When is it alright to use SYM-H as a storm index?

Recently many scientists have begun to use the SYM-H geomagnetic index as if it is a replacement for the classic storm index, viz. Dst. However, the indices are not only different in time resolution, but also in the number and location of magnetometer stations used, and especially in the method used to convolve station data into a final index. It is certainly not an a priori given that these two indices may be used interchangeably in any operational sense. 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 different convolution methods 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, and discuss sources of differences. 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.