTERESTING to NOTE that, with the exception of the 5701 Hz peak, all WE peaks listed here show a jump downwards at GPS 867615044 (July 4 20:10 UTC) that coincides with the relock after an intervention on WE (Elog 17432), which required opening the oven and caused a large temperature increase (+0.3 deg) inside the WE tower.
The size of the jump is variable: the 5692Hz jumps by -10Hz, the 5725Hz jumps by -0.5Hz, the 5763Hz jumps by -1Hz, the 5800Hz jumps by -2.5Hz.

Figure 5 shows the time evolution of the 5800 Hz peak and compares it to the temperature of WE mirror.

We can conclude that all these peaks are alias of mode of one arm mirror. Using the table of theretical predictions Michele provided me, the closest match seems to be the 212 mode,
whose thoretical value is 14470 Hz (its aliased value is 5530 Hz). The matching is not very good, but may be reasonable. But remains to explain the apparent splitting of this mode.