At What Distance are CME Deflections Determined?

Abstract:

Understanding the trajectory of a coronal mass ejection (CME), including any deflection from a radial path, is essential for space weather predictions. Kay et al. (2015a) developed a model, Forecasting a CME’s Altered Trajectory (ForeCAT), of CME deflections due to magnetic forces, not including the effects of reconnection. ForeCAT is able to reproduce the deflection of observed CMEs (Kay et al. 2015b). The deflecting CMEs tend to show a rapid increase of their angular momentum close to the Sun, followed by little to no increase at farther distances. Here we quantify the distance at which the CME deflection is “determined,” which we define as the distance after which the background solar wind has negligible influence on the total deflection. We consider a wide range in CME masses and radial speeds and determine that the majority of simulated CMEs obtain 90% of their total angular momentum at 1 AU below 2 Rs. The deflection of these CMEs can be well-described by assuming they propagate with constant angular momentum beyond 10 Rs. The assumption of constant angular momentum beyond 10 Rs yields underestimates of the total deflection at 1 AU of only 5% to 10%. Since the deflection from magnetic forces is determined by 10 Rs, non-magnetic forces must be responsible for any observed interplanetary deflections where the CME has increasing angular momentum.