Peculiar solar sources and geospace disturbances on 20-26 August 2018

TL;DR

This paper analyzes a rare, intense geomagnetic storm in August 2018 caused by a two-step filament eruption, highlighting its unusual features and the complex solar-terrestrial interactions involved.

Contribution

It provides a detailed case study of a peculiar solar event and its geospace impact, emphasizing the role of filament eruptions and cosmic ray anisotropy.

Findings

The storm was initiated by a two-step filament eruption with slow coronal mass ejections.

The interplanetary transient had limited expansion due to high-speed solar wind streams.

Unusual positive bursts in cosmic rays were observed during the event.

Abstract

On the approach to minimum of Solar Cycle 24, on 26 August 2018, an unexpectedly strong geomagnetic storm (GMS) suddenly occurred. Its Dst index reached -174 nT, that is the third of the most intense storms during the cycle. The analysis showed that it was initiated by a two-step long filament eruption, which occurred on 20 August in the central sector of the solar disk. The eruptions were accompanied by two large-scale divergent ribbons and dimmings of a considerable size and were followed by relatively weak but evident Earth-directed coronal mass ejections. In the inner corona, their estimated speed was very low of about 200-360 km/s. The respective interplanetary transients apparently propagated between two high-speed solar wind streams originated from a two-component coronal hole and therefore their expansion was limited. The resulting ejecta arrived at the Earth only on 25 August and brought an unexpectedly strong field of Bt ~ 18.2 nT with a predominantly negative Bz component of almost the same strength. The geospace storm also manifested itself in the form of a peculiar Forbush decrease (FD). Its magnitude was about 1.5%, which is rather small for the observed G3-class GMS. The main unusual feature of the event is that large positive bursts with an enhancement up to 3% above the pre-event level were recorded on the FD background. We argue that these bursts were mainly caused by an unusually large and changeable cosmic ray anisotropy combined with lowering of the geomagnetic cutoff rigidity in the perturbed Earth's magnetosphere under conditions of the cycle minimum.