Damping of vertical coronal loop kink oscillations through wave tunneling

The decay rate of vertical kink waves in a curved flux tube is modeled numerically. The full MHD equations are solved for a curved equilibrium flux tube in an arcade geometry and the decay of ψ, the integral over the flux tube of the modulus of the velocity perpendicular to the local magnetic field, is measured. These simulations are 2D and are thus restricted to kink oscillations in the loop plane. The decay rate is found to increase with increasing wavelength, increasing β and decreasing density contrast ratio. The wave tunneling effect is shown to be a possible mechanism for the high decay rate of the recent observed kink oscillation reported by Wang & Solanki (2004)

The emergence of magnetic flux through a partially ionised solar atmosphere

We present results from 2.5D numerical simulations of the emergence of magnetic flux from the upper convection zone through the photosphere and chromosphere into the corona. Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised, in particular the lower chromosphere. This leads to Cowling resistivities orders of magnitude larger than the Coulomb values, and thus to anisotropic dissipation in Ohm’s law. This also leads to localised low magnetic Reynolds numbers (R m < 1). We find that the rates of emergence of magnetic field are greatly increased by the partially ionised regions of the model atmosphere, and the resultant magnetic field is more diffuse. More importantly, the only currents associated with the magnetic field to emerge into the corona are aligned with the field, and thus the newly formed coronal field is force-free

Simulations of Alfvén and Kink wave driving of the solar chromosphere : efficient heating and spicule launching

Two of the central problems in our understanding of the solar chromosphere are how the upper chromosphere is heated and what drives spicules. Estimates of the required chromospheric heating, based on radiative and conductive losses, suggest a rate of ~0.1 erg cm−3 s−1 in the lower chromosphere and drops to ~10−3 erg cm−3 s−1 in the upper chromosphere. The chromosphere is also permeated by spicules, higher density plasma from the lower atmosphere propelled upwards at speeds of ~10–20 km s−1, for so-called Type I spicules, which reach heights of ~3000–5000 km above the photosphere. A clearer understanding of chromospheric dynamics, its heating, and the formation of spicules is thus of central importance to solar atmospheric science. For over 30 years it has been proposed that photospheric driving of MHD waves may be responsible for both heating and spicule formation. This paper presents results from a high-resolution MHD treatment of photospheric driven Alfvén and kink waves propagating upwards into an expanding flux tube embedded in a model chromospheric atmosphere. We show that the ponderomotive coupling from Alfvén and kink waves into slow modes generates shocks, which both heat the upper chromosphere and drive spicules. These simulations show that wave driving of the solar chromosphere can give a local heating rate that matches observations and drive spicules consistent with Type I observations all within a single coherent model

Leakage of waves from coronal loops by wave tunneling

To better understand the decay of vertically polarised fast kink modes of coronal loops by the mechanism of wave tunneling, simulations are performed of fast kink modes in straight flux slabs which have Alfvén speed profiles which include a tunneling region. The decay rates are found to be determined by the mode number of the trapped mode and the thickness of the tunneling region. Two analytical models are suggested to explain the observed decay. The first is a extension of the work of Roberts (1981, Sol. Phys., 69, 39) to include a finite thickness tunneling region, and the second is a simpler model which yields an analytical solution for the relationship between decay rate, period and the thickness of the tunneling region. The decay rates for these straight slabs are found to be slower than in observations and those found in a previous paper on the subject by Brady & Arber (2005, A&A, 438, 733) using curved flux slabs. It is found that the difference between the straight slabs used here and the curved slabs used in Brady & Arber (2005, A&A, 438, 733) can be represented as a geometric correction to the decay rate

Acoustic oscillations in solar and stellar flaring loops

Evolution of a coronal loop in response to an impulsive energy release is numerically modelled. It is shown that the loop density evolution curves exhibit quasi-periodic perturbations with the periods given approximately by the ratio of the loop length to the average sound speed, associated with the second standing harmonics of an acoustic wave. The density perturbations have a maximum near the loop apex. The corresponding field-aligned flows have a node near the apex. We suggest that the quasi-periodic pulsations with periods in the range 10-300 s, frequently observed in flaring coronal loops in the radio, visible light and X-ray bands, may be produced by the second standing harmonic of the acoustic mode

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

Magnetohydrodynamic (MHD) waves are widely considered as a possible source of heating for various parts of the outer solar atmosphere. Among the main energy dissipation mechanisms which convert the energy of damped MHD waves into thermal energy are collisional dissipation(resistivity) and viscosity. The presence of neutral atoms in the partially ionized plasmas of the solar photosphere, chromosphere and prominences enhances the efficiency of both these energy dissipation mechanisms. A comparative study of the efficiency of MHD wave damping in solar plasmas due to collisional and viscous energy dissipation mechanisms is presented here. The damping rates are taken from Braginskii 1965 and applied to the VAL C model of the quiet Sun (Vernazza et al. 1981). These estimations show which of the mechanisms are dominant in which regions. In general the correct description of MHD wave damping requires the consideration of all energy dissipation mechanisms via the inclusion of the appropriate terms in the generalized Ohm’s law, the momentum, energy and induction equations. Specific forms of the generalized Ohm’s Law and induction equation are presented that are suitable for regions of the solar atmosphere which are partially ionised

Sausage oscillations of coronal loops

Aims. Analytical theory predicts the existence of trapped global (or fundamental) sausage fast magnetoacoustic modes in thick and dense coronal loops only, with the periods estimated as the ratio of double the loop length and the Alfvén speed outside the loop. We extend this study to the leaking regime, considering global sausage modes of long loops with small density contrasts. Methods. Anti-symmetric fast magnetoacoustic perturbations (sausage, or m = 0 modes) of a low β plasma slab with the symmetric Epstein profile of plasma density are modelled numerically. Results. It was found that long loops with sufficiently small density contrast can support global sausage leaky modes of detectable quality. The periods of the leaky modes are found to be approximately determined by the loop length and the external Alfvén speed. If the loop length can be estimated from imaging observations, the observed period of this mode provides us with the information about the Alfvén speed outside the loop. For typical flaring coronal loops, the estimated periods of the global sausage modes are about 5−60 s

Phenomenon of Alfvénic vortex shedding

Generation of Alfvenic (magnetohydrodynamic) vortices by the interaction of compressible plasma flows with magnetic-field-aligned blunt obstacles is modeled in terms of magnetohydrodynamics. It is found that periodic shedding of vortices with opposite vorticity is a robust feature of the interaction in a broad range of plasma parameters: for plasma beta from 0.025 to 0.5, and for the flow speeds from 0.1 to 0.99 of the fast magnetoacoustic speed. The Strouhal number is the dimensionless ratio of the blunt body diameter to the product of the period of vortex shedding and the inflow speed. It is found to be consistently in the range 0.15-0.25 in the whole range of parameters. The induced Alfvenic vortices are compressible and contain spiral-armed perturbations of the magnetic field strength and plasma mass density up to 50%-60% of the background values. The generated electric current also has the spiral-armed structuring

Flare-generated acoustic oscillations in solar and stellar coronal loops

Long period longitudinal oscillations of a flaring coronal loop are studied numerically. In the recent work of Nakariakov et al. (2004) it has been shown that the time dependence of density and velocity in a flaring loop contain pronounced quasi-harmonic oscillations associated with the 2nd harmonic of a standing slow magnetoacoustic wave. In this work we investigate the physical nature of these oscillations in greater detail, namely, their spectrum (using the periodogram technique) and how heat positioning affects mode excitation. We found that excitation of such oscillations is practically independent of the location of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly a quarter of the period; it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating