Inner Structure of CME Shock Fronts Revealed by the Electromotive Force and Turbulent Transport Coefficients in Helios-2 Observations

TL;DR

This paper investigates the electromotive force and turbulent transport coefficients in the inner heliosphere during CMEs using Helios-2 data, revealing enhanced effects at shock fronts and proposing a method to identify magnetic transients.

Contribution

It introduces a one-dimensional CME shock-front model and a method to automatically detect magnetic transient events from spacecraft data.

Findings

Enhanced alpha, beta, and gamma terms at CME shock peaks

Strong electromotive force peaks during magnetic transients

Model validation against Helios-2 observations

Abstract

Electromotive force is an essential quantity in dynamo theory. During a coronal mass ejection (CME), magnetic helicity gets decoupled from the Sun and advected into the heliosphere with the solar wind. Eventually, a heliospheric magnetic transient event might pass by a spacecraft, such as the Helios space observatories. Our aim is to investigate the electromotive force, the kinetic helicity effect ($\alpha$ term), the turbulent diffusion ($\beta$ term) and the cross-helicity effect ($\gamma$ term) in the inner heliosphere below 1 au. We set up a one-dimensional model of the solar wind velocity and magnetic field for a hypothetic interplanetary CME. Because turbulent structures within the solar wind evolve much slower than this structure needs to pass by the spacecraft, we use a reduced curl operator to compute the current density and vorticity. We test our CME shock-front model against an observed magnetic transient that passes by the Helios-2 spacecraft. At the peak of the fluctuations in this event we find strongly enhanced $\alpha$, $\beta$ and $\gamma$ terms, as well as a strong peak in the total electromotive force. Our method allows us to automatically identify magnetic transient events from any in-situ spacecraft observations that contain magnetic field and plasma velocity data of the solar wind.