SuperMAG‐based partial ring current indices

Using the extensive set of stations in the SuperMAG collaboration, we introduce partial ring current indices, which provide new insights into ring current development. The indices are labeled SMR‐ 00, SMR‐ 06, SMR‐ 12 and SMR‐ 18 for their center local time range. These indices incorporate data from 98 mid and low latitude stations. The behavior of these local time indices during storms and substorms, on both an individual and superposed epoch basis, produces consistent patterns. The initial positive spike before a storm, which results from solar wind pressure enhancements, is seen simultaneously at all local times. Once the main phase of the storm begins, however, SMR ‐18 nearly always drops fastest and furthest in magnitude, while SMR ‐06 drops more slowly (i.e., has weaker ring current signatures), and never as far. Symmetry is, in fact, not reached until storm recovery is well underway, with a typical symmetry point of about 20–25 h after onset. If the main phase continues to new depths (larger magnitude negative SMR ) over a longer time period, the SMR ‐18 sector will continue to lead, and SMR ‐06 to lag. There has been controversy over the extent to which substorm auroral and cross‐tail currents perturb Dst and SYM‐H signatures. The signature of substorms can be seen very clearly as a positive spike of roughly 10 nT magnitude in SMR ‐00, and only to a much lesser extent elsewhere. The SMR global index, and thus also SYM‐H , experiences only a small immediate perturbation from a substorm onset, ending with a net drop of a few nT. Since there are typically only 1–2 substorms in a main phase, substorms are a minor factor in the development of the storm time ring current. Indeed, because even the peak perturbation of substorms currents in the most affected sector (which is midnight) is nearly an order of magnitude smaller than the storm perturbation (about 10 nT versus 80 nT), fluctuations in the cross‐tail and field‐aligned currents in general are not a major influence over SMR. The pattern of LT substorm responses, with a strong positive SMR ‐00 effect, weak positive SMR ‐06 effect, and negative SMR ‐18 effect implies it is field‐aligned currents and not the cross‐tail currents which create modest perturbations in the putatively ring current indices.