The drift seems to repeat daily in a regular way: between about 12 and 18 UTC the sidebands almost disappear. A similar behaviour is seen in the Central magnetometers (Figure 3).
It is also visible in the IPS CE monitor (see Figure 4) although sideband lines here have a smaller SNR.
I had a look at the day of May 6 when the ENEL power was cut off and we were running with diesel engines between 7 and 13 UTC. It is interesting that the sidebands drift did not stop, but continued during the ENEL cut off period.
This fact seems to favour the hypothesis that the 50Hz sidebands are created locally, and not coming with the ENEL. This would be a good news.
Additional informations that we have are:
(i) these sidebands disappear when there is no power on the IPS circuit (see Figure 5 and ENEL cutoff and 50 Hz sidebands), thus the hypothesis was made that they could be due to a modulation of the 50Hz by a few Hz noise from some heavy machine on the IPS circuit.
On the other hand, we also verified that (ii) they remain when we switched off all air conditioning devices in the april 2 test (see Figure 6 and Magnetic lines at 47 & 53 Hz).
Machines on the IPS that I know of, and we have not checked yet are the machines in the Tech Building, the MC air conditioning (it was not switched off for April 2 test), as well as all rain water and dirt water pumps. I think we can esily do this switch off test, before thinking of some more weird path.
