Solar Orbiter as Part of a Fleet of Spacecraft to Derive Power Laws for CME Evolution

(Science nugget #101 by Christian Möstl¹, Emma E. Davies¹, Eva Weiler^1,2, Hannah T. Rüdisser^1,2, Ute V. Amerstorfer¹, Andreas J. Weiss^3,4, Martin A. Reiss⁵, Satabdwa Majumdar¹, Timothy S. Horbury⁶, Stuart D. Bale⁷, and Daniel Heyner⁸)

Introduction

A major observational constraint on the evolution of interplanetary coronal mass ejections (ICMEs) from the Sun to Earth is given by in situ observations of ICMEs at different distances from the Sun. As Solar Orbiter is cruising in the inner heliosphere, it has so far observed 159 CMEs in situ, from mission start in early 2020 up to the middle of 2025. Together with Parker Solar Probe, this strongly increases the number of events observed in this solar wind spatial domain between the Sun and 1 au. As Parker Solar Probe observed CMEs even at 0.07 au, this is the first time we have a dataset of ICMEs covering almost the entire inner heliosphere. It is of high interest to derive from these observations power laws, in particular to describe how their total magnetic field magnitude decreases as they propagate away from the Sun.

A major application for these power laws, that has been revived recently, is that upstream monitors, spacecraft that are situated near the Sun–Earth line but closer to the Sun, could measure the magnetic field of an ICME in situ and thus solve the problem of predicting the southward magnetic field component and thus geomagnetic storm magnitude [1,2]. This has been demonstrated with Solar Orbiter [3,4] and STEREO-A [5,6]. The power laws can then be used to predict the field magnitude of the ICME in near-Earth space from a location closer to the Sun.

Figure 1. Solar Orbiter MAG and SWA observations of an ICME on 2023 April 10, at 0.29 au.

Figure 1 shows an example of an ICME observation at 0.29 au with Solar Orbiter, highlighting many features of an ICME observation. The three vertical lines, from left to right, show the shock arrival time, the start of the magnetic obstacle, and the end of the magnetic obstacle, which is defined by a smooth rotation of the magnetic field vector over several hours and low field variance.

Our team at the Austrian Space Weather Office at GeoSphere Austria has manually measured these ICMEs at Solar Orbiter and many other spacecraft, and built up a database, in its first version established a decade ago in the HELCATS project (https://www.helcats-fp7.eu/). The latest iteration, used in this paper, is dated to October 2025 and available at https://helioforecast.space/icmecat and on figshare [7]. The general requirement for an event to be added to the ICMECAT is that a magnetic obstacle is present. Our catalog has proven useful in many other contexts, for training AI models for automatic identification of ICMEs [8], or removing ICMEs to study the ambient wind [9], or multipoint SEP observation studies [10].

Figure 2. Overview of ICME observations in the ICMECAT. (a) Time and heliocentric distance, (b) a projection in HEEQ coordinates with the Earth fixed on the right side of the plot.

Figure 2 gives an overview of the time, distance and longitude of the observations in the ICME catalog. It can be seen that with the recent missions Parker Solar Probe, Solar Orbiter, and BepiColombo (only magnetic field observations during cruise phase), in situ observations of the plasma and magnetic field become much more numerous compared to those from Venus Express and MESSENGER a decade earlier.

Figure 3. Power laws for the mean and maximum magnetic field.

Results on Power Laws

The main result of our study are power laws for the CME total mean and maximum magnetic field inside the magnetic obstacles. The exponent is -1.57 for the mean and -1.53 for the maximum magnetic field. The result is similar when only ICME datapoints < 1 au are used, but the exponents drop to -1.33 for the domain > 1 au. That the power law is more "flat" for the outer heliosphere has been noted previously by many authors [11]. Another recent study [12] has also shown that the components also behave in a similar way, and are not differing when split up into axial or poloidal fields. When extending the power law to the solar photosphere, one notices that it falls short of the field magnitudes observed there, and a possible multipole extension to the power law that takes this into account is discussed in our paper.

Conclusion

The power laws we derived are the most complete of their kind yet, using the largest database with the number of 1976 events, and allow, among other applications, to "correct" the magnetic field magnitude observed by a sub-L1 monitor to Earth distance [4]. We know however that individual events can vary strongly around this average exponent and how these are influenced by the ambient wind need to be better studied [13].

Publication

This article has been published in The Astrophysical Journal with DOI: https://iopscience.iop.org/article/10.3847/1538-4357/ae50fe

Affiliations

(1) Austrian Space Weather Office, GeoSphere Austria, Graz, Austria

(2) Institute of Physics, University of Graz, Graz, Austria

(3) Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD 21250, USA

(4) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA

(5) Community Coordinated Modeling Center, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

(6) Imperial College London, South Kensington Campus, London SW7 2AZ, UK

(7) Physics Department and Space Sciences Laboratory, University of California, Berkeley, USA

(8) Technical University of Braunschweig, Braunschweig, Germany

Acknowledgements

This work is supported by ERC grant (HELIO4CAST, 10.3030/101042188), funded by the European Union. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. Solar Orbiter magnetometer operations are funded by the UK Space Agency (grant UKRI943). T.H. is supported by STFC grant ST/W001071/1. The PSP/FIELDS experiment was developed and is operated under NASA contract NNN06AA01C. D.H. was supported by the German Ministerium für Wirtschaft und Klimaschutz and the German Zentrum für Luft- und Raumfahrt under contract 50QW2202.

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