The Significance of Dipole Tilt for Substorm Onsets

One important factor that decides the location of substorm onset is the dipole tilt angle, defined as the angle between the earth's north dipole axis and the GSM z-axis. Naturally, the solar wind also plays an important role in the determination of the location and ionospheric footprint of the magnetospheric onset. A combination of ground-based signatures have been used to time substorm onsets from magnetometer and photometer data, viz. Pi2 onset, poleward motion and simultaneous brightening of 486.1 nm proton emissions and 557.7 nm electron emissions. The rapid poleward motion of brightening auroral arcs indicates acceleration of magnetotail electrons, likely from a reconnection event. When the onset locations are mapped to the magnetotail with Tsyganenko's T87, T89, T96, and T01 models, all the models indicate significant differences are found to depend strongly on dipole tilt. Near-earth onsets consistently feature large negative dipole tilts, with onsets moving progressively further downtail as tilt increases. These results are consistent with previous observations of the effect of dipole tilt on magnetotail boundaries.