A new noise is seen in the external magnetometer starting April 22 19:56 UTC. Figures 1 and 2. The noise has been detected and studied by Renato Romero and Federico. Here is a report of the investigation so far.
The noise is impulsive, as seen in Figures 3,4,5. Observed in the time domain it consists of a carrier at about 8Hz modulated in amplitude at approximately 0.28 Hz (period of 3.6s).
The noise is seen by all magnetometers site-wide. Internal magnetometers sees it a factor up to 10 larger than the external "N" magnetometer, in Figure 6 and Figure 7 (the EXT_N magnetometer is the gray spectrum in all 3 plots). Magnetometers that see it a bit more intense are those close to NI, WI and WE towers.
A close look to the spectral fine structure of the noise shows that it does not follow the mains, in Figure 8.
These hints suggest a DC powered PWM source (Federico).
Bruco runs using ENV_NI_MAG_V as target, and EXCLUDING all magnetometers (ENV_*MAG*) channels, show that this disturbance is not coherent with any other ENV channel (https://scientists.virgo-gw.eu/DataAnalysis/DetCharDev/bruco/users/fiori/ENV_NI_MAG_V_1397853318_ENV/) with the exception of a small coherence with the ENV_CEB_ELECTRIC. In particular no coherence, nor evidence of the noise is seen in all IPS and UPS probes. No coherence is also measured with any other Virgo signal (https://scientists.virgo-gw.eu/DataAnalysis/DetCharDev/bruco/users/fiori/ENV_NI_MAG_V_1397853318/).
Measurements will be done also to check for possible external sources, like it was galvanic currents in natural gas pipes (https://logbook.virgo-gw.eu/virgo/?r=55542).