Analysis of the Substorm Trigger Phase Using Multiple Ground-Based Instrumentation

During the substorm trigger phase, i.e. during the last few minutes before the onset, the slowly and linearly developing growth phase features strengthen exponentially. Often the enhanced auroral activity fades abruptly during a couple of minutes just prior to the breakup. On 5 October 1986, such fading was observed with an exceptionally good coverage of ground-based instruments (all-sky-camera, EISCAT radar and magnetometer cross, riometers and pulsation magnetometer) in northern Finland and Scandinavia. The global activity is characterized with IMF B-z, the Polar Cap index, and AU and AL indices. We present and analyze the local observations in detail and discuss the possible causes for the fading.