Spicule formation by ion-neutral damping

Abstract

The possible mechanism of generation of spicules by Alfvenic waves is studied in dissipative MHD where dissipation is mainly caused by ion-neutral collision damping, as suggested by Haerendel ([CITE]). Ion-neutral damping becomes non-negligible at the high cyclic frequencies involved, typically greater than $0.1~{\rm Hz}$, and the potential role played by this effect in both forming and supporting solar spicules is investigated. The propagation of high frequency Alfven waves on vertically open solar magnetic flux tubes is considered. The flux tubes are taken to be axisymmetric and initially untwisted with the field strength declining from $1600~{\rm G}$ in the photosphere to $20~{\rm G}$ in the corona. Their propagation is investigated by numerically solving a set of fully nonlinear, dissipative 1.5D MHD equations with the waves being generated by a continuous sinusoidal driver introduced into the equation of angular momentum in the low atmosphere of the Sun. Spicule-like structures with heights of around $5000{-}6000~{\rm km}$ were formed. The formation was found to be primarily caused by the impact of a series of slow shocks generated by the continuous interaction between the upward propagating driven wave train and the downward propagating train of waves created by reflection off the transition region. At the lower end of frequencies considered the heating due to ion-neutral damping was found to provide only a small benefit due to the increased thermal pressure gradient. At higher frequencies, whilst the heating effect becomes stronger, the much reduced wave amplitude reaching the transition region hinders spicule formation. The adiabatic results suggest that ion-neutral damping may not support spicules as described by Haerendel ([CITE]). However, the effect is highly sensitive to the level of ionisation and therefore the energy balance. Including the effects of thermal conduction and radiation may well lead to different results and thus it would be premature to dismiss the mechanism at this point.