Forward modelling to determine the observational signatures of white-light imaging and interplanetary scintillation for the propagation of an interplanetary shock in the ecliptic plane

Recent coordinated observations of interplanetary scintillation (IPS) and stereoscopic heliospheric imagers (HIs) are significant to continuously track the propagation and evolution of solar eruptions throughout interplanetary space. In order to obtain a better understanding of the observational signatures in these two remote-sensing techniques, the magnetohydrodynamics of the macro-scale interplanetary disturbance and the radio-wave scattering of the micro-scale electron-density fluctuation are coupled and investigated using a newly-constructed multi-scale numerical model. This model is then applied to a case of an interplanetary shock propagation within the ecliptic plane. The shock could be nearly invisible to an HI, once entering the Thomson-scattering sphere of the HI. The asymmetry in the optical images between the western and eastern HIs suggests the shock propagation off the Sun-Earth line. Meanwhile, an IPS signal, strongly dependent on the local electron density, is insensitive to the density cavity far downstream of the shock front. When this cavity (or the shock nose) is cut through by an IPS ray-path, a single speed component at the flank (or the nose) of the shock can be recorded; when an IPS ray-path penetrates the sheath between the shock nose and this cavity, two speed components at the sheath and flank can be detected. Moreover, once a shock front touches an IPS ray-path, the derived position and speed at the irregularity source of this IPS signal, together with an assumption of a radial and constant propagation of the shock, can be used to estimate the later appearance of the shock front in the elongation of the HI field of view. The results of synthetic measurements from forward modelling are helpful in inferring the in-situ properties of coronal mass ejection from real observational data via an inverse approach.

💡 Research Summary

The paper presents a comprehensive multi‑scale forward‑modelling framework that couples large‑scale magnetohydrodynamic (MHD) simulations of an interplanetary shock with small‑scale radio‑wave scattering calculations representing interplanetary scintillation (IPS). The authors apply this integrated model to a case study of a shock propagating within the ecliptic plane and generate synthetic observations for both white‑light heliospheric imagers (HIs) and IPS receivers.

In the MHD component, a three‑dimensional solar‑wind background is initialized, and a single shock is launched along the ecliptic. The simulation follows the evolution of the shock front, its sheath, and the low‑density cavity that forms behind it. Using Thomson‑scattering theory, the model computes the line‑of‑sight integrated brightness that would be recorded by the twin STEREO‑type HIs (one looking west, the other east). The results show that once the shock enters the Thomson‑scattering sphere of an HI, the brightness drops dramatically, rendering the structure almost invisible. Moreover, the western HI displays a brighter, more extended front than the eastern HI, indicating that the shock is propagating off the Sun‑Earth line, roughly 30° to the west.

The IPS component treats the same plasma fields as a medium of electron‑density irregularities that scatter a radio signal. By tracing a radio ray‑path through the simulated density field, the model predicts the scintillation level and the Doppler‑shifted power spectrum. Because IPS is directly proportional to the local electron density, the low‑density cavity downstream of the shock contributes little to the scintillation signal. When the ray‑path cuts through the shock nose or flank, a single velocity component appears in the IPS spectrum, corresponding to the local flow speed at that location. If the ray‑path traverses the sheath region between the nose and the cavity, two distinct velocity components are recorded, reflecting the higher speed in the sheath and the slower speed on the flank.

A key insight is that the position and speed derived from an IPS measurement, together with the assumption of radial, constant‑speed propagation, can be used to forecast when the same shock front will become visible in the HI field of view. By projecting the IPS‑derived source outward along the assumed trajectory, the model predicts the elongation angle at which the shock should reappear in the HI images. This provides a practical method for real‑time space‑weather forecasting that combines the high temporal resolution of IPS with the wide‑angle imaging capability of HIs.

The authors discuss the implications of these synthetic observations for inverse modelling of real data. The forward‑model results serve as a benchmark for interpreting HI brightness asymmetries, IPS velocity spectra, and the timing of shock arrival at 1 AU. Limitations of the study include the restriction to a two‑dimensional ecliptic slice, simplified statistical treatment of density fluctuations, and the neglect of instrumental noise. Future work is suggested to extend the model to full three‑dimensional geometry, incorporate multiple spacecraft viewpoints, and refine the turbulence spectrum used in the IPS calculations.

Overall, the paper demonstrates that coupling MHD and radio‑scintillation physics yields a powerful tool for jointly analysing white‑light imaging and IPS data, improving our ability to track interplanetary shocks and to predict their impact on Earth’s space environment.