The determination of substorm phases based on SuperMAG auroral electrojet indices and interplanetary magnetic field

This paper presents a refined methodology for identifying all three substorm phases (growth, expansion, and recovery) by optimizing the approach of Juusola et al. (2011) using the SuperMAG Lower (SML) electrojet index and Interplanetary Magnetic Field (IMF) Bz data. Phase determination criteria are established from the median values of SML (-66 nT) and its negative time derivative (-4 nT/min), derived from a 25-year dataset (1995-2019). The method distinguishes quiet (non-substorm and growth) from active (expansion and recovery) periods and employs specific thresholds for expansion (dSML/⪅−4 nT/min and SML ⪅ -66 nT), recovery (SML ⪅ -66 nT post-expansion), and growth (IMF Bz<0) phases. A 16.5-minute "characteristic substorm phase time" grouping window consolidates short-duration intervals, and a priority system (expansion ⪆ recovery ⪆ growth) resolves phase overlaps. The SML-based method's performance was evaluated against an AL-based approach and the Forsyth et al. (2015) onset list under both geomagnetically quiet and strong storm conditions. Results demonstrate superior identification accuracy for the SML-based method, capturing all Forsyth onsets during quiet periods and a portion during storms, unlike the AL-based method, which captured none in the analyzed cases. Statistical comparisons of average phase durations and occurrence ratios further highlight the enhanced performance and distinct characteristics of SML-based identification.