Deriving the Topological Properties of the Magnetic Field of Coronal Mass Ejections from In Situ Measurements: Techniques

TL;DR

This paper introduces a new method to quantify magnetic helicity in coronal mass ejections using in situ measurements, revealing that writhe and mutual helicity are significant components near 1 au, which impacts reconstruction techniques.

Contribution

A novel technique for directly measuring magnetic helicity components in CMEs from in situ data, accounting for complex geometries beyond flux rope models.

Findings

Significant helicity is stored in writhe and mutual helicity, not just twist.

The method uses synthetic in situ measurements from 3D CME simulations.

Implications for improving CME magnetic field reconstructions.

Abstract

Coronal mass ejections (CMEs) are magnetized plasma systems with highly complex magnetic topology and evolution. Methods developed to assess their magnetic configuration have primarily focused on reconstructing three-dimensional representations from one-dimensional time series measurements taken in situ using techniques based on the "highly twisted magnetic flux rope" approximations. However, the magnetic fields of CMEs is know to have more complicated geometries. Their structure can be quantified using measures of field line topology, which have been primarily used for solar physics research. In this work, we introduce a novel technique of directly quantifying the various form of magnetic helicity within a CME in the interplanetary space using synthetic in situ measurements. We use a relatively simple three-dimensional simulation of a CME initiated with a highly-twisted flux rope. We find that a significant portion of the magnetic helicity near 1~au is contained in writhe and mutual helicity rather than just in twist. We discuss the implications of this finding for fitting and reconstruction techniques.