Peculiar solar sources and geospace disturbances on 20-26 August 2018 (1912.08153v1)

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.

• The paper investigates a unique geomagnetic storm triggered by a two-step filament eruption and slow, Earth-directed CMEs during August 2018.
• The paper quantifies the storm's intensity with a Dst index of -174 nT and highlights an unusual Forbush decrease alongside large cosmic ray bursts.
• The paper emphasizes monitoring non-active region eruptions and cosmic ray anisotropy to enhance space weather forecasting and predictive models.

Analysis and Insights on Peculiar Solar Sources and Geospace Disturbances (20–26 August 2018)

The paper entitled "Peculiar solar sources and geospace disturbances on 20–26 August" provides a detailed investigation into a significant geomagnetic storm (GMS) event driven by solar activities during the declining phase of Solar Cycle 24. The paper delivers insights into the underlying solar phenomena and the resulting geospace impacts, with a focus on the unexpected intensity of the geomagnetic disturbances and associated cosmic ray variability.

Summary of Findings

1. Solar Source and Dynamics:
   • The key solar events leading up to the geomagnetic storm were a series of eruptions on 20 August, identified as a two-step filament eruption in a non-active region (non-AR) of the solar disk.
   • These eruptions were characterized by significant filament eruptions and slow coronal mass ejections (CMEs) at speeds of about 200–360 km/s. The CMEs were Earth-directed, albeit with modest visibility and speed, which informed the eventual geospace impact due to their interaction with solar wind streams.
2. Geomagnetic and Cosmic Ray Impacts:
   • Upon their arrival at Earth on 25 August, the interplanetary manifestations of these CMEs resulted in an intense geomagnetic storm with a Dst index reaching -174 nT, marking it as the third most severe storm in Solar Cycle 24, unexpected for a cycle approaching minimum activity.
   • Of particular note was the unusual Forbush decrease (FD), a measure of cosmic ray suppression, which was relatively weak (about 1.5%) given the storm's intensity but exhibited large positive bursts.
3. Cosmic Ray Anisotropy:
   • The paper highlights an anomalous and dynamic cosmic ray anisotropy during this period, contributing to the cosmic ray bursts observed on the Forbush decrease background. This anisotropy, coupled with variations in geomagnetic cutoff rigidity, underscores the complex geospace responses to solar events.

Implications and Future Considerations

• Space Weather Forecasting: The paper stresses the importance of monitoring non-AR filament eruptions and their associated CMEs, even if they appear minor or slow-moving. These events can result in significant geospace disturbances under the right conditions, such as those characterized by favorable Earth impact and southward magnetic orientations.
• Cosmic Ray Monitoring: The findings point to the value of real-time cosmic ray data and anisotropy measurements as precursors for GMSs, offering insights into the approaching interplanetary disturbances that may not be apparent from direct solar observations alone.
• Theoretical Contributions: This research contributes to understanding the relationship between solar eruptions, cosmic ray modulation, and geomagnetic storms. It challenges existing models of CME expansion and interaction with solar winds, suggesting scenarios where constrained CME expansion can amplify their geospace consequences.

The paper underscores that even at solar minimums, significant and unexpected geospace disturbances can arise from seemingly inconspicuous solar activity. These insights are pivotal for refining space weather models, enhancing predictive capabilities, and developing strategies for mitigating the effects of extreme space weather events on Earth. Further research could explore the intricate dynamics of CME-solar wind interactions and their cascading effects on cosmic ray anisotropy and geomagnetic responses.