Correct normalization of the <i>Dst</i> index

Abstract

The Dst index has been one of the most important solar-terrestrial indices for decades, and it is used in numer- ous studies as a measure of the temporal development and intensity of magnetic storms and the ring current. Here we discuss two issues related to the relative and absolute normal- ization that are problematic to the Dst index. We show for the first time quantitatively that the magnetic disturbances at the four Dst stations are ordered according to the latitudinal pro- jection of an equatorial disturbance upon the local horizontal of the geomagnetic field. Therefore, the distur- bances observed at each station should be first normalized by the cosine of the geomagnetic latitude of the station before they are averaged to form the Dst index. Perhaps surpris- ingly, the recipe to calculate the Dst index does not include this normalization and, therefore, must be revised on this part. We also discuss the effects of correcting the quiet-time seasonal variation, the so called non-storm component in the Dst index. This correction is seasonally varying, being largest around equinoxes and smallest at solstices, leading to an average correction (increase) of about 6 nT, i.e. about 25-30%, for annual averages of the Dst index. This increase also leads to significantly improved correlations between the corrected Dst index, the so called Dcx index, and many other indices of solar-terrestrial disturbance. We show here in de- tail that the correlation between the geomagnetic Ap index and the Dcx index (cc = 0.83) is much higher than between Ap and Dst (cc = 0.60). These results give further evidence that the Dcx index is a more truthful measure of magnetic storminess than the original Dst index.