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

TL;DR

This study leverages simultaneous SEP observations by BepiColombo (0.6 AU) and STEREO-A (1.0 AU) to probe transport along a common magnetic field line. It introduces a data-assimilation approach to the focused transport equation with pitch-angle diffusion, yielding a time-series estimate of the parallel mean free path $\lambda_{\parallel}$ that evolves through the event. The results show a transition from ballistic-like rise to diffusion-dominated decay, with $\lambda_{\parallel}$ decreasing to roughly $0.5$ AU for $1.5$ MeV protons and $1.0$ AU for $5.9$ MeV protons during decay, corroborated qualitatively by magnetic-field fluctuations at 1.0 AU. Differences between SEP-derived $\lambda_{\parallel}$ and estimates from field fluctuations are discussed in the context of turbulence geometry and the need to account for cross-field diffusion in future work.

Abstract

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.

Figures (9)

• Figure 1: Spacecraft location and magnetic field connection at (a) 17:30 UT on March 30, 2022, and (b) 08:00 UT on March 31, 2022, created by the Solar-Mach Python tool 2023FrASS...958810G. The flare and its magnetic connection are denoted as the black arrow and the dashed line, respectively. Solar wind speeds of 700 ${\rm km \; s^{-1}}$in (a) and 500 ${\rm km \; s^{-1}}$ in (b) are used at the BepiColombo and STEREO-A locations, and 400 ${\rm km \; s^{-1}}$ elsewhere.
• Figure 2: Time profiles of the SEP differential flux (particles cm$^{-2}$ s$^{-1}$ sr$^{-1}$ MeV$^{-1}$) during 2022/03/30 17:20 UT - 2022/03/31 23:20 UT. (a-c) 600 seconds cadence data taken by MPO/BERM at 0.6 AU in 1.5-5.9 MeV, 5.9-9.1 MeV, and 13.0-20.7 MeV. (d-f) 60 seconds cadence data taken by STEREO-A/LET at 1.0 AU in 1.8-3.6 MeV, 4.0-6.0 MeV, and 6.0-10.0 MeV. Vertical dashed lines indicate the inferred onset time at each channel. Vertical dashed-dotted lines in the LET profile indicate the onset time of the long-lasting component estimated from the velocity dispersion analysis of the BERM time profile.
• Figure 3: Velocity dispersion analysis of the MPO/BERM (plus) and STEREO-A/LET (asterisk) data. The error bars indicate the temporal resolution of the instruments. Solid lines represent the regression line for each instrument.
• Figure 4: Time profiles of the SEP differential fluxes (particles cm$^{-2}$ s$^{-1}$ sr$^{-1}$ MeV$^{-1}$) taken by the GOES-16 SGPS-X during 2022/03/30 17:20 UT - 2022/03/31 23:20 UT. (a-e) 300 seconds cadence data in 1.9-2.3 MeV, 2.3-3.4 MeV, 3.4-6.5 MeV, 6.5-12 MeV, and 12-25 MeV.
• Figure 5: Time profiles of 60 seconds cadence (a-b) solar wind density and velocity taken by STEREO-A/PLASTIC at 1.0 AU, (c-e) magnetic field vector $(B_R, B_T, B_N)$ taken by STEREO-A/IMPACT, and (f) total magnetic field strength taken by IMPACT (solid line) and Mio/MGF at 0.6 AU (dashed line). Time windows for the Fourier analysis are indicated by vertical dashed lines.