Can ion-neutral damping help to form spicules?

Abstract

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.

Chromospheric heating and generation of plasma outflows by impulsively generated two-fluid magnetoacoustic waves

“…Hence, an additional mechanism of heating is required to compensate for these radiative losses (Narain & Ulmschneider 1996). One of the sources of this heating may be associated with collisions between particles, which lead to wave damping (Erdélyi & James 2004). The idea of heating the solar chromosphere by acoustic waves is very old and was first developed by Biermann (1946) and Schwarzschild (1948), who suggested that acoustic waves play a major role in the heating of this atmospheric layer.…”

Buoyancy-driven Magnetohydrodynamic Waves in a Partially Ionized Plasma

“…The effect of partial ionization on Alfvén waves have been studied by, e.g., Haerendel (1992), de Pontieu & Haerendel (1998), James & Erdélyi (2002), James et al (2003), Erdélyi & James (2004), Leake et al (2005), and Vranjes et al (2008) to name but a few. Some of these works suggested the damping of Alfvén waves as a mechanism for the formation of spicules.…”

Magnetic-diffusion-driven shear instability of solar flux tubes

“…This could have been anticipated on the grounds that in the photosphere ( 500 km) and chromosphere ( 2500 km) the plasma number density is much smaller than the neutral number density and thus, high frequency (with respect to the ion-neutral collision frequency) MHD waves are severely damped by collisional dissipation of the wave energy. A way out of this difficulty is to retain the MHD momentum equation for the ionized component and include the effect of collisions with the neutrals by modifying the induction and energy equations to incorporate a conductivity tensor (Erdélyi & James 2004;Leake & Arber 2006), an approach often employed to study the lower ionosphere of the Earth. However, the derivation of the time-independent conductivity tensor neglects time derivatives in the electron and ion momentum equations, i.e.…”