Central position of the source region of storm-time chorus

We describe an analysis method to estimate the central position of the source region of electromagnetic waves. The method is based on simultaneous measurements of the Poynting flux in several points in space. We analyze measurements of whistler-mode chorus by the four spacecraft of the Cluster mission close to their perigee at a radial distance of 4 Earth radii. The data were recorded on the nightside during disturbed periods on 31 March 2001 and 18 April 2002. The derived central position of the chorus source region is close to the geomagnetic equator, as it has been shown by previous studies. This global centrum of the source region is defined by the equilibrium of the Poynting flux parallel and antiparallel to the field line. The central position is found to randomly move back and forth with an amplitude of a few thousands of kilometers on time scales of minutes. The typical order of magnitude of the speed of this motion is 100 km/s. We exclude the possibility of flapping of a tail-like configuration of the ambient magnetic field. These random fluctuations of the central position of the source region are thus connected to the generation mechanism itself.