Determining the Magnetic Field Orientation of Coronal Mass Ejections from Faraday Rotation

TL;DR

This paper presents a method using Faraday rotation to determine the magnetic field orientation of coronal mass ejections (CMEs), enabling early space weather forecasting by mapping CME geometry days before Earth impact.

Contribution

It introduces a novel approach to measure CME magnetic orientation using Faraday rotation profiles, validated with observations and simulations, advancing space weather prediction capabilities.

Findings

FR profiles indicate flux-rope geometry of CMEs

Magnetic orientation can be determined 2-3 days before impact

FR mapping can resolve CME geometry back to the Sun

Abstract

We describe a method to measure the magnetic field orientation of coronal mass ejections (CMEs) using Faraday rotation (FR). Two basic FR profiles, Gaussian-shaped with a single polarity or "N"-like with polarity reversals, are produced by a radio source occulted by a moving flux rope depending on its orientation. These curves are consistent with the Helios observations, providing evidence for the flux-rope geometry of CMEs. Many background radio sources can map CMEs in FR onto the sky. We demonstrate with a simple flux rope that the magnetic field orientation and helicity of the flux rope can be determined 2-3 days before it reaches Earth, which is of crucial importance for space weather forecasting. An FR calculation based on global magnetohydrodynamic (MHD) simulations of CMEs in a background heliosphere shows that FR mapping can also resolve a CME geometry curved back to the Sun. We discuss implementation of the method using data from the Mileura Widefield Array (MWA).