On the magnetic field evolution of interplanetary coronal mass ejections from 0.07 to 5.4 au

TL;DR

This study analyzes the evolution of interplanetary coronal mass ejections' magnetic fields from near the Sun to 5.4 au, revealing a power-law decay and proposing a multipole model to connect solar surface fields with interplanetary observations.

Contribution

It provides the most extensive catalog and analysis of ICME magnetic field evolution, introducing a novel multipole power law model linking solar surface fields to interplanetary magnetic fields.

Findings

Magnetic field strength follows a power law with distance, with an exponent of approximately -1.57.

A multipole model with two exponents better explains the magnetic field decay from the Sun to 5.4 au.

The observed ICME magnetic fields are inconsistent with quiet Sun and active region fields when extended to the photosphere.

Abstract

A central question for understanding interplanetary coronal mass ejection (ICME) physics and improving space weather forecasting is how ICMEs evolve in interplanetary space. We have updated one of the most comprehensive in situ ICME catalogs to date, which now includes 1976 events from 11 space missions covering over 34 years, from December 1990 to August 2025. We have combined existing catalogs including magnetic obstacles and identified and added boundaries of an additional 807 (40.8%) events ourselves. With this catalog, we demonstrate the most extensive analysis to date of total ICME magnetic field values as a function of heliocentric distance. Parker Solar Probe has observed 6 ICMEs at $< 0.23$ au (until April 2025), and Solar Orbiter and BepiColombo have added more events near 0.3 au, bridging the major observational gap towards the solar corona. Our main result is that a single power law can describe the evolution of the mean total magnetic field (exponent value of $k=-1.57$) and maximum field ($k=-1.53$) for ICMEs with magnetic obstacles (MOs), from 0.07 to 5.4 au. Extending the power law to the solar photosphere reveals a strong inconsistency with magnetic field magnitudes observed in the quiet Sun and active regions by 2 and 4 orders of magnitude, respectively. We introduce a multipole-type power law with two exponents, $k_1=-1.57$, and $k_2=-6$, relating the ICME magnetic field magnitude to an average solar active region field strength. These results present important observational constraints for the evolution of ICMEs from the Sun to the heliosphere.