Earth's Ground-Based Magnetic Perturbations in Response to Substorms

We have produced large-scale maps of the surface perturbations in the Earth's magnetic field following the onset of substorms. Data from 72 ground magnetometer stations in the northern hemisphere at geomagnetic latitudes over 60 degrees were used. Upstream interplanetary magnetic field (IMF) data from the ACE satellite were also used for sorting ground magnetometer data by orientation of the IMF. Both datasets were compiled into 5-minute increments for an eight-year time period (1998-2005). This is the first time that such a large data set has been synthesized. Pseudo-AU, AL, and AE indices were calculated from these data for the eight-year period. These indices, derived from a greater number of stations than the original, true AE indices, were used to generate a list of substorms that extended from 1998-2005. The initiation times of the substorm events were used to select the data for a spherical harmonic fit analysis, to produce a statistically averaged response of the ground-based perturbations in the Earth's magnetic field following the onset of substorms. This response was sorted by IMF orientation and peak magnitude of the substorm AL index. Maps of the three vector components of the averaged magnetic perturbations for each IMF orientation will be presented. These results can be used to supplement an existing model of geomagnetic perturbations, in order to show the effects that could be expected from a substorm having specified values for the peak AL index and IMF orientation.