Ion-neutral Collision Frequency Research Articles

Practical expression for an effective ion-neutral collision frequency in flowing plasmas of some noble gases

AbstractA simple expression for the effective ion-neutral collision frequency in weakly ionized drifting plasmas of helium, neon, and argon is suggested. This expression can be useful for practical estimations related to the problems of particle charging and ion drag force in complex (dusty) plasmas.

Simulated ion velocity distribution and its incoherent scattering spectrum in the high-latitude ionosphere

If the relatively larger convection electric field and small the collision frequency between ions and neutrals in the high-latitude ionosphere are considered, the integral solution of the non-Maxwellian ion velocity distribution function for the relaxation collision model is simplified, and a torus distribution function of the ion velocity is obtained. Also, the feature of the torus velocity distribution function is analyzed and simulated with different parameters in high-latitude ionosphere. Furthermore, by using the electromagnetic radiation theory of Sheffield and taking into account the ion temperature anisotropy and its variation with the electric field, we simulate the incoherent scattering spectrum of ions in different directions. The research shows that the ionospheric convection electric field, the ion-neutral collision frequency and the ion anisotropic temperature distribution all have an effect on the incoherent scattering spectrum.

Self-similar space-time evolution of an initial density discontinuity

The space-time evolution of an initial step-like plasma density variation is studied. We give particular attention to formulate the problem in a way that opens for the possibility of realizing the conditions experimentally. After a short transient time interval of the order of the electron plasma period, the solution is self-similar as illustrated by a video where the space-time evolution is reduced to be a function of the ratio x/t. Solutions of this form are usually found for problems without characteristic length and time scales, in our case the quasi-neutral limit. By introducing ion collisions with neutrals into the numerical analysis, we introduce a length scale, the collisional mean free path. We study the breakdown of the self-similarity of the solution as the mean free path is made shorter than the system length. Analytical results are presented for charge exchange collisions, demonstrating a short time collisionless evolution with an ensuing long time diffusive relaxation of the initial perturbation. For large times, we find a diffusion equation as the limiting analytical form for a charge-exchange collisional plasma, with a diffusion coefficient defined as the square of the ion sound speed divided by the (constant) ion collision frequency. The ion-neutral collision frequency acts as a parameter that allows a collisionless result to be obtained in one limit, while the solution of a diffusion equation is recovered in the opposite limit of large collision frequencies.

Ion–neutral coupling in Earth's thermosphere, estimated from concurrent radar and optical observations above Alaska

During March–April 2011 a campaign of coordinated observations was undertaken between the Poker Flat Advanced Modular Incoherent Scatter Radar and the Poker Flat Scanning Doppler Imager. These instruments provide horizontally resolved maps of plasma and neutral parameters in Earth's thermosphere. We report on data collected during the campaign, and use these data to investigate two key aspects of ion–neutral coupling, namely Joule heating and the ion–neutral collision frequency. Volumetric Joule heating rates were often well correlated with measured ion temperature enhancements. The contribution of the neutral wind dynamo to the observed heating rates was positive when the absolute horizontal magnetic field perturbation (|ΔH|) was less than approximately 40nT, and negative above that level. The total momentum–transfer ion–neutral collision frequency was estimated to be 1.02−0.152+0.179s−1 at an altitude of 260km, which, for a neutral composition of 75% atomic oxygen, yielded an estimate of the O+–O collision frequency of 0.766−0.114+0.134s−1.

ALFVÉN WAVES IN A PARTIALLY IONIZED TWO-FLUID PLASMA

Alfv\'en waves are a particular class of magnetohydrodynamic waves relevant in many astrophysical and laboratory plasmas. In partially ionized plasmas the dynamics of Alfv\'en waves is affected by the interaction between ionized and neutral species. Here we study Alfv\'en waves in a partially ionized plasma from the theoretical point of view using the two-fluid description. We consider that the plasma is composed of an ion-electron fluid and a neutral fluid, which interact by means of particle collisions. To keep our investigation as general as possible we take the neutral-ion collision frequency and the ionization degree as free parameters. First, we perform a normal mode analysis. We find the modification due to neutral-ion collisions of the wave frequencies and study the temporal and spatial attenuation of the waves. In addition, we discuss the presence of cut-off values of the wavelength that constrain the existence of oscillatory standing waves in weakly ionized plasmas. Later, we go beyond the normal mode approach and solve the initial-value problem in order to study the time-dependent evolution of the wave perturbations in the two fluids. An application to Alfv\'en waves in the low solar atmospheric plasma is performed and the implication of partial ionization for the energy flux is discussed.

