# On the Spatial Coherence of Magnetic Ejecta: Measurements of Coronal Mass Ejections by Multiple Spacecraft Longitudinally Separated by 0.01 AU

## TL;DR

This study investigates the three-dimensional magnetic structure of coronal mass ejections by analyzing multipoint spacecraft measurements at small radial but larger longitudinal separations, revealing a magnetic coherence scale of 0.25-0.35 au.

## Contribution

It provides new insights into the longitudinal magnetic coherence scale of CMEs using multipoint measurements from Wind and ACE spacecraft.

## Key findings

- Magnetic field correlation remains high (above 0.97) at separations ≤0.008 au.
- Magnetic coherence length is approximately 0.25-0.35 au for magnetic magnitude.
- Magnetic field components have a coherence scale of about 0.06-0.12 au.

## Abstract

Measurements of coronal mass ejections (CMEs) by multiple spacecraft at small radial separations but larger longitudinal separations is one of the ways to learn about the three-dimensional structure of CMEs. Here, we take advantage of the orbit of the Wind spacecraft that ventured to distances of up to 0.012 astronomical units (au) from the Sun-Earth line during the years 2000 to 2002. Combined with measurements from ACE, which is in a tight halo orbit around L1, the multipoint measurements allow us to investigate how the magnetic field inside magnetic ejecta (MEs) changes on scales of 0.005 - 0.012 au. We identify 21 CMEs measured by these two spacecraft for longitudinal separations of 0.007 au or more. We find that the time-shifted correlation between 30-minute averages of the non-radial magnetic field components measured at the two spacecraft is systematically above 0.97 when the separation is 0.008 au or less, but is on average 0.89 for greater separations. Overall, these newly analyzed measurements, combined with 14 additional ones when the spacecraft separation is smaller, point towards a scale length of longitudinal magnetic coherence inside MEs of 0.25 - 0.35 au for the magnitude of the magnetic field but 0.06 - 0.12 au for the magnetic field components. This finding raises questions about the very nature of MEs. It also highlights the need for additional "mesoscale" multi-point measurements of CMEs with longitudinal separations of 0.01 - 0.2 au.