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

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 ($k=-1.53$) in the magnetic obstacle (MO), 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.

Figures (5)

• Figure 1: Overview of ICME observations. (a) Heliocentric distance at which all events in the ICMECAT catalog were observed over time. Each dot represents an ICME observation, with colors corresponding to the different spacecraft by which they were observed in situ. (b) Longitude and radial distance (HEEQ) of all spacecraft positions at time of ICME observation, up to 1.6 au. (c) Heliocentric distance coverage of ICME events observed in the inner heliosphere ($\sim$$<$ 1 au) since the launch of PSP in 2018, with an extension of the PSP orbit up to 2030 (gray line). (d) Coverage in latitude (HEEQ) of ICME events observed in the inner heliosphere from 2018 onwards with the Solar Orbiter trajectory (gray line) extended up to 2030.
• Figure 2: An example in situ observation of an ICME in the presented ICMECAT catalog: Solar Orbiter observed this event at a minimum distance of 0.29 au on 2023 April 10. From left to right, the vertical lines state the shock arrival time and the beginning and end times of the magnetic obstacle, which in this case is a well defined magnetic flux rope of south-east-north type. The panels from top to bottom show magnetic field components (in RTN coordinates) and total field, proton bulk speed, proton density, and proton temperature.
• Figure 3: In situ CME observations close to the Sun: The total magnetic field and components of all six ICMEs observed by PSP/FIELDS until 2025 April 30 at distances $< 0.23$ au, starting with the closest event in 2022 September 5. Each panel shows the magnetic field in RTN coordinates, with the event start date and the minimum heliocentric distance PSP reached during the full ICME observation (between ICME start time and MO end time). From left to right, the vertical black lines delineate the ICME start time, the MO start time and MO end time. When there are only two vertical lines in panels (b) and (c), they mark the MO start time and MO end time.
• Figure 4: Evolution of ICME total magnetic field strength in their magnetic obstacles with increasing heliocentric distance. (a) Each dot represents the mean magnetic field of the magnetic obstacle $\langle B_{MO}(R) \rangle$ of an observed ICME by one of the 11 spacecraft indicated in the legend. The solid black line is a direct power law fit to $\langle B_{MO}(R) \rangle$ versus heliocentric distance, $R$. The grey dashed lines show the $3\sigma$ spread in the fit results (see text). The dashed-dotted red line is a power law fit to the maximum magnetic field in the CME magnetic obstacle $\max(B_{MO}(R))$ versus $R$. The resulting power law formulas are indicated on the plot. (b) Similar plot in log10-log10 space with a linear fit restricted to events $< 1.02$ au. (c) Plot in log10-log10 space and fit for all events (0.07 to 5.4 au).
• Figure 5: Connecting the power law for the ICME magnetic obstacle (MO) to solar magnetic field strengths. The power law for the MO mean magnetic field with an exponent of $k=-1.57$ (solid black line) is compared to continuous in situ observations of the total magnetic field by PSP/FIELDS (thin green line, all data between 2018 October 2 and 2025 April 30) and Solar Orbiter/MAG (thin blue line, 2020 April 15 to 2025 May 31 May). The mean magnetic field in the MO is indicated for ICME events in the ICMECAT by MESSENGER, BepiColombo, PSP and Solar Orbiter as dots (see legend). The multipole power law fit for active regions is represented by the orange solid line, with $k_1=-1.57$ and $k_2=-6$. The multipole power law for the quiet Sun is given by the blue solid line ($k_1=-1.57$ and $k_2=-4$). The black, blue and orange solid lines all merge into a single line around 0.05 au.