I. Zhelyazkov

Axial structure of a plasma column produced by a large‐amplitude electromagnetic surface wave

Electromagnetic surface waves in the rf and microwave frequency range can be used to produce long large/small‐diameter plasma columns. We propose a theory of surface‐wave‐produced plasmas at two different low‐pressure gas‐discharge regimes, diffusion and recombination, respectively. For a given regime the axial plasma density profile is specified by two quantities: a numerical parameter σ=ωR/c (ω being the wave frequency, R the tube radius, and c denoting the speed of light) and the collision frequency ν (ν≪ω). Our theory predicts the magnitude of the axial electron number density gradient. The very good agreement between the experimentally measured plasma density gradients and the theoretically calculated ones for diffusion‐controlled surface‐wave‐produced plasma columns confirms the adequacy of the proposed model.

Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind

Context. Solar wind plasma is supposed to be structured in magnetic flu x tubes carried from the solar surface. Tangential velocity discontinuity near the boundaries of individual tubes may result in Kelvin-Helmholtz instability, which may contribute into the solar wind turbulence. While the axial magnetic field may stabilize the instability, a sma ll twist in the magnetic field may allow to sub-Alfvenic moti ons to be unstable. Aims. We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tube in the solar wind with di fferent configurations of external magnetic field. Methods. We use magnetohydrodynamic equations in the cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tube moving along its axis in the cases o f untwisted and twisted external fields. Then we solve the dis persion equations analytically and numerically and found thresholds for Kelvin-Helmholtz instability in both cases of external field. Results. Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by external axial magnetic field for sub-Alfvenic motions. However, even sma ll twist in the external magnetic field allows the Kelvin-Hel mholtz instability to be developed for any sub-Alfvenic motions. The unstable harmonics correspond to vortices with high azimuthal mode numbers, which are carried by the flow. Conclusions. Twisted magnetic flux tubes can be unstable to Kelvin-Helmho ltz instability when they move with small speed relative to main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute into the solar wind turbulence.

Propagation of a large-amplitude surface wave in a plasma column sustained by the wave

The axial electron number density distribution of a plasma produced by a rf surface wave launcher has been found theoretically. The results obtained are expressed in the form of universal curves which are applicable to a wide range of experimental conditions (wave frequency, collision frequency, tube radius). The theory agrees well with the available experimental data.

Self-consistent kinetic model of a surface-wave-sustained discharge in nitrogen

A self-consistent kinetic model based on a set of coupled equations consisting of the local electron Boltzmann equation and the rate balance equations for the most important excited species (vibrationally and electronically excited molecular states) and charged particles in a nitrogen discharge has been developed. The system under analysis is a plasma column produced by a travelling, azimuthally symmetric surface wave. Electron collisions of first and second kind with nitrogen molecules and electron - electron collisions are accounted for in the Boltzmann equation. Therefore, this equation is coupled to the set of equations for electronic and vibrational populations through both inelastic and superelastic collisions. The field strength necessary for the discharge steady-state operation is obtained from the balance between the total rate of ionization (including associative, direct and step-wise ionization) and the total rate of electronic losses (due to diffusion to the wall and bulk recombination). The model determines, as a function of the discharge operating parameters (pressure, tube radius, wave frequency, degree of ionization), the electron energy distribution, the populations of the vibrational levels of the electronic ground state and the most important electronic states as well as the concentrations of and ions, consistently with the discharge maintaining electric field. Theoretical results for the electron energy distribution function and some of its moments are compared with experimental ones obtained in a low-pressure surface-wave-sustained discharge at a wave frequency of 500 MHz.

Observation of standing kink waves in solar spicules

We analyze the time series of Ca ii H-line obtained from Hinode/SOT on the solar limb. The time-distance analysis shows that the axis of spicule undergos quasi-periodic transverse displacement at different heights from the photosphere. The mean period of transverse displacement is ∼180 s and the mean amplitude is 1 arc sec. Then, we solve the dispersion relation of magnetic tube waves and plot the dispersion curves with upward steady flows. The theoretical analysis shows that the observed oscillation may correspond to the fundamental harmonic of standing kink waves.

Populations of excited atomic states along argon surface-wave plasma columns at low and intermediate pressures

The axial distributions of the electrons and 3p54s and 3p54p excited atoms in argon plasma columns sustained by traveling electromagnetic waves have been studied both experimentally and theoretically in the gas pressure range of 0.2–2.8 Torr. Various diagnostic methods (surface-wave interferometry, emission and absorption spectroscopy) have been used in data gathering. The theoretical model includes a self-consistent solution to the electron Boltzmann equation, electron energy balance equation, a set of balance equations for excited atoms and charged particles, the gas thermal balance equation, the wave dispersion relation, and the wave energy balance equation. The agreement between experimental data and theoretical results is very good.

