Coronal mass ejections are not coherent magnetohydrodynamic structures.

Area of Science:

• * Solar physics
• * Space weather
• * Plasma physics

Background:

• * Coronal mass ejections (CMEs) are major solar eruptions impacting space weather.
• * Current interpretations often model CMEs as coherent, solid-like structures.
• * Understanding CME evolution is crucial for space weather prediction.

Purpose of the Study:

• * To investigate the coherence of CMEs during their propagation from the Sun.
• * To determine the distance at which CMEs lose their magnetohydrodynamic coherence.
• * To re-evaluate the assumptions used in interpreting CME observations.

Main Methods:

• * Analysis of CME propagation dynamics and geometric expansion.
• * Comparison of theoretical models with observational data from over 400 CMEs.
• * Assessment of CME interaction with the ambient solar wind.

Main Results:

• * Supersonic radial propagation causes CME plasma parcels to expand faster than local wave speeds.
• * CMEs lose their coherence as magnetohydrodynamic structures within 0.3 AU of the Sun.
• * Observed coherence is likely due to similar initial conditions and a homogeneous medium.

Conclusions:

• * CMEs are not coherent structures during much of their interplanetary journey.
• * Accurate reconstruction of the ambient solar wind is essential for interpreting CME observations.
• * Simple assumptions about CME shape and coherence are often invalid, especially with varying solar wind conditions.