Modeling the Multiple CME Interaction Event on 6–9 September 2017 with WSA‐ENLIL+Cone

Abstract

A series of coronal mass ejections (CMEs) erupted from the same active region between 4–6 September 2017. Later, on 6–9 September, two interplanetary (IP) shocks reached L1, creating a complex and geoeffective plasma structure. To understand the processes leading up to the formation of the two shocks, we model the CMEs with the Wang‐Sheeley‐Arge (WSA)‐ENLIL+Cone model. The first two CMEs merged already in the solar corona driving the first IP shock. In IP space, another fast CME presumably interacted with the flank of the preceding CMEs and caused the second shock detected in situ. By introducing a customized density enhancement factor (dcld) in the WSA‐ENLIL+Cone model based on coronagraph image observations, the predicted arrival time of the first IP shock was drastically improved. When the dcld factor was tested on a well‐defined single CME event from 12 July 2012 the shock arrival time saw similar improvement. These results suggest that the proposed approach may be an alternative to improve the forecast for fast and simple CMEs. Further, the slowly decelerating kilometric type II radio burst confirms that the properties of the background solar wind have been preconditioned by the passage of the first IP shock. This likely caused the last CME to experience insignificant deceleration and led to the early arrival of the second IP shock. This result emphasizes the need to take preconditioning of the IP medium into account when making forecasts of CMEs erupting in quick succession.

Space weather: the solar perspective

“…Top to bottom: Total magnetic field and vector components, dynamical pressure, proton temperature, and plasma-beta. Image reproduced with permission from Werner et al (2019), copyright by AGU connectivity, generated a GLE. Due to the eruption site located close to the limb, measurements of the CME kinematics were less strongly affected by projection effects.…”

The GIC and Geomagnetic Response Over Fennoscandia to the 7–8 September 2017 Geomagnetic Storm

“…The interplanetary shock observed at L1 at ∼ 23:50 UTC on 6 September was produced by two Earth‐directed coronal mass ejections (CMEs) launched in short succession on 4 September, which likely interacted close to the Sun. Another interplanetary shock arrived at 23:00 UTC on 7 September, which was caused by a fast CME that erupted on 6 September (Shen et al, ; Werner et al, ).…”

“…launched in short succession on 4 September, which likely interacted close to the Sun. Another interplanetary shock arrived at 23:00 UTC on 7 September, which was caused by a fast CME that erupted on 6 September (Shen et al, 2018;Werner et al, 2019).…”

Modeling the Geomagnetic Response to the September 2017 Space Weather Event Over Fennoscandia Using the Space Weather Modeling Framework: Studying the Impacts of Spatial Resolution

“…We selected this event since the period between September 7 and 8, 2017 was particularly active from a space weather context, resulting in global geomagnetic disturbances which have received deserved attention over the past few years (Clilverd et al., 2018; Dimmock et al., 2019; Piersanti et al., 2019). A key factor that contributed to the interest in this event was that the upstream conditions originated from the interaction between multiple Interplanetary Coronal Mass Ejections (ICMEs; Werner et al., 2019), which created a highly complex period of external driving in terms of shocks, turbulence, IMF rotations, and plasma fluctuations. This event is ideal for addressing (1–5) listed above since Fennoscandia measured significant extended geomagnetic disturbances (Dimmock et al., 2019), multiple substorms were reported, and it is expected to pose a challenge to numerical models.…”