Strong variations of cosmic ray intensity during thunderstorms and   associated pulsations of the geomagnetic field
  Strong variations of the intensity of secondary cosmic rays during thunderstorms are found to be accompanied in some cases by very clear pulsations of the geomagnetic field. The experiment is carried out in the Baksan Valley, North Caucasus, the Carpet air shower array being used as a particle detector. Magnetic field measurements are made with high-precision magnetometers located deep underground in the tunnel of the Baksan Neutrino Observatory, several kilometers apart from the air shower array. 
K. Kh. Kanonidi, N. S. Khaerdinov, A. S. Lidvansky, and L. E.   Sobisevich
physics.geo-ph