High‐resolution global storm index: Dst versus SYM‐H

The classic view is that Dst and SYM‐H time series differ mainly due to the dissimilarity in the method to determine the base values. Dst and SYM‐H are both indices designed to measure the intensity of the storm time ring current. They are calculated in similar but not identical manners. Since SYM‐H has the distinct advantage of having 1‐min time resolution compared to the 1‐hour time resolution of Dst , it is worth determining if the differences introduced by using different ground stations and slightly different methods of baseline subtraction produce statistically significant differences in the values of the indices. We have examined data from these indices collected over more than 20 years to determine the extent to which Dst and SYM‐H are equivalent or different. We found that a simple combination of linear trends with a break at SYM‐H = −300 nT provides an excellent comparison with the Dst index. For quiet times and for small storms the deviations are typically no more than 10 nT. Moderate storms feature deviations typically only slightly more than 10 nT, and intense storms have deviations that are usually less than 20 nT. We conclude that the classic view is accurate and recommend that in future studies the SYM‐H index be used as a de facto high‐resolution Dst index.