Why the Shock-ICME Complex Structure is Important: Learning From the Early 2017 September CMEs (1805.05763v1)

Abstract: In the early days of 2017 September, an exceptionally energetic solar active region AR12673 aroused great interest in the solar physics community. It produced four X class flares, more than 20 CMEs and an intense geomagnetic storm, for which the peak value of the Dst index reached up to -142nT at 2017 September 8 02:00 UT. In this work, we check the interplanetary and solar source of this intense geomagnetic storm. We find that this geomagnetic storm was mainly caused by a shock-ICME complex structure, which was formed by a shock driven by the 2017 September 6 CME propagating into a previous ICME which was the interplanetary counterpart of the 2017 September 4 CME. To better understand the role of this structure, we conduct the quantitative analysis about the enhancement of ICME's geoeffectiveness induced by the shock compression. The analysis shows that the shock compression enhanced the intensity of this geomagnetic storm by a factor of two. Without shock compression, there would be only a moderate geomagnetic storm with a peak Dst value of -79 nT. In addition, the analysis of the proton flux signature inside the shock-ICME complex structure shows that this structure also enhanced the solar energetic particles (SEPs) intensity by a factor of ~ 5. These findings illustrate that the shock-ICME complex structure is a very important factor in solar physics study and space weather forecast.

The Significance of Shock-ICME Complex Structures: Insights from September 2017 CMEs

The paper "Why the Shock-ICME Complex Structure is Important: Learning From the Early 2007 September CMEs" presents a comprehensive analysis of an intense geomagnetic storm caused by a complex interaction between coronal mass ejections (CMEs) and interplanetary shocks. The paper examines the series of solar events in September 2017, focusing on the mechanisms by which the shock-ICME complex structure enhances geomagnetic and solar energetic particle (SEP) effects.

Key Findings

The paper identifies and analyzes three distinct ICMEs and their associated solar sources, with multiple step disturbances observed in geomagnetic indices. The data was gathered using a combination of in situ spacecraft observations from Wind and DSCOVR, supplemented by coronagraph images from STEREO-A and SOHO.

1. Interaction-Driven Geomagnetic Disturbance: The analysis concludes that the intense geomagnetic storm, recording a peak Dst index value of -142 nT, was predominantly driven by the interaction of the second ICME with a subsequent shock due to its high-energy compression effects. Without this interaction, the storm would likely have been characterized as moderate, evidencing only a peak Dst value of approximately -79 nT.
2. Quantifying Shock Compression Influence: Utilizing the shock recovery method from Wang et al., the paper quantifies the enhancement, attributing a factor of 2 increase in geomagnetic intensity due to shock compression. When simulated without shock impact, forecast Dst values align with observations confirming substantial amplification caused by the shock.
3. SEP Enhancement within Shock-ICME Structures: The paper also reports a significant enhancement of SEP proton flux within the shock-ICME structure, approximately five times greater than the surrounding background. This points to the critical role complex interactions play not only in geomagnetic storm intensity but also in SEP propagation.

Implications and Future Research

The research underlines fundamental implications for space weather predictability and solar-terrestrial physics by confirming that interactions involving multiple CMEs, particularly shock-induced compressions, can significantly magnify space weather impacts. This suggests that forecasting frameworks need to integrate interactions between shocks and preceding CMEs to effectively predict both geomagnetic storm severity and SEP flux intensification.

For future research, significant emphasis should be placed on enhancing shock-interaction prediction abilities based on solar observations. Understanding which parameters most influence such interactions' geoeffectiveness will be crucial, potentially through statistical analyses across broader datasets. Particularly, the recurrent implications on SEP intensification demand further exploration to establish generalized principles across diverse solar events.

In summary, this paper contributes a detailed case analysis that reinforces the critical role of shock-ICME interactions in solar physics, stressing the need for sophisticated predictive models that account for complex intersolar magnetic interactions. These findings are essential for both theoretical advancements and practical applications in space weather forecasting, laying the groundwork for future investigations into interplanetary shock dynamics and their terrestrial impacts.