Effect of partial ionization on wave propagation in solar magnetic flux tubes

Observations show that waves are ubiquitous in the solar atmosphere and may play an important role for plasma heating. The study of waves in the solar corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully ionized plasma. However, the plasma in the photosphere and the chromosphere is only partially ionized. Here we investigate theoretically the impact of partial ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid model. We derive the general dispersion relation that takes into account the effects of neutral-ion collisions and the neutral gas pressure. We take the neutral-ion collision frequency as an arbitrary parameter. Particular results for transverse kink modes and slow magnetoacoustic modes are shown. We find that the wave frequencies only depend on the properties of the ionized fluid when the neutral-ion collision frequency is much lower that the wave frequency. For high collision frequencies realistic of the solar atmosphere ions and neutrals behave as a single fluid with an effective density corresponding to the sum of densities of both fluids and an effective sound velocity computed as the average of the sound velocities of ions and neutrals. The MHD wave frequencies are modified accordingly. The neutral gas pressure can be neglected when studying transverse kink waves but it has to be taken into account for a consistent description of slow magnetoacoustic waves. The MHD waves are damped due to neutral-ion collisions. The damping is most efficient when the wave frequency and the collision frequency are of the same order of magnitude. For high collision frequencies slow magnetoacoustic waves are more efficiently damped than transverse kink waves. In addition, we find the presence of cut-offs for certain combinations of parameters that cause the waves to become non-propagating.

Nonlinear structure of coupled drift ion acoustic waves in dissipative pair-ion-electron plasmas

The Kadomtsev-Petviashvili-Burgers (KPB) equation is derived for coupled drift acoustic shock waves in a partially ionized non-uniform pair-ion-electron (PIE) plasma in the presence of both density and temperature gradients respectively. Both linear and nonlinear studies are presented. The nonlinear KPB equation is derived in the small amplitude approximation method and its solution is found using the tanh method. The numerical calculations also presented for PIE plasmas of fullerene plasma for illustration keeping in view the recent experiments. The effect of density and temperature inhomogeneities on the nature of the shock is also highlighted. The role of the velocity of the nonlinear structure with regard to the density and temperature gradients driven drift velocities is also pointed out and the effect of ion-neutral collision frequency is also investigated. This work may be useful for future laboratory experimental investigations on pair-ion-electron plasmas.

RAYLEIGH-TAYLOR INSTABILITY IN PARTIALLY IONIZED COMPRESSIBLE PLASMAS

We study the modification of the classical criterion for the linear onset and growing rate of the Rayleigh-Taylor instability (RTI) in a partially ionized plasma in the two-fluid description. The plasma is composed of a neutral fluid and an electron-ion fluid, coupled by means of particle collisions. The governing linear equations and appropriate boundary conditions, including gravitational terms, are derived and applied to the case of the RTI in a single interface between two partially ionized plasmas. The limits of collisionless, no gravity, and incompressible fluids are checked before addressing the general case. We find that both compressibility and ion-neutral collisions lower the linear growth rate, but do not affect the critical threshold of the onset of the RTI. The configuration is always unstable when a lighter plasma is below a heavier plasma regardless the value of the magnetic field strength, the ionization degree, and the ion-neutral collision frequency. However, ion-neutral collisions have a strong impact on the RTI growth rate, which can be decreased by an order of magnitude compared to the value in the collisionless case. Ion-neutral collisions are necessary to accurately describe the evolution of the RTI in partially ionized plasmas such as prominences. The timescale formore » the development of the instability is much longer than in the classical incompressible fully ionized case. This result may explain the existence of prominence fine structures with life times of the order of 30 minutes. The timescales derived from the classical theory are about one order of magnitude shorter and incompatible with the observed life times.« less

