ИСТИНА

Most of space experiment data suffer from a bias effect due to a limitation of dynamic and spatial ranges for measurements. Many empirical models just neglect this effect that lead to wrong predictions of extreme events. We discuss a classical problem of biased data such as magnetopause crossings of geosynchronous orbit, a circular orbit with the radius of 6.6 Re. The magnetopause moves to the distances below 6.6 Re under strong solar wind dynamic pressure Pd and/or large southward IMF Bz. However, there is a lower limit of Pd and Bz values under which the magnetopause does not cross the geosynchronous orbit at a given latitude and longitude. These limit values forms a so-called envelope boundary, which can be represented as a surface in multi-dimension space of parameters Pd, Bz, latitude and longitude. Determining this surface allows developing a model for prediction whether or not the magnetopause crosses the geosynchronous orbit. Variations of the envelope boundary with a storm-time geomagnetic index Dst give important information about influence of internal magnetospheric process to the dynamics of the magnetopause.