Solar wind control of the low‐latitude asymmetric magnetic disturbance field

The technique of empirical linear prediction filters is used to investigate the extent to which the low latitude dawn‐dusk magnetic asymmetry is controlled by the dawn‐dusk solar wind motional electric field VB s and/or by substorm processes measured by the westward auroral electrojet index AL. The dawn‐dusk asymmetry is measured by a new index defined as the difference between dawn and dusk deviations in the X (geomagnetic Northward) magnetic field component. The empirically determined filters obtained from this analysis provide quantitative information which characterizes the coupling processes. For example, the VB s to AL filter is a delayed pulse beginning after a delay of about 15 minutes, peaking at 60 minutes and returning to zero at 120 min. The filter has the characteristics of a low pass filter with a cutoff frequency at 10 −4 Hz. The VB s to ASYM filter is also a delayed pulse with similar constants. The VB s to ASYM filter, however, also has a long tail which gradually decays reaching zero near 5 or 6 hours lag time. The AL to ASYM filter is acausal, being nonzero at future lag times. The peak of the filter occurs at a delay of about 10 min. and decays exponentially becoming nearly zero after 4 hours lag. Our results indicate that some currents are directly driven by the solar wind‐magnetosphere interaction and that their magnetic perturbations contribute to both the AL and ASYM indices. A portion of the AL index that is uncorrelated with VB s is, however, correlated with ASYM suggesting that internal magnetospheric processes contribute to AL and ASYM as well.