CME Deflection Predictions Using ForeCAT (Forecasting a CME's Altered Trajectory)

The accurate prediction of the path of coronal mass ejections (CMEs) plays an important role in space weather forecasting, and knowing the source location of the CME does not always suffice. During solar minimum polar coronal holes (CHs) deflect high latitude CMEs toward the ecliptic and when CHs extend to lower latitudes other deflections can occur. To predict whether a CME will impact Earth, these nonideal effects must be taken into account. Our previous simulations of an erupting flux rope placed near a CH in the low corona indicate magnetic forces as the key driver behind these nonradial motions close to the Sun's surface. Here, we present a newly developed IDL routine ForeCAT (Forecasting a CME's Altered Trajectory) to predict the path of a CME. Given the background solar wind conditions, the launch site of the CME, and the properties of the CME (such as its magnetic energy), we forecast the deflection of the CME. Our model incorporates the effects of magnetic tension and magnetic pressure gradient forces acting on opposite edges of the CME as the primary drivers of the deflection, and the CME expands according to its magnetic energy. The strength of the magnetic pressure and tension forces results from the CME size and location with respect to various solar features such as CHs, active regions, or streamer regions. We also include the effects of drag as the edges propagate outward against the solar wind. For each edge, we numerically integrate the forces leading to a change in edge position. We present comparisons with previously observed deflection events and studies of the model's sensitivity to input parameters.