TWO-DIMENSIONAL RADIATIVE MAGNETOHYDRODYNAMIC SIMULATIONS OF THE IMPORTANCE OF PARTIAL IONIZATION IN THE CHROMOSPHERE

The bulk of the solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the chromospheric plasma. We investigate the importance of introducing neutral particles into the MHD equations using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span the solar atmosphere from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-gray and non-LTE radiative transfer, and thermal conduction along the magnetic field. The effects of partial ionization are implemented using the generalized Ohm's law, i.e., we consider the effects of the Hall term and ambipolar diffusion in the induction equation. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic diffusion, Hall term, and ambipolar diffusion show strong variations in the chromosphere. These strong variations of the various magnetic diffusivities are absent or significantly underestimated when, as has been common for these types of studies, using the semi-empirical VAL-C model as a basis for estimates. In addition, we find that differences in estimating the magnitude of ambipolar diffusion arise depending on which method is used to calculate the ion–neutral collision frequency. These differences cause uncertainties in the different magnetic diffusivity terms. In the chromosphere, we find that the ambipolar diffusion is of the same order of magnitude or even larger than the numerical diffusion used to stabilize our code. As a consequence, ambipolar diffusion produces a strong impact on the modeled atmosphere. Perhaps more importantly, it suggests that at least in the chromospheric domain, self-consistent simulations of the solar atmosphere driven by magnetoconvection can accurately describe the impact of the dominant form of resistivity, i.e., ambipolar diffusion. This suggests that such simulations may be more realistic in their approach to the lower solar atmosphere (which directly drives the coronal volume) than previously assumed.

E region electric fields at the dip equator and anomalous conductivity effects

Zonal and vertical electric fields were estimated at E region heights in the Brazilian sector. Zonal electric fields are obtained from the vertical electric fields based on their relation through the Hall-to-Pedersen ionospheric conductivities ratio. The technique for obtaining the vertical electric field is based on its proportionality to the Doppler velocities of type 2 irregularities as detected by coherent radars. The 50MHz backscatter coherent (RESCO) radar was used to estimate the Doppler velocities of the type 2 irregularities embedded in the equatorial electrojet. A magnetic field-line integrated conductivity model was developed to provide the conductivities. It considers a multi-species ionosphere and a multi-species neutral atmosphere, and uses the IRI 2007, the MISIS 2000 and the IGRF 10 models as input parameters for ionosphere, neutral atmosphere and Earth’s magnetic field, respectively. The ion-neutral collision frequencies of all the species are combined through the momentum transfer collision frequency equation, and different percentages of electron-neutral collisions were artificially included for studying the implication of such increase in the zonal electric field, which resulted ranging from 0.13 to 0.49mV/m between the 8 and 18h (LT), under quiet magnetic conditions.

Creating an evanescent ion/ion reaction region within a low-pressure linear ion trap

We present a new environment for conducting ion/ion reactions within a linear ion trap (LIT) mass spectrometer. Normally, these reactions are performed in a dedicated high-pressure (P>mTorr) region that uses some form of radio frequency (RF) potentials to trap the reacting cations and anions in the axial and radial dimensions. Generally, a low-pressure (P≤10−5Torr) environment is not well suited for ion/ion reactions due to the low ion-neutral gas collision frequency that results in higher kinetic energies for the ions – a deleterious condition for efficient ion/ion reactions. However, with some minor modifications, a low-pressure LIT can be transformed into an evanescent (or temporary) ion/ion reaction region. This is done by adding DC and RF voltages to auxiliary electrodes and a pulse valve for the addition of cooling gas. These features serve to restrict axial and radial motion of the reacting ions. We compared the performance of this ion/ion reaction region to a static high-pressure region (∼10−3Torr) and a static low-pressure region (∼10−5Torr). In terms of reaction efficiency, cycle times, etc., the performance of the evanescent ion/ion reaction environment compares favorably with the dedicated static high-pressure ion/ion reaction region.

Incoherent scatter spectrum of ionospheric plasma with an anisotropic temperature ion distribution

The expression of anisotropic temperature ion distribution function under the 13-moment approximation is obtained by solving a set of moment equations based on the Boltzmann equation for a relaxation collision model and with consideration of the anisotropic temperature ion distribution. And the incoherent scatter spectrum with an anisotropic temperature ion distribution is simulated in different directions based on the electromagnetic radiation theory of Sheffield. The effects of different electrical field strengths, ratios of electron temperature to ion temperature, and ion-neutral collision frequencies on the incoherent scatter spectrum are all discussed. Finally, the value of theoretical simulation is compared with the measured value of incoherent scattering spectrum. The result show that the incoherent scatter spectrum of ions seriously deviates from the form of the Maxwellian distribution in the equilibrium state. This phenomenon can be attributed to the effects of anisotropic temperature ion distribution, the larger convection electric field, and other factors in high latitude ionosphere.

