Mode conversion of two-fluid shocks in a partially-ionised, isothermal, stratified atmosphere

Abstract

Context. The plasma of the lower solar atmosphere consists of mostly neutral particles, whereas the upper solar atmosphere is mostly ionised particles and electrons. A shock that propagates upwards in the solar atmosphere therefore undergoes a transition where the dominant fluid is either neutral or ionised. An upwards propagating shock also passes a point where the sound and Alfvén speed are equal. At this point the energy of the acoustic shock can separated into fast and slow components. How the energy is distributed between the two modes depends on the angle of magnetic field. Aims. The separation of neutral and ionised species in a gravitationally stratified atmosphere is investigated. The role of two-fluid effects on the structure of the shocks post-mode-conversion and the frictional heating is quantified for different levels of collisional coupling. Methods. Two-fluid numerical simulations are performed using the (PIP) code of a wave steepening into a shock in an isothermal, partially-ionised atmosphere. The collisional coefficient is varied to investigate the regimes where the plasma and neutral species are weakly, strongly and finitely coupled. Results. The propagation speeds of the compressional waves hosted by neutral and ionised species vary, therefore velocity drift between the two species is produced as the plasma attempts to propagate faster than the neutrals. This is most extreme for a fast-mode shock. We find that the collisional coefficient drastically changes the features present in the system, specifically the mode conversion height, type of shocks present, and the finite shock widths created by the two-fluid effects. In the finitely-coupled regime fast-mode shock widths can exceed the pressure scale height leading to a new potential observable of two-fluid effects in the lower solar atmosphere.

Characterization of shock wave signatures at millimetre wavelengths from Bifrost simulations

“…The formation of shock waves and their propagation through the atmosphere have been studied by means of detailed one-dimensional (1D) numerical simulations (e.g. [2][3][4][5][6][7][8][9][10][11][12][13][14][15]).…”

Stability of two-fluid partially-ionised slow-mode shock fronts

“…However, it is known that for switch-off shocks, the finite width has a dependence on the neutral fraction and plasma−𝛽 (Hillier et al 2016). Also, the physical width of the shocks studied in this paper is much smaller than the > 300 km shocks in Snow & Hillier (2020) As such, one would expect that the stability range changes for different shock types. Further work is needed to calculate this stable range for different types of partially-ionised shocks.…”

Chromospheric Heating by Magnetohydrodynamic Waves and Instabilities

“…In a multi-fluid approach, the heating due to neutrals is expressed via the frictional heating term (Braginskii 1965;Leake et al 2014). The influence of multi-fluid effects on the formation and dissipation of chromospheric shock waves has been scarcely studied, see e.g., Hillier et al (2016); Snow and Hillier (2019), Snow and Hillier (2020). Hillier et al (2016) modeled the formation and evolution of slow-mode shocks driven by reconnection in a partially ionized plasma.…”

“…The influence of multi‐fluid effects on the formation and dissipation of chromospheric shock waves has been scarcely studied, see e.g., Hillier et al. (2016); Snow and Hillier (2019, 2020). Hillier et al.…”