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Coronal mass ejections (CMEs) can be continuously tracked through a largeportion of the inner heliosphere by direct imaging in visible and radiowavebands. White-light (WL) signatures of solar wind transients, such as CMEs,result from Thomson scattering of sunlight by free electrons, and thereforedepend on both the viewing geometry and the electron density. The Faradayrotation (FR) of radio waves from extragalactic pulsars and quasars, whicharises due to the presence of such solar wind features, depends on theline-of-sight magnetic field component $B_\parallel$, and the electron density.To understand coordinated WL and FR observations of CMEs, we perform forwardmagnetohydrodynamic modelling of an Earth-directed shock and synthesise thesignatures that would be remotely sensed at a number of widely distributedvantage points in the inner heliosphere. Removal of the background solar windcontribution reveals the shock-associated enhancements in WL and FR. While theefficiency of Thomson scattering depends on scattering angle, WL radiance $I$decreases with heliocentric distance $r$ roughly according to the expression $I\propto r^{-3}$. The sheath region downstream of the Earth-directed shock iswell viewed from the L4 and L5 Lagrangian points, demonstrating the benefits ofthese points in terms of space weather forecasting. The spatial position of themain scattering site $\mathbf{r}_{\rm sheath}$ and the mass of plasma at thatposition $M_{\rm sheath}$ can be inferred from the polarisation of theshock-associated enhancement in WL radiance. From the FR measurements, thelocal $B_{\parallel {\rm sheath}}$ at $\mathbf{r}_{\rm sheath}$ can then beestimated. Simultaneous observations in polarised WL and FR can not only beused to detect CMEs, but also to diagnose their plasma and magnetic fieldproperties.

astrophysical journal/the astrophysical journal

Using Coordinated Observations in Polarised White Light and Faraday  Rotation to Probe the Spatial Position and Magnetic Field of an  Interplanetary Sheath