Including and explaining the important early positive part of main phase in the treatment of geomagnetic storms

The important early positive part of main phase (MP) from positive main phase onset (MPO) to 0‑level of SYM-H (and Dst, up to over 200 nT) was somehow missed in the treatment of geomagnetic storms. In this paper, we include the missed positive part in MP for over 1000 (out 1360) storms (SYM-HMin ≤-25 nT) having positive MPO identified in the SYM-H index in 1981-2024 by a computer algorithm using 5 selection criteria. The missed part is included by raising the 0-level of SYM-H to MPO-level, which increases the revised storm intensity (SYM-HMin^r) by up to -145 nT and revised storm impulsive strength (IpsSYM-H^r) by up to -139 nT. The inclusion of the positive part of MP therefore seems important for all aspects of global space weather. For example, IpsSYM-H^r identifies all 3 severe space weather (SvSW) events that caused power outage with a large separation of 52 nT and identifies all 9 minor-system-damage space weather (MSW) events that caused capacitor stripping and high induced voltage in power transformers from over 1000 normal space weather (NSW) events that did not cause any such damages since 1981. The included positive part of MP is explained statistically using long-term simultaneous SYM-H, solar wind dynamic pressure P and Y-component of interplanetary electric field IEFy (or VBz computed from solar wind velocity V and Bz component of IMF) data available for 156 storms since 1998. In addition, a 1-minute resolution SYM-H model developed from the existing Dst models is used to investigate the combined and relative contributions of P and IEFy on the included positive part of MP. The data statistics and model results reveal that the positive part of MP in majority (74%) of storms is contributed mainly (≥75%) by positive IEFy or increase in ring current; in a small number (6%) of storms, it is contributed mainly (≥75%) by sudden decrease in P; and in the remaining 20% of storms, it is contributed by both decrease in P and positive IEFy with major contribution from positive IEFy. In short, the important positive part of MP is caused mainly by the increase in ring current.