Azimuthal expansion of the dipolarization at geosynchronous orbits associated with substorms

Abstract. Dipolarization, one of the main indicators of substorm expansion onset, represents topological changes in the magnetotail. It is believed that dipolarization is initiated at a longitudinally confined region in the tail, followed by the azimuthal expansion. There are very limited studies in the literature regarding the azimuthal propagation of the dipolarization front in the magnetotail. In this study we have used ten years of GOES data and POLAR and IMAGE data to study the characteristics of the propagation of the dipolarization fronts at the geosynchronous orbit. We have identified a number of dipolarization events in the GOES magnetic field data and substorm onsets from POLAR UVI and IMAGE-FUV measurements. Using the delay of dipolarization signatures at the two GOES satellites and onset times from POLAR and IMAGE measurements we have estimated the propagation speed of the dipolarization fronts. The calculated speeds vary between 10 km/s and 420 km/s and show a power law distribution.