Quantitative model for the generic 3D shape of ICMEs at 1 AU

TL;DR

This paper develops statistical methods to derive a quantitative 3D shape model of ICMEs at 1 AU, integrating observational data and theoretical shock models to improve understanding of their structure and space weather impact.

Contribution

It introduces a novel statistical approach combining observational and theoretical models to determine a generic 3D shape of ICMEs, including shock, sheath, and flux rope structures.

Findings

ICMEs are compatible with symmetric or mildly asymmetric shock shapes.

No global trend in sheath thickness and ICME velocity along the front.

The model constrains ICME structure for better space weather prediction.

Abstract

Interplanetary imagers provide 2D projected views of the densest plasma parts of interplanetary coronal mass ejections (ICMEs) while in situ measurements provide magnetic field and plasma parameter measurements along the spacecraft trajectory, so along a 1D cut. As such, the data only give a partial view of their 3D structures. By studying a large number of ICMEs, crossed at different distances from their apex, we develop statistical methods to obtain a quantitative generic 3D shape of ICMEs. In a first approach we theoretically obtain the expected statistical distribution of the shock-normal orientation from assuming simple models of 3D shock shapes, including distorted profiles, and compare their compatibility with observed distributions. In a second approach we use the shock normal and the flux rope axis orientations, as well as the impact parameter, to provide statistical information across the spacecraft trajectory. The study of different 3D shock models shows that the observations are compatible with a shock symmetric around the Sun-apex line as well as with an asymmetry up to an aspect ratio around 3. Moreover, flat or dipped shock surfaces near their apex can only be rare cases. Next, the sheath thickness and the ICME velocity have no global trend along the ICME front. Finally, regrouping all these new results and the ones of our previous articles, we provide a quantitative ICME generic 3D shape, including the global shape of the shock, the sheath and the flux rope. The obtained quantitative generic ICME shape will have implications for several aims. For example, it constrains the output of typical ICME numerical simulations. It is also a base to develop deeper studies of the transport of high energy solar and cosmic particles during an ICME propagation, as well as for modeling and forecasting space weather conditions near Earth.