Anomalous cosmic rays within the inner heliosphere: first observations of ACR helium by Solar Orbiter/HET

(Solar Orbiter Nugget #95 by Z. Xu^1,2, R. F. Wimmer-Schweingruber¹, L. Berger¹, P. Kühl¹, A. Kollhoff¹, B. Heber¹, S. I. Böttcher¹, L. Yang¹, V. Heidrich-Meisner¹, R. D. T. Strauss³, R. Gomez-Herrero⁴, J. Rodriguez-Pacheco⁴, D. Pacheco⁵, and R. A. Leske²)

Introduction

Anomalous cosmic rays (ACRs) originate from interstellar neutral atoms that become ionised in the heliosphere, are transported outward with the solar wind, and are subsequently accelerated at the termination shock to energies of tens of MeV per nucleon. After acceleration, these particles re-enter the inner heliosphere, where their transport is governed by diffusion, convection, adiabatic cooling, and gradient and curvature drifts in the heliospheric magnetic field.

The radial gradient of ACR intensities – describing how particle flux varies with heliocentric distance – is a key observable for understanding these transport processes, particularly the role of particle drifts, which reverse with the ~11-year solar magnetic polarity cycle.

Recent measurements by Parker Solar Probe revealed unexpectedly large radial gradients for ACR oxygen within 1 au during the 2018–2020 solar minimum (A+ polarity). These values were comparable to those expected during the opposite polarity phase, challenging existing drift models and motivating independent measurements with other spacecraft and particle species. Solar Orbiter, travelling between 0.28 and 1 au since its launch in February 2020, provides such an opportunity.

Observations with Solar Orbiter/HET

We analyse helium measurements from the High Energy Telescope (HET), part of the Energetic Particle Detector (EPD) suite on Solar Orbiter, covering the energy range 11.1–49 MeV per nucleon. By combining all four HET viewing directions and rebinning into broader energy channels, we improve counting statistics and track subtle intensity variations along the spacecraft’s orbit.

To isolate radial effects, time periods affected by solar energetic particle events and interplanetary coronal mass ejections were excluded. The remaining quiet-time intensities were averaged over Carrington rotations. To account for long-term solar modulation effects during the rise of solar cycle 25, Solar Orbiter data were normalised to simultaneous SOHO/EPHIN measurements at 1 au.

Figure 1. Quiet-time helium energy spectra from Solar Orbiter/HET and other instruments, showing good agreement and confirming the reliability of the measurements.

Key Results: radial gradients of ACR helium

By analysing the ratio of Solar Orbiter to SOHO intensities as a function of radial distance over the first three spacecraft orbits (February 2020 to December 2021), we derive the radial gradient of ACR helium between 0.5 and 1 au.

The average gradient in the 11–49 MeV per nucleon range is 22 ± 4% per au, indicating that the ACR helium intensity increases by approximately 22% for every additional astronomical unit away from the Sun. After correcting for the contribution of galactic cosmic rays using the BON2020 model, the gradient increases to 32 ± 8% per au for the 11–41 MeV range.

These values are in very good agreement with independent Parker Solar Probe measurements from the same solar minimum period, providing strong confirmation of enhanced ACR radial gradients within the inner heliosphere.

Figure 2. Energy dependence of ACR helium radial gradients, showing consistency between Solar Orbiter and Parker Solar Probe results and the effect of removing galactic cosmic ray contributions.

Temporal evolution of the gradient

An important result is the observed temporal evolution of the radial gradient. Over the first three Solar Orbiter orbits, the gradient increases from approximately 15% per au to about 30% per au, following the rise of solar activity during solar cycle 25.

This trend is consistent with theoretical expectations: as the heliospheric current sheet (HCS) tilt angle increases, drift access of ACRs into the inner heliosphere becomes more restricted, resulting in steeper radial intensity gradients.

Implications and outlook

The large ACR radial gradients measured within 1 au — significantly higher than those typically observed beyond 1 au — indicate that particle transport in the inner heliosphere differs fundamentally from that in the outer heliosphere. In particular, the stronger radial magnetic-field component and modified diffusion conditions likely play a key role.

These observations provide important constraints for models of cosmic ray transport. Looking ahead, Solar Orbiter is moving to higher heliographic latitudes following its 2025 Venus gravity assist. Combined with Parker Solar Probe measurements near the ecliptic and future observations at L1, this will enable the first determinations of latitudinal gradients of ACRs in the inner heliosphere.

This nugget is based on the following work: Xu et al., A&A 708, A36 (2026)

Acknowledgements

The authors acknowledge the use of data from Solar Orbiter, SOHO, ACE, Parker Solar Probe, and Chang’E-4. Solar Orbiter is a mission of international cooperation between ESA and NASA. Funding support from various agencies and institutions is gratefully acknowledged.

Affiliations

(1) Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, Germany

(2) California Institute of Technology, Pasadena, USA

(3) Centre for Space Research, North-West University, South Africa

(4) University of Alcalá, Space Research Group, Spain

(5) University of Science and Technology of China, Hefei, China

References

[1] Xu et al. (2026) 

[2] Rankin et al. (2021)

[3] Rankin et al. (2022) 

[4] Wimmer-Schweingruber et al. (2020) 

[5] Xu et al. (2020) 

[6] Xu et al. (2022) 

[7] Slaba & Whitman (2020)