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On the approach to minimum of Solar Cycle 24, on 26 August 2018, anunexpectedly strong geomagnetic storm (GMS) suddenly occurred. Its Dst indexreached -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. Theeruptions were accompanied by two large-scale divergent ribbons and dimmings ofa considerable size and were followed by relatively weak but evidentEarth-directed coronal mass ejections. In the inner corona, their estimatedspeed was very low of about 200-360 km/s. The respective interplanetarytransients apparently propagated between two high-speed solar wind streamsoriginated from a two-component coronal hole and therefore their expansion waslimited. The resulting ejecta arrived at the Earth only on 25 August andbrought an unexpectedly strong field of Bt ~ 18.2 nT with a predominantlynegative Bz component of almost the same strength. The geospace storm alsomanifested itself in the form of a peculiar Forbush decrease (FD). Itsmagnitude 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 anenhancement up to 3% above the pre-event level were recorded on the FDbackground. We argue that these bursts were mainly caused by an unusually largeand changeable cosmic ray anisotropy combined with lowering of the geomagneticcutoff rigidity in the perturbed Earth's magnetosphere under conditions of thecycle minimum.

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