The Heliocentric Distance Where the Deflections and Rotations of Solar Coronal Mass Ejections Occur

TL;DR

This study identifies the heliocentric distance (~10 solar radii) where magnetic forces primarily influence CME deflections and rotations, showing that beyond this point, CMEs propagate with constant angular momentum, simplifying space weather modeling.

Contribution

The paper quantifies the distance at which CME deflections are set, demonstrating that magnetic forces dominate within 10 Rs and that beyond this, CMEs follow a constant angular momentum trajectory.

Findings

Deflections are determined within 10 Rs from the Sun.

Assuming constant angular momentum beyond 10 Rs underestimates deflections by 1-5%.

Magnetic forces are responsible for deflections within 10 Rs, non-magnetic forces dominate beyond.

Abstract

Understanding the trajectory of a coronal mass ejection (CME), including any deflection from a radial path, and the orientation of its magnetic field is essential for space weather predictions. Kay et al. (2015b) developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), of CME deflections and rotation due to magnetic forces, not including the effects of reconnection. ForeCAT is able to reproduce the deflection of observed CMEs (Kay et al. 2015a). 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 deflection and rotation 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 1% to 5% and underestimates of the rotation of 10%. Since the deflection from magnetic forces is determined by 10 Rs, non-magnetic forces must be responsible for any observed interplanetary deflections or rotations where the CME has increasing angular momentum.