Investigating the Magnetic Structure of Interplanetary Coronal Mass Ejections using Simultaneous Multi-Spacecraft In situ Measurements

TL;DR

This study uses multi-spacecraft in situ measurements to analyze the spatial variability and ageing effects in interplanetary CMEs, revealing significant variations and the limitations of single-spacecraft data for magnetic structure analysis.

Contribution

It provides a comprehensive analysis of spatial and temporal variations in CME magnetic fields using multi-spacecraft data, improving understanding of in situ measurement limitations.

Findings

Average magnetic field strength varies by 18% between spacecraft.

Ageing effects can be significantly reduced with multi-spacecraft data.

Single-spacecraft measurements near 1 au are strongly affected by ageing.

Abstract

In situ measurements from spacecraft typically provide a time series at a single location through coronal mass ejections (CMEs) and they have been one of the main methods to investigate CMEs. CME properties derived from these in situ measurements are affected by temporal changes that occur as the CME passes over the spacecraft, such as radial expansion and ageing, as well as spatial variations within a CME. This study uses multi-spacecraft measurements of the same CME at close separations to investigate both the spatial variability (how different a CME profile is when probed by two spacecraft close to each other) and the so-called ageing effect (the effect of the time evolution on in situ properties). We compile a database of 19 events from the past four decades measured by two spacecraft with a radial separation <0.2 au and an angular separation <10{\deg}. We find that the average magnetic field strength measured by the two spacecraft differs by 18% of the typical average value, which highlights non-negligible spatial or temporal variations. For one particular event, measurements taken by the two spacecraft allow us to quantify and significantly reduce the ageing effect to estimate the asymmetry of the magnetic field strength profile. This study reveals that single-spacecraft time series near 1 au can be strongly affected by ageing and that correcting for self-similar expansion does not capture the whole ageing effect.