Penetration electric fields driving main phase Dst

On the basis of three selection criteria we have analyzed the main phase electrodynamics of 17 magnetic storms that occurred between 1999 and mid‐2005: (1) the availability of solar wind measurements from the Advanced Composition Explorer (ACE) satellite, (2) a minimum Dst ≤ −100 nT, and (3) no significant recovery episode between the beginning and maximum epoch of the main phase ( dDst / dt > 0). We combined the Volland‐Stern and Siscoe‐Hill models to estimate the intensities of electric fields ( E VS ) in the equatorial plane as the polar cap potential divided by the width of the magnetosphere along the dawn‐dusk axis. A survey of provisional Dst and E VS traces over the 5.5 years of interest shows that all sustained excursions of E VS above quiet time levels of 0.22 ± 0.08 mV/m were closely tied to the main phases of storms. In every instance, E VS returned to background at or near the beginning of the recovery phase. The data indicate very high correlations between Dst and I VS = ∫ E VS dt during the main phase of all selected storms.