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 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 switching on-off with a cycle of approximately 4.4s (0.23 Hz): on for 2.2s and off for 2.2s.
The noise is seen by all magnetometers site-wide. Internal magnetometers sees it a factor up to about 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 narrow spectral 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/). While s is a good news for the interferometer, it does not help much locating the source.
Measurements will be done also to check for possible external sources, like it was for galvanic currents in the natural gas pipelines (https://logbook.virgo-gw.eu/virgo/?r=55542).