ANALYSIS OF THE SUBSTORM TRIGGER PHASE USING MULTIPLE GROUND-BASED INSTRUMENTATION

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