Dependence of Great Geomagnetic Storm ($\Delta$SYM-H$\le -200$ nT) on Associated Solar Wind Parameters

Abstract

We use $\Delta$SYM-H to capture the variation in the SYM-H index during the main phase of a geomagnetic storm. We define great geomagnetic storms as those with $\Delta$SYM-H $\le -200$ nT. After analyzing the data that were not obscured by solar winds, we determined that 17 such storms occurred during Solar Cycles 23 and 24. We calculated time integrals for the southward interplanetary magnetic field component $I(B_{s})$, the solar wind electric field $I(E_{y})$, and a combination of $E_{y}$ and the solar wind dynamic pressure $I(Q)$ during the main phase of a great geomagnetic storm. The strength of the correlation coefficient (CC) between $\Delta$SYM-H and each of the three integrals $I(B_{s})$ (CC = 0.73), $I(E_{y})$ (CC = 0.86), and $I(Q)$ (CC = 0.94) suggests that $Q$, which encompasses both the solar wind electric field and the solar wind dynamic pressure is the main driving factor that determines the intensity of a great geomagnetic storm. The results also suggest that the impact of $B_{s}$ on the great geomagnetic storm intensity is much more significant than that of the solar wind speed and the dynamic pressure during the main phase of an associated great geomagnetic storm. The better estimation of the intensity of an extreme geomagnetic storm intensity based on solar wind parameters is also discussed.

Appendix

The solar wind parameters responsible for the main phases of 17 great geomagnetic storms and 15 intense geomagnetic storms ($\Delta$SYM-H$>-200$ nT and $\Delta$SYM-H$\le -100$ nT) are listed in Table 1. In the table, column 1 to column 10 are No., date, the minimum of Dst, the minimum of SYM-H, $\Delta$SYM-H, the averaged southward interplanetary magnetic field ($\overline{B_{s}}$), the averaged solar wind dynamic pressure ($\overline{P_{d}}$), $I(B_{s})$, $I(E_{y})$ and $I(Q)$, respectively.