Spicule formation by ion-neutral damping

Abstract

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.

Wave propagation in multiple flux tubes and chromospheric heating

“…(a) We have identified an efficient mechanism for the generation of longitudinal motions from purely transverse motions at the base of tubes -similar results were found by James and Erdélyi (2002) for Alfvén waves in spicules; (b) These longitudinal motions in the chromosphere are associated with slow magnetoacoustic waves that are responsible for heating the atmosphere which is localized to the regions close to the tube axis -a similar inference was made by De Pontieu et al (2004) in a different context; (c) We find that the main contribution to the heating comes from motions generated within the tube and not from the ambient medium or from waves from neighbouring tubes.…”

Predicting Observational Signatures of Coronal Heating by Alfvén Waves and Nanoflares

“…The amplitudes of the driver are such that a hv 2 i 1=2 $ 1:1 km s À1 azimuthal photospheric velocity field is generated. model for creating spicules and other chromospheric dynamical events has already been shown (Kudoh & Shibata 1999;James & Erdélyi 2002).…”

Collisional and viscous damping of MHD waves in partially ionized plasmas of the solar atmosphere

“…The energy and momentum transfer from the Alfvén waves damped in the partially ionized chromospheric plasma has been proposed as a driving factor for solar spicules (Haerendel 1992;DePontieu & Haerendel 1998;James & Erdélyi 2002;James et al 2003). The leakage of Alfvén wave energy through the footpoints of a coronal loop, caused by wave dissipation in the partially ionized chromosphere (DePontieu et al 2001;Ofman 2002), is considered as one of possible explanations of the recently observed rapid damping of coronal loop oscillations (Aschwanden et al 1999b;Nakariakov et al 1999).…”