Ben Snow

Shocks are a fundamental feature of the solar atmosphere. The can form naturally as an upwards propagating compressional wave steepens due to stratification, becoming nonlinear and resulting in a shock. Shocks can also undergo mode conversion when the magnetic field is inclined, with slow and fast mode components separating at the height at which sound and Alfven speed are equal. This mode conversion height can occur in the lower solar atmosphere where the medium is partially ionised. In this poster, I study the mode conversion of stratified shocks in a two-fluid medium.
Our numerical model consists of a 1.5D isolthermal stratified atmosphere with both ionised and neutral species. The atmosphere is mostly neutral at the base of our domain, becoming more ionised towards the upper boundary because of the different pressure scale heights of neutral and ionised species. The magnetic field is inclined. A linear perturbation in velocity, pressure and density is prescribed at the base of our domain that steepens into a shock well below the mode conversion height. The simulations are performed using the (PIP) code. The collisional coupling has a free parameter that allows us to study different levels of coupling between the two species.
For a single-fluid MHD simulation, the shock separates into slow and fast components at the mode conversion height, with the slow component being smoothed, and the fast component remaining sharp. For the two fluid cases, the result depends on the level of coupling of the system. For weak coupling, only the slow plasma component is affected, with the fast-mode shock decoupled. At moderate coupling, the neutrals start to respond to the plasma fast shock, resulting in a large finite width due to the two-fluid interaction. As one tends towards infinite coupling, the finite-width of the fast-mode shock tends to zero as the system approached a fully-coupled MHD solution.
The finite width of the fast-mode shock is a function of the coupling of the system. For a large range of coupling coefficients the finite-width of the shock can exceed the pressure scale height. This leads to a potential observational consequence of two-fluid effects, namely a gradual rise in Doppler velocity for fast-mode shocks. In dimensional units, The shock width would be roughly 300 km, with a gradual Doppler velocity increase over approximately 6 seconds.
The full paper is published as Snow & Hillier, 2020. A&A, 637, A97