The relation between the northern polar cap and auroral electrojet geomagnetic indices in the wintertime

Abstract

The polar cap (PC) index is a measure of the high‐latitude geomagnetic disturbances due to Hall and field‐aligned currents. The index is well correlated with the auroral electrojet AL and AUindices (correlation with the PC index is 76% and 66%, resp.). Here we obtain several types 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 the precipitating particles of the field‐aligned currents. The new models predict AL and AU from PC with correlations much higher than those found by earlier studies. Thus linear moving‐average filters reproduce the observed AL with a correlation of 88% (AU: 75%) while linear autoregressive moving‐average (ARMA) models based on the PC index produce in‐sample single‐step predictions with 98% and 97% correlations with AL and AU respectively. For long‐term, out‐of‐sample prediction, the linear ARMA prediction from the PC index has an asymptotic prediction error which is at least 25% more accurate than the prediction from solar wind input. Nonlinear models are slightly more accurate than their linear counterparts, indicating a weak nonlinearity in the relation between 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.