Can We Estimate the Intensities of Great Geomagnetic Storms (ΔSYM-H ≤ −200 nT) with the Burton Equation or the O’Brien and McPherron Equation?

Abstract

We input the solar wind parameters responsible for the main phases of 15 great geomagnetic storms (GGSs; ΔSYM-H ≤ −200 nT) into the empirical formulae created by Burton et al. (hereafter the Burton equation) and by OBrien & McPherron (hereafter the OM equation) to evaluate whether these two equations can correctly estimate the intensities of GGSs. The results show that the intensities of most GGSs estimated by the OM equation are much smaller than the observed intensities. The rms error between the intensities estimated by the OM equation and the observed intensities is 203 nT, implying that the estimated storm intensity deviates significantly from the observed one. The rms error between the intensities estimated by the Burton equation and the observed intensities is 130.8 nT. The relative error caused by the Burton equation for storms with intensities ΔSYM-H < −400 nT. The results indicate that the two equations cannot predict the intensities of GGSs correctly. On the contrary, the intensity of a GGS estimated by the empirical formula created by Wang et al. can approximate observations better if we select the right weight for the solar wind dynamic pressure, proving that solar wind dynamic pressure is an important factor of GGS intensity. This pressure is overlooked by the ring current injection terms of the Burton and OM equations. This is the reason why the two equations do not effectively estimate GGSs.