Relative Contributions of Solar Wind Dynamic Pressure and Interplanetary Electric Field on the Early Positive Part of Main Phase of Geomagnetic Storms

Abstract The important early declining part of the main phase (MP) of geomagnetic storms in SYM‐H (and Dst) from positive main phase onset (MPO) to 0‐level of SYM‐H was somehow missed in the treatment of the storms. We included the missed part (or revised the storms) by raising the 0‐level of SYM‐H to MPO‐level, and showed that the inclusion is important for most aspects of global space weather. However, what drives the missed part is not yet understood. We take up such a study using data and a SYM‐H model. The relative contributions of the solar wind dynamic pressure P and interplanetary electric field IEFy on the missed part are studied statistically using good quality P and IEFy data available for 116 revised intense storms in 1998–2024. The results reveal that (a) the missed part in majority (62%) of storms is mainly (≥75%) due to positive IEFy; (b) in a small number of storms (12%), it is mainly (≥75%) due to decrease in P; and (c) in the remaining 26% of storms, it is due to both decrease in P and positive IEFy. The SYM‐H model is developed from the existing Dst models and incorporates the delay of SYM‐H response to IEFy turning positive. The model reproduces the combined effect of P and positive IEFy nearly agreeing with SYM‐H data, and illustrates their relative contributions for one storm each in the three groups. In short, the missed positive part in majority of storms seems due to positive IEFy.