Magnetotail topology around substorm onset time

The time evolution of the magnetotail topology during minutes preceding and following substorm onset is an important question of the magnetospheric dynamic. The substorm onset is a local and explosive phenomenon, which makes it difficult to describe with in situ data. In this study, we investigate this problem thanks the remote sensing data provided by the FUV camera onboard the IMAGE spacecraft. In the first part of this study, we use a set of IMAGE-FUV and GOES-8 simultaneous observations obtained during substorms periods to develop a model of the magnetic field elevation angle at geosynchronous orbit. In the second part, we use a set of 259 substorms observed by the IMAGE spacecraft between 2000 and 2002. Taking advantage of the global scale of the observations provided by IMAGE-FUV, we describe the time evolution of the magnetotail topology provided by the model. An interesting result shows that the field is symmetrically stretched around the onset position before the onset time and this symmetry is broken after onset, with a more important stretching duskward to the onset.