# Propagation Characteristics of Two Coronal Mass Ejections From the Sun Far into Interplanetary Space

## TL;DR

This study investigates the propagation of two significant coronal mass ejections from the Sun into interplanetary space, revealing their deceleration, interaction, and impact on geomagnetic storms through multi-instrument observations and MHD modeling.

## Contribution

It provides new insights into CME deceleration, interaction in interplanetary space, and the role of sheath regions in geomagnetic storm generation, based on comprehensive observational and modeling data.

## Key findings

- Both CMEs decelerated significantly within 1 AU and gradually thereafter.
- The two CMEs interacted in interplanetary space despite different solar launch times.
- Intense geomagnetic storms were caused by southward magnetic fields in the sheath regions.

## Abstract

Propagation of coronal mass ejections (CMEs) from the Sun far into interplanetary space is not well understood due to limited observations. In this study we examine the propagation characteristics of two geo-effective CMEs, which occurred on 2005 May 6 and 13, respectively. Significant heliospheric consequences associated with the two CMEs are observed, including interplanetary CMEs (ICMEs) at the Earth and Ulysses, interplanetary shocks, a long-duration type II radio burst, and intense geomagnetic storms. We use coronagraph observations from SOHO/LASCO, frequency drift of the long-duration type II burst, in situ measurements at the Earth and Ulysses, and magnetohydrodynamic (MHD) propagation of the observed solar wind disturbances at 1 AU to track the CMEs from the Sun far into interplanetary space. We find that both of the two CMEs underwent a major deceleration within 1 AU and thereafter a gradual deceleration when they propagated from the Earth to deep interplanetary space due to interactions with the ambient solar wind. The results also reveal that the two CMEs interacted with each other in the distant interplanetary space even though their launch times on the Sun were well separated. The intense geomagnetic storm for each case was caused by the southward magnetic fields ahead of the CME, stressing the critical role of the sheath region in geomagnetic storm generation, although for the first case there is a corotating interaction region involved.