Consequences of the neural network investigation for Dst-AL relationship

Several recent studies have suggested that most of the D-st main phase variations and of the AL variations similarly respond to a certain type of solar wind condition although the processes are independent of each other. This similarity suggests that some consistency between the D-st main phase development and AL variations exists, regardless of the existence of causality. In what situations this consistent relationship really exists or collapses has been examined with the technique of an Elman recurrent neural network. The network was trained with the D-st and hourly averaged AL indices for 70 storm events from 1967 to 1981, and tested for nine storms that occurred in 1982. The result shows that the D-st-AL relationship can be categorized into two types: high correlative mapping for which 80% and more of the D-st peak in the main phase is reproduced by AL, and partially correlative mapping where only about a half of the D-st peak is reproduced. It is found that whether the correlation is high or partial is determined by whether the D-st main phase develops smoothly or with a discontinuity, i.e., for storms having a discontinuity in the main phase, the coherency collapses. The discontinuity ire the D-st main phase is associated with the rapid southward IMF change after the northward excursion. We suggest that it is this IMF variation to which storms and/or substorms respond in a highly complex manner and that such a complex response can be associated with about a half of the maximum ring current intensity.