Separating DP1 and DP2 Current Pattern Contributions to Substorm-Like Intensifications in SML

Abstract

Substorms have been identified from negative bays in the AL/SML index, which traces the minimum northward ground magnetic deflection at auroral latitudes, produced by enhancements of the westward electrojet. For substorms, negative bays are caused by the closure of the Substorm Current Wedge through the ionosphere, typically localized to the nightside and centered around 23-00 magnetic local time (MLT). In this case, the equivalent current pattern that causes the magnetic deflections is given the name Disturbance Polar (DP) 1. However, negative bays may also form when the westward electrojet is enhanced by increased convection, driving Pedersen and Hall currents in the auroral zone. Convection enhancements also strengthen the eastward electrojet, monitored by AU/SMU index. In this case, the equivalent current pattern that produces the magnetic deflections is called DP2. Unlike other substorm identification methods, the Substorm Onsets and PHases from Indices of the Electrojets technique by Forsyth et al. (2015), https://doi.org/10.1002/2015ja021343 attempts to distinguish between the DP1 and DP2 enhancements that cause substorm-like SML bays. Despite this, we find evidence that between 1997 and 2019 up to 59% of the 30,329 events originally identified as substorms come from enhancements of DP2 on top of the 2,627 convection enhancement events already identified. We explore ways to improve substorm identification using auroral indices to fully separate the DP1 and DP2 bays but conclude that there is insufficient information in the auroral indices to achieve this. In reality, any “substorm” list is a list of magnetic enhancements, auroral enhancements, etc., which may or may not correspond to substorm activity and should be treated that way.