  To predict whether a coronal mass ejection (CME) will impact Earth, the
effects of the background on the CME's trajectory must be taken into account.
We develop a model, ForeCAT (Forecasting a CME's Altered Trajectory), of CME
deflection due to magnetic forces. ForeCAT includes CME expansion, a three-part
propagation model, and the effects of drag on the CME's deflection. Given the
background solar wind conditions, the launch site of the CME, and the
properties of the CME (mass, final propagation speed, initial radius, and
initial magnetic strength), ForeCAT predicts the deflection of the CME. Two
different magnetic backgrounds are considered: a scaled background based on
type II radio burst profiles and a Potential Field Source Surface (PFSS)
background. For a scaled background where the CME is launched from an active
region located between a CH and streamer region the strong magnetic gradients
cause a deflection of 8.1 degrees in latitude and 26.4 degrees in longitude for
a 1e15 g CME propagating out to 1 AU. Using the PFSS background, which captures
the variation of the streamer belt position with height, leads to a deflection
of 1.6 degrees in latitude and 4.1 degrees in longitude for the control case.
Varying the CME's input parameters within observed ranges leads to the majority
of CMEs reaching the streamer belt within the first few solar radii. For these
specific backgrounds, the streamer belt acts like a potential well that forces
the CME into an equilibrium angular position.
