A study of solar energetic particle transport on 30 March 2022 using multi-spacecraft data assimilation

We analyze a unique solar energetic particle event observed simultaneously by the BepiColombo and STEREO-A spacecraft on March 30, 2022. The two spacecraft at heliocentric distances of 0.6 and 1.0 AU are expected to be aligned approximately along the same magnetic field line, providing a valuable opportunity to investigate particle transport processes in the inner heliosphere. Protons with energies above 1.0 MeV exhibit velocity dispersion during the rise phase, suggesting that the energetic particles are produced close to the Sun, possibly associated with a coronal mass ejection. In contrast, protons during the decay phase are characterized by long-lasting time profiles with longer time scales at 1.0 AU than at 0.6 AU, suggesting that the particles deviate from ballistic propagation. By assimilating these multi-spacecraft observation data into numerical simulations of the focused transport equation, for the first time, we estimate the mean free path parallel to the magnetic field as a time series. The inferred mean free path decreases over time and approaches around 0.5-1.0 AU at the STEREO-A location during the decay phase, suggesting an increasing influence of scattering on particle transport. This interpretation is qualitatively supported by independent STEREO-A observations that showed increasing magnetic field fluctuations, suggesting the connection between the particle transport and the local field fluctuations. However, only a fraction of these fluctuations is expected to contribute to particle scattering, which may be due to the multidimensional nature of magnetic field fluctuations.

💡 Research Summary

The paper presents a comprehensive analysis of a solar energetic particle (SEP) event that occurred on 30 March 2022, observed simultaneously by the BepiColombo spacecraft at 0.6 AU and STEREO‑A at 1.0 AU. Because the two probes were nearly aligned along the same Parker‑spiral magnetic field line, the event offers a rare opportunity to study particle transport in the inner heliosphere with two magnetically connected observation points.

During the rise phase, proton fluxes measured by BepiColombo’s Environment Radiation Monitor (BERM) and STEREO‑A’s Low‑Energy Telescope (LET) show clear velocity dispersion: higher‑energy particles arrive earlier, indicating a prompt release close to the Sun and near‑ballistic propagation along the field line. A velocity‑dispersion analysis yields path lengths of 0.56 ± 0.07 AU (BERM) and 0.92 ± 0.02 AU (LET), consistent with a release region located roughly 0.05–0.13 AU above the solar surface. However, the inferred release times differ by about 1.5 h (≈19:42 UT at BepiColombo vs. ≈18:12 UT at STEREO‑A), suggesting that the two spacecraft may have sampled slightly different particle populations during the early phase.

In the decay phase, both spacecraft record long‑lasting fluxes, but the characteristic decay time (τ_d) grows with heliocentric distance: τ_d ≈ 3.5–7.2 h at 0.6 AU versus 13–23 h at 1.0 AU. This systematic increase cannot be explained by simple ballistic transport and points to enhanced scattering as particles travel outward.

To quantify the scattering, the authors assimilate the multi‑spacecraft observations into numerical solutions of the focused transport equation (FTE). BERM fluxes are used as the inner‑boundary condition, while the parallel mean free path λ∥ is treated as a time‑dependent parameter that is adjusted to reproduce the LET measurements at 1 AU. This data‑assimilation approach yields a λ∥ that starts at several astronomical units during the rise phase and declines steadily to 0.5–1.0 AU during the decay phase. The decreasing λ∥ indicates that the influence of magnetic turbulence on particle propagation strengthens with time.

Independent magnetic‑field measurements from STEREO‑A’s magnetometer show an increase in magnetic‑field fluctuations (B_rms) during the same interval, providing qualitative support for the inferred reduction in λ∥. Nevertheless, only a fraction of the observed fluctuations appears to contribute to particle scattering. The authors argue that this discrepancy arises because the fluctuations are multidimensional (e.g., anisotropic, non‑slab components), limiting the resonant interaction with the particles—a conclusion that aligns with earlier critiques of quasi‑linear theory that often over‑estimate scattering rates.

The paper also examines solar‑wind conditions. Prior to 00:40 UT on 31 March, STEREO‑A experienced a relatively steady fast wind (≈600–700 km s⁻¹) with modest density and magnetic‑field variability, coinciding with the rise phase. After this time, the wind slowed to ≈500 km s⁻¹, density rose, and multiple magnetic‑field discontinuities appeared, creating a more turbulent environment that likely contributed to the observed increase in scattering.

Overall, the study demonstrates that multi‑spacecraft data assimilation into focused‑transport models can retrieve a time‑varying parallel mean free path, offering a more realistic description of SEP propagation than static‑λ∥ assumptions. The findings have direct implications for space‑weather forecasting: incorporating time‑dependent scattering parameters could improve predictions of SEP arrival times and intensities at different heliocentric distances. The authors suggest future work should integrate three‑dimensional turbulence models and expand the spacecraft network to map spatial variations of λ∥, thereby linking transport properties more tightly to the underlying acceleration processes.