  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.
