Spicules are dynamic jets propelled upwards (at speeds of approximately 20 km s(-1)) from the solar 'surface' (photosphere) into the magnetized low atmosphere of the Sun. They carry a mass flux of 100 times that of the solar wind into the low solar corona. With diameters close to observational limits (< 500 km), spicules have been largely unexplained since their discovery in 1877: none of the existing models can account simultaneously for their ubiquity, evolution, energetics and recently discovered periodicity. Here we report a synthesis of modelling and high-spatial-resolution observations in which numerical simulations driven by observed photospheric velocities directly reproduce the observed occurrence and properties of individual spicules. Photospheric velocities are dominated by convective granulation (which has been considered before for spicule formation) and by p-modes (which are solar global resonant acoustic oscillations visible in the photosphere as quasi-sinusoidal velocity and intensity pulsations). We show that the previously ignored p-modes are crucial: on inclined magnetic flux tubes, the p-modes leak sufficient energy from the global resonant cavity into the chromosphere to power shocks that drive upward flows and form spicules.

Abstract. The possible mechanism of generation of spicules by Alfvénic waves 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 at the high cyclic frequencies involved, 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 high frequency Alfvén 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 G in the photosphere to 20 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 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 (1992). 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.

Abstract. The possible mechanism of generation of spicules by Alfvénic waves 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 at the high cyclic frequencies involved, 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 high frequency Alfvén 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 G in the photosphere to 10−40 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 up to 7000 km were formed. The formation was found to be 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 and aided by the increased thermal pressure gradient caused by Joule heating due to ion-neutral collisions. The adiabatic results suggest that ion-neutral damping may not support spicules as described by Haerendel (1992). 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. In addition, the relatively high chromospheric temperatures obtained, even at frequencies for which ion-neutral damping and heating might be expected to be unimportant, suggest intriguing possibilities for combining the mechanism with others that are better able to recreate spicule dynamics but suffer from unrealistically low temperatures.

Abstract. The possible mechanism of generation of spicules by Alfvénic 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 Alfvénic 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 photosphere 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 momentum 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 thermal 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.