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Abstract

The geomagnetic storm occurred on 25–26 August 2018 as a surprise to forecasters. The arrival of a weak coronal mass ejection did not show a sudden impulse in the magnetic data; however, when the Interplanetary magnetic field Bz turned southward, it intensified and further remained unchangeably negative for the next 9 hr, causing a major storm with the minimum SYM‐H excursion of −205 nT. In this work, we study the thermospheric, ionospheric, and electrodynamic behavior during this storm. We use a set of space‐borne (the Swarm constellation, GUVI/TIMED) and ground‐based (GPS receivers, magnetometers, SuperDARN) instruments. We particularly focus on storm effects in the American and East Pacific sectors, where unprecedented hemispheric asymmetries occurred in the thermosphere and ionosphere during the main and the recovery phases of the storm. At the beginning of the storm, a strong positive ionospheric storm was observed in the Northern Hemisphere, while in the Southern Hemisphere, surprisingly, no storm effect occurred. During the recovery phase, the thermospheric composition ratio O/N2 showed an extreme expansion of the bulge into the opposite hemisphere. Our analysis shows that in each case the asymmetry was produced by a unique combination of drivers that acted at particular moment of time and in particular place. The seasonal asymmetry in the high‐latitude plasma and neutral mass density distributions along with the asymmetries in the geomagnetic field and the timing of these impacts played the decisive role.