On the usage of geomagnetic indices for data selection in internal field modelling

We present a review on geomagnetic indices describing global geomagnetic storm activity (Kp, am, Dst and dDst/dt) and on indices designed to characterize high latitude currents and substorms (PC and AE-indices andtheir variants). The focus in our discussion is in main field modelling, where indices are primarily used in data selection criteria for weak magnetic activity.The publicly available extensive data bases of index values are used to derive joint conditional Probability Distribution Functions (PDFs) for different pairs of indices in order to investigate their mutual consistency in describing quiet conditions.

This exercise reveals that Dst and its time derivative yield a similar picture as Kp on quiet conditions as determined with the conditions typically used in internal field modelling. Magnetic quiescense at high latitudes is typically searched with the help of Merging Electric Field (MEF) as derived from solar wind observations.

We use in our PDF analysis the PC-index as a proxy for MEF and estimate the magnetic activity level at auroral latitudes with the AL-index. With these boundary conditions we conclude that the quiet time conditions that are typically used in main field modelling (PC < 0:8, Kp < 2and IDstI < 30 nT) correspond to weak auroral electrojet activity quite well:Standard size substorms are unlikely to happen, but other type of activations(e.g. pseudo breakups AL > -300 nT) can take place, when these criteria prevail.

Although AE-indices have been designed to probe electrojet activity only in average conditions and thus their performance is not optimal during weak activity, we note that careful data selection with advanced AE-variants may appear to be the most practical way to lower the elevated RMS-values which still exist in the residuals between modelled and observed values at high latitudes.

Recent initiatives to upgrade the AE-indices, either with a better coverage of observing stations and improved baseline corrections (the Super-MAG concept) or with higher accuracy in pinpointing substorm activity (the Midlatitude Positive Bay -index) will most likely be helpful in these efforts.