Do All Interplanetary Coronal Mass Ejections Have A Magnetic Flux Rope Structure Near 1 AU?

TL;DR

This study investigates whether all interplanetary coronal mass ejections (ICMEs) possess a magnetic flux rope structure near 1 AU, using geometric analysis to evaluate the effects of observational perspective and interactions.

Contribution

The paper provides evidence supporting that all ICMEs have a magnetic flux rope structure near 1 AU, emphasizing the role of geometric effects in observing non-magnetic cloud ICMEs.

Findings

Narrow sheath regions in MCs support geometric selection effects.

NMC sizes are consistent across solar cycles 23 and 24.

NMCs are smaller than MCs in cycle 23 but larger in cycle 24.

Abstract

Interplanetary coronal mass ejections (ICMEs) often consist of a shock wave, sheath region, and ejecta region. The ejecta regions are divided into two broad classes: magnetic clouds (MC) that exhibit the characteristics of magnetic flux ropes and non-magnetic clouds (NMC) that do not. As CMEs result from eruption of magnetic flux ropes, it is important to answer why NMCs do not have the flux rope features. One claims that NMCs lose their original flux rope features due to the interactions between ICMEs or ICMEs and other large scale structures during their transit in the heliosphere. The other attributes this phenomenon to the geometric selection effect, i.e., when an ICME has its nose (flank, including leg and non-leg flanks) pass through the observing spacecraft, the MC (NMC) features will be detected along the spacecraft trajectory within the ejecta. In this Letter, we examine which explanation is more reasonable through the geometric properties of ICMEs. If the selection effect leads to different ejecta types, MCs should have narrower sheath region compared to NMCs from the statistical point of view, which is confirmed by our statistics. Besides, we find that NMCs have the similar size in solar cycles 23 and 24, and NMCs are smaller than MCs in cycle 23 but larger than MCs in cycle 24. This suggests that most NMCs have their leg flank pass through the spacecraft. Our geometric analyses support that all ICMEs should have a magnetic flux rope structure near 1 AU.