  To understand the global-scale physical processes behind coronal mass
ejection (CME)-driven geomagnetic storms and predict their intensity as a space
weather forecasting measure, we develop an interplanetary CME flux
rope-magnetosphere interaction module using 3D magnetohydrodynamics. The
simulations adequately describe ICME-forced dynamics of the magnetosphere
including the imposed magnetotail torsion. These interactions also result in
induced currents which is used to calculate the geomagnetic perturbation.
Through a suitable calibration, we estimate a proxy of geoeffectiveness -- the
Storm Intensity index (STORMI) -- that compares well with the Dst/SYM-H Index.
Simulated impacts of two contrasting coronal mass ejections quantified by the
STORMI index exhibit a high linear correlation with the corresponding Dst and
SYM-H indices. Our approach is relatively simple, has fewer parameters to be
fine-tuned, is time-efficient compared to complex fluid-kinetic methods.
Furthermore, we demonstrate that flux rope erosion does not significantly
affect our results. Thus our method has the potential to significantly extend
the time window for predictability -- an outstanding challenge in geospace
environment forecasting -- if early predictions of near-Earth CME flux rope
structures based on near-Sun observations are available as inputs. This study
paves the way for early warnings based on operational predictions of CME-driven
geomagnetic storms.
