Abstract
We report on the solar and interplanetary (IP) causes of the third
largest geomagnetic storm (2018 August 26) in solar cycle 24. The 2018
August 20 coronal mass ejection (CME) originating from a quiescent
filament region becomes a magnetic cloud (MC) at 1 au after
~5 days. The CME accelerates for about a day as
evidenced by the time evolution of the CME speed, the intensity of the
associated post-eruption arcade, and the reconnected flux. 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 in the
MC second half of the MC. We confirm that the enhanced strength of the
2018 August 26 storm is a direct result of the enhanced MC density by
comparing with two other events: the 2010 May 28 MC that has similar
properties except for no density enhancement and the 2014 April 11 MC
with a complex density structure. The Comprehensive Inner
Magnetosphere-Ionosphere (CIMI) model applied to the three events
confirm that higher ring current energy results from larger dynamic
pressure in the MCs. A complex temporal structure develops in the storm
main phase if the underlying MC has a complex density structure during
intervals of southward interplanetary magnetic field.