THE RELATION BETWEEN THE NORTHERN POLAR-CAP AND AURORAL ELECTROJET GEOMAGNETIC INDEXES IN THE WINTERTIME

The polar cap (PC) index is a measure of the high-latitude geomagnetic disturbances due to Hall and field-aligned currents. The index is wel l correlated with the auroral electrojet AL and AU indices (correlatio n with the PC index is 76% and 66%, resp.). Here we obtain several typ es of data-based models that relate the PC to the AL and AU indices in the wintertime, when the ionospheric conductivity is mostly due to th e precipitating particles of the field-aligned currents. The new model s predict AL and AU from PC with correlations much higher than those f ound by earlier studies. Thus linear moving-average filters reproduce the observed ill with a correlation of 88% (AU: 75%) while linear auto regressive moving-average (ARMA) models based on the PC index produce in-sample single-step predictions with 98% and 97% correlations with A L and AU respectively. For long-term, out-of-sample prediction, the li near ARMA prediction from the PC index has an asymptotic prediction er ror which is at least 25% more accurate than the prediction from solar wind input. Nonlinear models are slightly more accurate than their li near counterparts, indicating a weak nonlinearity in the relation betw een the polar cap and auroral zone indices. The prediction-observation correlations are sufficiently high that models based on the PC index can be used for specification of the auroral geomagnetic activity.