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

TL;DR

This study demonstrates that the shock-ICME complex structure significantly amplifies geomagnetic storm intensity and solar energetic particle levels, emphasizing its importance in space weather prediction and solar physics research.

Contribution

It provides a detailed analysis of the shock-ICME complex structure's role in enhancing geomagnetic storms and SEPs, highlighting its significance for space weather forecasting.

Findings

Shock compression doubled the geomagnetic storm intensity.

Shock-ICME structure increased SEP flux by about 5 times.

Without shock, the storm would be moderate with Dst of -79 nT.

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.