What Is Unusual About the Third Largest Geomagnetic Storm of Solar Cycle 24?

We report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (26 August 2018) in solar cycle 24. The underlying coronal mass ejection (CME) originating from a quiescent filament region becomes a 440 km/s magnetic cloud (MC) at 1 au after ∼5 days. The prolonged CME acceleration (for ∼24 hr) coincides with the time profiles of the post‐eruption arcade intensity and reconnected flux. Chen et al. (2019, https://doi.org/10.3847/1538-4357/ab3f36) obtain a lower speed since they assumed that the CME does not accelerate after ∼12 hr. The presence of multiple coronal holes near the filament channel and the high‐speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high‐inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase (prominence material) in the second half of the MC. Simulations using the Comprehensive Inner Magnetosphere‐Ionosphere model show that a higher ring current energy results from larger dynamic pressure (density) in MCs. Furthermore, the Dst index is highly correlated with the main‐phase time integral of the ring current injection that includes density, consistent with the simulations. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward IP magnetic field. We conclude that the high intensity of the storm results from the prolonged CME acceleration, complex rotation of the CME flux rope, and the high density in the 1‐au MC. Plain Language Summary: Powerful coronal mass ejections (CMEs) are responsible for very intense geomagnetic storms due to the out of the ecliptic component of the magnetic field in the CME flux rope or in the sheath if shock‐driving (Gosling, 1993, https://doi.org/10.1029/93ja01896). The 26 August 2018 storm was very intense, but the CME was inconspicuous and weak near the Sun. However, over an extended period of time the CME accelerated slowly and picked up adequate speed to cause an intense storm. Due to the presence of coronal holes near the eruption region, the CME rotated in such a way that the CME magnetic field and Earth's magnetic field can efficiently couple to transfer energy into the magnetosphere to cause the geomagnetic storm. The energy transfer is expedited by the presence of dense material deep inside the CME. Key Points: A coronal mass ejection characterized by prolonged acceleration, rotation, and high‐density content results in the intense geomagnetic stormSimulations with a kinetic code confirm that the ring current injection involves both solar wind dynamic pressure and electric fieldEmpirical formulas for predicting Dst based on solar wind electric field work when the magnetic cloud has no high‐density enhancement [ABSTRACT FROM AUTHOR]