Game of drivers and a surprise ionospheric storm

A major geomagnetic storm occurred on 25-26 August 2018 as a surprise to forecasters. A weak coronal mass ejection (CME) was launched from the Sun on 20 August 2018 and it collided with the Earth's magnetic field on 25 August. With no significant change in the solar wind speed, this collision did not show a sudden impulse in the magnetic data. However, when the IMF Bz turned southward, it intensified and further remained unchangeably negative for the next 9 hours. This caused an intensive G3- storm with the minimum SYM-H excursion of -205 nT. While generally gradually starting storms are known as less intensive, the August 2018 storm has become the third strongest storm in the 24^thsolar cycle, after the 2015 St. Patrick's Day storm and the 22-23 June 2015 storm.

In this work, we study the thermospheric, ionospheric and electrodynamic behavior during the August 2018 surprise storm. For this purpose, we use a set of space-borne (the Swarm constellation, GUVI/TIMED) and ground-based (GPS receivers, magnetometers, SuperDARN) instruments. In the thermosphere, the 25-26 August 2018 storm caused 300-500% increase with respect to the quiet-time values. In the ionosphere, the storm-time effects were different in different regions. In the American sector, where we observe very pronounced hemispheric asymmetries in the ionospheric response during the main and the recovery phases of the storm. Our analysis shows that a play of several drivers, such as thermospheric winds and composition, storm-time electric fields, as well as the asymmetry in the high-latitude convection pattern led to the observed distinctive asymmetries at different stages of the storm. In the Indian sector, a strong hemispherically symmetric increase was observed during the recovery phase. This positive effect was attributed to the thermospheric winds.