Nonlinear Evolution of the Dust Acoustic Instability in Artificially Created Dusty Space Plasmas

A 2-D hybrid electrostatic model is used to investigate the dust acoustic instability driven by an ion beam. To understand the generation and the nonlinear electrostatic waves, we investigate possible parameter regimes for the dust acoustic instability. The principal application involves the Charged Aerosol Release Experiment in the ionosphere, but also potential applications are for the planetary ring system of Saturn. The plasma background ions and charged dust are treated as discrete particles, whereas electrons are dealt with as a Boltzmann fluid. A Monte Carlo collision model is adopted to deal with the ion-neutral collisions. The kinetic dispersion relation is numerically solved to investigate effects of the ion drift velocity and the dust charge and density on the wave growth rate and the wave propagation direction. The simulation results agree with linear analysis of the instability, which also shows that the most unstable dust acoustic wave may obliquely propagate to the streaming direction. This may be important for radar diagnostics. The wave saturates by trapping dust. The waves damp out as the ion-neutral collision frequency increases comparable with the wave growth rate, and more importantly, such collision may lead to unstable waves propagating even further off the streaming direction.

Nonlinear electrostatic shock waves in inhomogeneous plasmas with nonthermal electrons

Density inhomogeneity driven linear and nonlinear ion drift waves are investigated in a plasma consisting of heavy ions and non-thermal electrons. The dissipation is introduced in the system by the ion-neutral collision frequency. The nonlinear Korteweg de Vries Burgers (KdVB) and Burgers like equations are derived in the small amplitude limit, and the solution is obtained using the tangent hyperbolic method. It is found that the system under consideration admits rarefactive shock structures. It is observed that the ion-neutral collision frequency, nonthermal electron population, inverse density inhomogeneity scalelength, and the ambient magnetic field affect the propagation characteristics of the drift shock waves. The present study may be applicable in regions of space where nonthermal electrons and heavy ions have been observed.

Dusty space plasma diagnosis using temporal behavior of polar mesospheric summer echoes during active modification

Abstract. The objective of this paper is to study the effect of different plasma and dust parameters on Polar Mesospheric Summer Echoes (PMSE) temporal behavior after turn-on and turn-off of radio wave heating and to use these responses to diagnose the properties of the dust layer. The threshold radar frequency and dust parameters for the enhancement or suppression of radar echoes after radio wave heating turn-on are investigated for measured mesospheric plasma parameters. The effect of parameters such as the electron temperature enhancement during heating, dust density, dust charge polarity, ion-neutral collision frequency, electron density and dust radius on the temporal evolution of electron irregularities associated with PMSE are investigated. The possible diagnostic information for various charged dust and background plasma quantities using the temporal behavior of backscattered radar power in active experiments is discussed. The computational results are used to make predictions for PMSE active modification experiments at 7.9, 56, 139, 224 and 930 MHz corresponding to existing radar facilities. Data from a 2009 VHF (224 MHz) experiment at EISCAT is compared with the computational model to obtain dust parameters in the PMSE.

Damping of Alfvén waves in solar partially ionized plasmas: effect of neutral helium in multi-fluid approach

Chromospheric and prominence plasmas contain neutral atoms, which may change the plasma dynamics through collision with ions. Most of the atoms are neutral hydrogen, but a significant amount of neutral helium may also be present in the plasma with a particular temperature. Damping of MHD waves due to ion collision with neutral hydrogen is well studied, but the effects of neutral helium are largely unknown. We aim to study the effect of neutral helium in the damping of Alfven waves in solar partially ionized plasmas. We consider three-fluid magnetohydrodynamic (MHD) approximation, where one component is electron-proton-singly ionized helium and other two components are the neutral hydrogen and neutral helium atoms. We derive the dispersion relation of linear Alfven waves in isothermal and homogeneous plasma. Then we solve the dispersion relation and derive the damping rates of Alfven waves for different plasma parameters. The presence of neutral helium significantly enhances the damping of Alfven waves compared to the damping due to neutral hydrogen at certain values of plasma temperature (10000-40000 K) and ionization. Damping rates have a peak near the ion-neutral collision frequency, but decrease for the higher part of wave spectrum. Collision of ions with neutral helium atoms can be of importance for the damping of Alfven waves in chromospheric spicules and in prominence-corona transition regions.