Magnetohydrodynamic waves and their stability status in solar spicules

Aims. We investigate conditions under which magnetohydrodynamic waves propagating along spicules become unstable because of the Kelvin-Helmholtz instability. Methods. We employ the dispersion relations of normal modes (kink and sausage waves) derived from the linearised magnetohydrodynamic equations. We assume real wave numbers and complex angular wave frequencies, namely complex wave phase velocities. The dispersion relations are solved numerically at fixed input parameters and various flow velocities. Results. It is shown that the stability of the waves depends upon three parameters, the density contrast between spicules and their environment, the ratio of the background magnetic field outside to that inside spicules, and the value of the Alfven-Mach number (the ratio of the jet velocity to Alfven speed inside the spicules). At certain densities and magnetic fields, an instability of the Kelvin-Helmholtz type can arise if the Alfven-Mach number exceeds a critical value ‐ in our case it is equal to 12.6, which means that for an Alfven speed inside the spicules of 70 kms −1 the jet velocity should be larger than 882 kms −1 . Conclusions. It is found that only kink waves can become unstable, while the sausage ones are always unaffected by the Kelvin-Helmholtz instability.

Kelvin-Helmholtz instability of kink waves in photospheric twisted flux tubes

Aims. We investigate conditions under which kink magnetohydrodynamic waves propagating along photospheric uniformly twisted flux tubes with axial mass flows become unstable as a consequence of the Kelvin-Helmholtz instability. Methods. We employed the dispersion relations of kink waves derived from the linearised magnetohydrodynamic equations. We assumed real wave numbers and complex angular wave frequencies, namely complex wave phase velocities. The dispersion relations were solved numerically at fixed input parameters and several mass flow velocities. Results. We show that the stability of the waves depends upon four parameters, the density contrast between the flux tube and its environment, the ratio of the background magnetic fields in the two media, the twist of the magnetic field lines inside the tube, and the value of the Alfven-Mach number (the ratio of the jet velocity to Alfven speed inside the flux tube). We assume that the azimuthal component of the magnetic field in the tube is proportional to the distance from the tube axis and that the tube is only weakly twisted (i.e., the ratio of the azimuthal and axial components of the magnetic field is low). At certain densities and magnetic field twists, an instability of the Kelvin-Helmholtz type of kink (m = 1) mode can arise if the Alfven-Mach number exceeds a critical value. In particular, for an isolated twisted magnetic flux tube (magnetically free environment) at a density contrast (the ratio of the mass density of the environment to that of the tube itself) equal to 2 and a magnetic field twist (defined as the ratio of azimuthal magnetic field component at the inner surface of the tube to the background magnetic field strength) equal to 0.4, the threshold Alfven-Mach number has a magnitude of 1.250075, which means that for an Alfven speed inside the tube of 10 kms −1 the jet velocity should be higher than 12. 5k ms −1 to ensure the onset of the Kelvin-Helmholtz instability of the kink (m = 1) mode. Speeds of that order can be detected in photospheric tubes. Conclusions. The observed mass flows may trigger the Kelvin-Helmholtz instability of the kink (m = 1) mode in weakly twisted photospheric magnetic flux tubes at critical Alfven-Mach numbers lower that those in untwisted tubes if the magnetic field twist lies in the range 0.36−0.4 and the flow speed exceeds a critical value. A weak external magnetic field (with a ratio to the magnetic field inside the tube in the range 0.1−0.5) slightly increases that critical value.

Theoretical study of a plasma column sustained by an electromagnetic surface wave in the dipolar mode

This paper presents a theoretical model of a plasma column sustained by an electromagnetic surface wave in the dipolar ( m =1) mode for two different gas-discharge regimes: free-fall/diffusion and recombination respectively. The dispersion characteristics of the wave and the axial profiles of the plasma density, wave power, wavenumber and wave-field components for a given regime are specified by one numerical parameter σ = ωR/C , where ω is the angular wave frequency, R the plasma radius and c the speed of light, irrespective of the gas nature and pressure. It is established that there exists a ‘critical’ value of this parameter, σ cr = 0·3726, below which a plasma is not likely to be sustained. A comparison between the axial structures of plasma columns sustained by electromagnetic waves in the dipolar and azimuthally symmetric modes is made. The model is in agreement with the available experimental results.

Modeling of a plasma column produced and sustained by a traveling electromagnetic surface wave

We present an improved model of a plasma column produced and sustained by a traveling azimuthally symmetric electromagnetic surface wave. The axial density profile of the column as well as the wave dispersion and field and power characteristics are now specified by five parameters: (i) the collision frequency for momentum transfer ν, ν<ω (ω being the wave angular frequency), (ii) the number σ=ωR/c (R is the plasma column radius, c the speed of light), (iii) the number β indicating the gas‐discharge regime (B=0 for the diffusion regime and 0<β≤2 for the bulk recombination one), (iv) the permittivity of the container (glass tube) ed, and (v) the number γ=1+d/R (d denoting the thickness of the tube), the last two parameters having been neglected earlier. The influence of ed and γ on the wave dispersion properties and the axial profiles of the plasma density, wave power, and wave electric field components has been studied numerically. Our improved model is tested with the experimental data [A. Sola, A. Gamero...