Can ion-neutral damping help to form spicules?

Abstract

The possible mechanism of generation of spicules by Alfvenic disturbances is studied in dissipative MHD where dissipation is mainly caused by ion-neutral collision damping, as suggested by Haerendel (1992). Ion-neutral damping becomes non-negligible in the upper chromosphere at high cyclic frequencies of typically greater than 0.1 Hz, and the potential role played by this effect in both forming and supporting solar spicules is investigated. The propagation of randomly generated Alfvenic disturbances 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 G in the photo- sphere to 20 G in the corona. Their propagation is investigated by numerically solving a set of fully nonlinear, dissipative 1.5D MHD equations with waves being generated by a continuous random driver introduced into the equation of angular mo- mentum in the low atmosphere of the Sun. This work is a continuation of James et al. (2003) which studied the results for a continuous, monochromatic sinusoidal driver. As with the previous study, spicule-like structures were formed. The formation was again found to be primarily caused by the impact of a series of slow shocks generated by the continuous interaction between the upward propagating driven disturbance and the downward propagating disturbances reflected by the transition region. The formation was aided by the increased ther- mal pressure gradient caused by Joule heating due to ion-neutral collisions. There is some indication that an analogue of the momentum transfer effect suggested by Haerendel (1992) for simple sinusoidal waves is at work, but this effect on it's own is at best only of a similar order as the reduction in height caused by including damping in the first place. However, the effect is highly sensitive to the level of ionisation and therefore to 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 completely at this point. Significant damping and heating was again observed, strengthening the previously made suggestions that ion-neutral damping may play a more important role in the dynamics of the upper chromosphere than normally assumed in numerical simulations (where it is often neglected completely), although a treatment of radiative losses must be included before this can be confirmed. The heating provided by ion-neutral damping may be an appropriate counter to the low temperatures suffered by other mechanisms better able to reproduce spicule dynamics.