Peculiar Solar Sources and Geospace Disturbances on 20–26 August 2018

Abstract

On the approach to minimum of Solar Cycle 24, on 26 August 2018, an unexpectedly strong geomagnetic storm (GMS) suddenly occurred. Its D st 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 -1 . 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 B t ≈ 18.2 nT with a predominantly negative B z 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.

A Peculiar ICME Event in August 2018 Observed With the Global Muon Detector Network

“…Such doublet structures in the solar wind speed enhancement are often observed in corotating interaction regions in association with longitudinally elongated and complex coronal hole(s), as was also discussed in Abunin et al. (2020). We also note in Figure 2a that the cosmic‐ray density I 0 ( t ) starts decreasing after the MFR period.…”

“…Figure 2a also shows a significant variation of I 0 (t) around noon on 24 August when the disturbance is recorded in the solar wind parameters. We do not analyze this variation in this paper, but Abunin et al (2020) considered it as an indication of another ICME which they identified. Figure 2c shows magnitudes of  ( ) w t ξ and  ( ) w t ξ , the parallel and perpendicular components of the anisotropy in Equation 9, while Figure 2d displays |ξ w (t)| as a function of the pitch angle (θ) between ξ w (t) and the IMF.…”

“…Recently, Abunin et al. (2020) have also analyzed cosmic‐ray data observed by neutron monitors in August 2018. Neutron monitors have their maximum response to cosmic rays with P m ∼ 10 GV which is ∼1/5 of P m for muon detectors.…”

First Look at a Geomagnetic Storm With Santa Maria Digisonde Data: F Region Responses and Comparisons Over the American Sector

“…An unexpected and unusual strong geomagnetic storm (G3 class storm accordingly to the NOAA Space Weather Prediction Center) occurred on August 25, 2018. The analysis performed by Abunin et al (2020) revealed that the storm was initiated by a two-step long filament eruption in the central sector of the solar disk on August 20. It was accompanied by two consecutive ICMEs of a rather small size and low speed and followed by the HSS.…”

“…The selected disturbed period is the first one in which Santa Maria Digisonde data are available in all phases of the storm. The August 25, 2018 geomagnetic storm is considered very complex by several authors (Abunin et al., 2020; Astafyeva et al., 2020) since it was caused by two consecutive Interplanetary Earth‐directed Coronal Mass Ejections (ICMEs) and a followed High‐Speed Solar Wind Stream (HSS).…”