We carried out the investigation of probability density function features for magnetic field fluctuations in the Earth's magnetosphere tail on different time scales on the basis of the satellite Geotail data for 1997, 1999 and 2000. The changes of shape and parameters of probability density function for the periods before and during the current disruption were studied. As an evolution characteristic for different time scales the changes of maximum probability density function values P(0) were investigated. Two asymptotic modes for P(0) corresponding to two different power lows were found: the changes of probability density function maximums correspond to the Gauss process on large time scales and to the Levy distribution on small time scales. The intersection of the two asymptotes corresponds to a time scale value of 1 sec. The proposed approach is universal and can be used for the analysis of fluctuations of other parameters being of other nature.