Similarities and differences in low- to middle-latitude geomagnetic indices

Several versions of low‐ to middle‐latitude geomagnetic indices are examined throughout a 24 year interval and during storm time with respect to a normalized epoch timeline based on several key storm features. In particular, we conduct a quantitative comparison of the storm time superpositioning of the Dst, SYM‐H, and 1 min U.S. Geological Survey Dst indices using error analysis and employing descriptive statistics to assess the similarities and differences between them. The events are then categorized by storm intensity and examined as a function of the storm phase. While the indices are highly correlated with each other, dramatic deviation between the indices exists at certain storm epoch times. In particular, the error increases at storm peak and especially for more intense storms. The differences at storm peak are, on average, 20% of the peak value of the indices. These differences arise from the choice of magnetometer stations to include in each index and the various methodologies used to compile the individual perturbation measurements into a global value. The conclusions are that multiple indices should be considered when determining low‐ to middle‐latitude magnetic perturbations and that the difference between the indices should be considered as an error estimate on these values. Key Points Three low‐latitude geomagnetic indices (Dst, SYM‐H, USGS‐Dst) are compared. RMS errors between them are ~10 nT at quiet times, much more during storms. At storm peak, the error is ~20% of the index magnitudes between the three.