Evolutionary phases of equatorial spreadFincluding L band scintillations and plumes in the context of GPS total electron content variability: A case study

[1] The evolution of large-scale (few kilometers), medium-scale (few hundreds of meters), and small-scale (meters) size plasma density irregularities in the postsunset equatorial F region, in the context of characteristic GPS total electron content (GTEC) variations, are reported from Indian longitudes. The ionograms and GTEC from a GPS receiver installed as a part of the GPS Aided Geo Augmentation Network (GAGAN) project for satellite-based navigation are obtained from an equatorial station at Trivandrum (8.5°N, 76.91°E, dip latitude 0.5°N). The variations in the GTEC with respect to TEC are considered to represent the seed perturbations for the plasma instability that results in the equatorial spread F (ESF) irregularities and are treated as a perturbation factor (P). The VHF radar at Gadanki (13.5°N, 79.17°E, dip latitude 6.4°N) provided the small-scale structures of ESF. The background thermospheric conditions that affect the growth of the plasma instability through ion-neutral collision frequency (νin) are estimated using the F region base height (h′F)and the representative scale height of the neutral atmosphere and are represented by a growth factor (G). The present case study reveals a close coupling between the background ionospheric conditions and the baseline perturbations in deciding the evolutionary phases of ESF. It has been shown that although large-scale (kilometer scale) irregularities are formed without any constraints when the background ionospheric-thermospheric conditions are favorable in the presence of fluctuations in GTEC, consistently, the medium-scale and small-scale irregularities show remarkable similarity with the variations in the product of the perturbation and growth factors.

The effects of ion-neutral collision frequency on the plasma sheath dynamics for oblique entrance of ions into the sheath

Recently it has been shown that the magnetic field has significant effects on the characteristics of ions which enter the collisionless plasma sheath with an oblique incidence angle. Here, we have investigated these effects in collisional plasma sheath. Considering the ion-neutral collision, the basic equations of fluid model in a plasma sheath have been numerically solved in the presence of an external magnetic field where the ion collision frequency has a power law dependency to the ion flow velocity. The results show that the effects of magnetic field on the plasma sheath dynamics strongly depend on the ion-neutral collision frequency and its dependency to ion flow velocity.

Mesoscale observations of Joule heating near an auroral arc and ion-neutral collision frequency in the polar capEregion

[1] We report on the first mesoscale combined ionospheric and thermospheric observations, partly in the vicinity of an auroral arc, from Svalbard in the polar cap on 2 February 2010. The EISCAT Svalbard radar employed a novel scanning mode in order to obtain F and E region ion flows over an annular region centered on the radar. Simultaneously, a colocated Scanning Doppler Imager observed the E region neutral winds and temperatures around 110 km altitude using the 557.7 nm auroral optical emission. Combining the ion and neutral data permits the E region Joule heating to be estimated with an azimuthal spatial resolution of ∼64 km at a radius of ∼163 km from the radar. The spatial distribution of Joule heating shows significant mesoscale variation. The ion-neutral collision frequency is measured in the E region by combining all the data over the entire field of view with only weak aurora present. The estimated ion-neutral collision frequency at ∼113 km altitude is in good agreement with the MSIS atmospheric model.

Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach

We derive the dynamics of magnetohydrodynamic waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Two-fluid magnetohydrodynamic equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear magnetohydrodynamic waves are derived for simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. We found that two- and single-fluid descriptions give similar results for low frequency waves. However, the dynamics of MHD waves in two-fluid approach is significantly changed when the wave frequency becomes comparable or higher than ion-neutral collision frequency. Alfven and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 ion-neutral collision frequency for 50 % of neutral Hydrogen) in wave spectrum. The damping rates are reduced for higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales smaller than ion-neutral collision time. Therefore, two-fluid approximation should be used for the description of relatively fast processes. Some results of single-fluid description, for example the damping of high-frequency Alfven waves in the solar chromosphere due to ion-neutral collisions, should be revised in future.