Analysis of the bi-modal nature of solar wind-magnetosphere coupling

It has been shown that the optimal linear prediction filter relating the solar wind electric field and the geomagnetic activity, as measured by the AL index, is both bi-modal and dependent on the level of activity in the magnetosphere. Further studies truncated the prediction filter to a five parameter model containing two low-pass filtered delta functions of arbitrary amplitude and delay time. The present study elaborates on the nature of the bi-modal response by using the five parameter model to quantify the effects of the level of geomagnetic activity on each of the modes of the filter individually. The authors find that at all levels of activity, the second mode, occurring at approximately one hour, is relatively unchanged. The first mode, however, has a one parameter dependence on the level of activity in the magnetosphere. The amplitude of the first mode is shown to have a significant increase with respect to activity