Observation of a substorm onset on 12 September 2001

The orbits of Polar and Cluster spacecraft in 2001-2003 are extremely well suited to investigate substorms. Here, we discuss a substorm onset on 12 September 2001 between 1800 and 1915 UT. At this time, the Polar spacecraft was at 8 Re, 0034 LT, 70° invariant latitude (IL) and the Cluster spacecraft was at 19.3 Re, 2330 LT, 77° IL. The geostationary spacecraft LANL-97A was close to local midnight. The IMAGE spacecraft was 7 Re above the northern aurora. The substorm onset was observed by these spacecraft as well as by several magnetic ground stations, but exact timing could not be retrieved due to the very localized nature of the flows and that Cluster and Polar were not on magnetic conjunctions.