Effects of Thomson-Scattering Geometry on White-Light Imaging of an Interplanetary Shock: Synthetic Observations from Forward Magnetohydrodynamic Modelling

TL;DR

This study uses forward magnetohydrodynamic simulations to analyze how Thomson-scattering geometry affects white-light imaging of interplanetary shocks, highlighting implications for 3D reconstruction of electron density in the heliosphere.

Contribution

It provides a quantitative analysis of Thomson-scattering geometry effects on white-light imaging of shocks using synthetic observations from MHD models.

Findings

Shock appears as an inclined brightness streak in images.

Brightness contributions vary with observer longitude and shock position.

Polarization measurements can help determine shock location and improve 3D reconstructions.

Abstract

Stereoscopic white-light imaging of a large portion of the inner heliosphere has been used to track interplanetary coronal mass ejections. At large elongations from the Sun, the white-light brightness depends on both the local electron density and the efficiency of the Thomson-scattering process. To quantify the effects of the Thomson-scattering geometry, we study an interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-light imaging. Identifiable as an inclined streak of enhanced brightness in a time-elongation map, the travelling shock can be readily imaged by an observer located within a wide range of longitudes in the ecliptic. Different parts of the shock front contribute to the imaged brightness pattern viewed by observers at different longitudes. Moreover, even for an observer located at a fixed longitude, a different part of the shock front will contribute to the imaged brightness at any given time. The observed brightness within each imaging pixel results from a weighted integral along its corresponding ray-path. It is possible to infer the longitudinal location of the shock from the brightness pattern in an optical sky map, based on the east-west asymmetry in its brightness and degree of polarization. Therefore, measurement of the interplanetary polarized brightness could significantly reduce the ambiguity in performing three-dimensional reconstruction of local electron density from white-light imaging.