Detection of Phase Jumps of Free Core Nutation of the Earth and their   Concurrence with Geomagnetic Jerks
  We detected phase jumps of the Free Core Nutation (FCN) of the Earth directly from the analysis of the Very Long Baseline Interferometer (VLBI) observation of the Earth rotation for the period 1984-2003 by applying the Weighted Wavelet Z-Transform (WWZ) method and the Short-time Periodogram with the Gabor function (SPG) method. During the period, the FCN had two significant phase jumps in 1992 and 1998. These epochs coincide with the reported occurrence of geomagnetic jerks. 
Toshimichi Shirai, Toshio Fukushima, Zinovy Malkin