Can ion-neutral damping help to form spicules?

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 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.

Inference of the magnetic field in spicules from spectropolarimetry of He I D$_\mathsf{3}$

“…In James et al (2003) on the numerical simulation of Alfvén waves generating and supporting spicules, the authors compute maximum spicule heights for different magnetic fields in the corona, and for several different wave periods from 2 to 10 s. Our inferred field strengths hardly allow coronal field strengths that reach a 20 G value, which seems right at the limit of what those authors would need to reproduce spicule heights. On the other hand, the spicule heights that they compute for 10 G coronal fields, which seems a more appropriate value for the range of field strengths in spicules that we inferred, appear to be too low compared to observed spicular heights.…”

Collisional dissipation of Alfvén waves in a partially ionised solar chromosphere

“…Previous work on the generation and propagation of Alfvén waves has included the simulation of high frequency waves generated by photospheric motions and the damping of these waves in the partially ionised chromosphere above. This damping was suggested as an explanation of the formation of spicules (DePontieu & Haerendel 1998;DePontieu 1999;James et al 2003). Spicules are high, thin jet like structures consisting of chromospheric plasma.…”

The Nonlinear Evolution of a Twist in a Magnetic Shocktube

“…Incorporating gravity and a magnetic flux tube that expands as a function of height, such that rB z ≈constant, where r is the flux tube radius, leads to increased torsional velocities as an Alfvén wave propagates forwards (Hollweg 1992;James et al 2003;. These additions to the model will only serve to further increase the amplitude of the torsional motions seen, and possibly strengthen the effects of the Alfvénic instability along the shocktube.…”

“…In addition, non-thermal broadenings have been observed within the chromosphere (Beckers 1968(Beckers , 1972Pishkalo 1994;Jess et al 2009Jess et al , 2015. These non-thermal broadenings are usually interpreted as Alfvén waves and have been considered as a possible mechanism responsible for Type-I spicule formation (Hollweg et al 1982;Sterling & Hollweg 1988;Hollweg 1992;Kudoh & Shibata 1999;James et al 2003; though it is possible that other mechanisms such as leakage of p-mode oscillations along inclined field lines are responsible (De Pontieu et al 2004). For a review, see Sterling (2000).…”

“…Studies by James et al (2003) and have shown that partial ionization of the atmosphere may aid Alfvén waves with the heating and formation of chromospheric structures such as spicules.…”