  The level of background signals in modern cryogenic resonant mass
gravitational wave antenna is discussed caused by (a) the geomagnetic field
pulsations and (b) an atmosferic of very low frequency band, generated by a
lightning flash. The analysis of our results show that the signals of this
origin will generally exceed the signals from the gravitational wave sources.
To suppress these artifacts in such gravitational antenna, it is necessary to
use the magnetometer included as anti-coincidence protection and a system of
magnetic screens.
