  Forecasting the geomagnetic effects of solar storms, known as coronal mass
ejections (CMEs), is currently severely limited by our inability to predict the
magnetic field configuration in the CME magnetic core and by observational
effects of a single spacecraft trajectory through its 3D structure. CME
magnetic flux ropes can lead to continuous forcing of the energy input to the
Earth's magnetosphere by strong and steady southward-pointing magnetic fields.
Here, we demonstrate in a proof-of-concept way a new approach to predict the
southward field Bz in a CME flux rope. It combines a novel semi-empirical model
of CME flux rope magnetic fields (3-Dimensional Coronal ROpe Ejection or
3DCORE) with solar observations and in situ magnetic field data from along the
Sun-Earth line. These are provided here by the MESSENGER spacecraft for a CME
event on 2013 July 9-13. 3DCORE is the first such model that contains the
interplanetary propagation and evolution of a 3D flux rope magnetic field, the
observation by a synthetic spacecraft and the prediction of an index of
geomagnetic activity. A counterclockwise rotation of the left-handed erupting
CME flux rope in the corona of 30 degree and a deflection angle of 20 degree is
evident from comparison of solar and coronal observations. The calculated Dst
matches reasonably the observed Dst minimum and its time evolution, but the
results are highly sensitive to the CME axis orientation. We discuss
assumptions and limitations of the method prototype and its potential for real
time space weather forecasting and heliospheric data interpretation.
