Influence of the ambient solar wind flow on the propagation behavior of interplanetary CMEs

TL;DR

This study investigates how ambient solar wind flow influences the propagation of interplanetary CMEs, analyzing three events with remote sensing and in situ data, and comparing observations with solar wind models to improve transit time predictions.

Contribution

It provides new insights into the distance at which solar wind drag dominates CME propagation, using combined observational data and MHD modeling.

Findings

CME speed adjusts to solar wind speed at varying distances, from below 30 Rs to beyond 1 AU.

ENLIL model effectively characterizes background solar wind structure during low solar activity.

The distance of CME speed adjustment depends on CME and ambient solar wind properties.

Abstract

We study three CME/ICME events (2008 June 1-6, 2009 February 13-18, 2010 April 3-5) tracked from Sun to 1 AU in remote-sensing observations of STEREO Heliospheric Imagers and in situ plasma and magnetic field measurements. We focus on the ICME propagation in IP space that is governed by two forces, the propelling Lorentz force and the drag force. We address the question at which heliospheric distance range the drag becomes dominant and the CME gets adjusted to the solar wind flow. To this aim we analyze speed differences between ICMEs and the ambient solar wind flow as function of distance. The evolution of the ambient solar wind flow is derived from ENLIL 3D MHD model runs using different solar wind models, namely Wang-Sheeley-Arge (WSA) and MHD-Around-A-Sphere (MAS). Comparing the measured CME kinematics with the solar wind models we find that the CME speed gets adjusted to the solar wind speed at very different heliospheric distances in the three events under study: from below 30 Rs, to beyond 1 AU, depending on the CME and ambient solar wind characteristics. ENLIL can be used to derive important information about the overall structure of the background solar wind, providing more reliable results during times of low solar activity than during times of high solar activity. The results from this study enable us to get a better insight into the forces acting on CMEs over the IP space distance range, which is an important prerequisite in order to predict their 1 AU transit times.