An attempt to locate substorm onsets using Pi1 signatures

Pi1 observations, because of their higher frequency, hold the promise of providing better temporal resolution for accurate timing of substorm onsets, thus continuing to be a matter of considerable importance for evaluation of competing substorm mechanisms. In this presentation we show that the same Pi1 signatures detected by the ground magnetometer array can be used also for the spatial location of substorm onsets. We have used data from Antarctic search-coil magnetometers. To locate an ionospheric source of Pi1 signatures we have applied a method of emission tomography that was previously used in seismology. The source image reconstruction algorithm uses scanning of the volume under investigation; for each of the grid points a coherency measure for multi-channel data is calculated. For the source image reconstruction we have introduced a coherency measure, that may be coined the nonlinear semblance. Though the Antarctic stations are elongated in one direction, that is not favorable for tomography methods, the results obtained seems to be very promising for locating substorm onsets with ground magnetometer data.

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