Neutral Velocity Research Articles

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.

Measurement of the plasma and neutral gas flow velocities in a low-pressure hollow-cathode plasma jet sputtering system

We present results from two mutually independent measurements of plasma and neutral gas flow velocities in a plasma jet deposition system. Operation of a hollow-cathode discharge within the plasma jet nozzle in the pulse regime enables the simple use of a classical Langmuir probe for the plasma flow velocity measurement. In this method, we assume that the plasma is generated solely inside the nozzle during the power impulse and we measure the time of flight of the ions along a known distance between the nozzle end and the probe. The plasma velocity at the plasma jet axis is then determined by differentiation of the dependence of the distance covered by ions on time. As the second method the well-known Pitot tube is used for measurement of the neutral gas velocity. By comparison of both methods we have experimentally proved that the neutral gas flow velocity is almost unaffected by the presence of the plasma, i.e. it does not substantially depend on whether the discharge is switched on or off. The results of both methods correspond well; detected differences are qualitatively explained. It is documented that the plasma jet can be operated both in a subsonic and in a supersonic regime. We present the dependences of the plasma and neutral gas flow velocity on the distance from the cathode, on the pressure in the reactor chamber, and on the flow rate of the working gas.

Splitting over physical processes as applied to the construction of a numerical method for solving the system of kinetic equations governing a hall thruster rarefied plasma jet

A numerical method is constructed for solving the system of kinetic equations describing the behavior of a rarefied plasma jet exhausted from a Hall thruster. A similar problem was previously considered in the steady case. Now the same problem is solved in the unsteady formulation. The numerical method is based on a generalization of the splitting method with respect to physical processes, which is widely used in rarefied gas dynamics. The basic difficulty faced in the natural generalization of this method as applied to thruster jets is that a boundary condition has to be taken into account when the ion distribution function is determined. This difficulty is overcome by analytically selecting the corresponding term at the stage of free-molecular motion. Another difficulty that can be coped with by the splitting method is that the ion and neutral velocity spaces have widely different scales. Techniques for making the method conservative are described, and situations in which this is necessary are discussed. Qualitative characteristics of some numerical computations are compared with experimental data. The comparison shows that the numerical method constructed adequately reproduces the behavior of the modeled object.

Benchtop Energetics: Hyperthermal Species Detection

We propose a novel scheme for monitoring the transition between deflagration and “detonation-like” behavior of small-scale explosive samples-in-vacuum subjected to short duration shock stimuli. Our approach relies on measuring the chemical identities and velocity distributions of the gaseous species produced by such samples; e.g. the relatively low velocity expansion-quenched reaction intermediates produced by deflagration versus the hyperthermal thermodynamically stable molecules generated by the termination of a detonation wave at an explosive-vacuum interface. We demonstrate our ability to detect such species by time-of-flight mass spectrometry (TOFMS) using fast Al atoms produced by laser ablation of aluminum metal. Extensive SIMION simulations of ion trajectories in our mass spectrometer lead to a semi-quantitative model connecting the system operating parameters and the velocity-dependent neutral species detection efficiency. We present a method for correcting our data for these detection biases, and for transforming them into neutral species velocity and kinetic energy distributions. We also present preliminary TOFMS data of hyperthermal organic molecular species produced by direct laser ablation/ignition of thin-film nitrocellulose samples.

Comparison of nighttime zonal neutral winds and equatorial plasma bubble drift velocities over Brazil

We present results from the first extended period of coincident observations of thermospheric zonal neutral winds and equatorial plasma bubble (EPB) zonal drift velocities over northeastern Brazil during the October to December months of 2009 and 2010. The EPB zonal drift velocities are estimated utilizing images of the O I 630.0 nm emissions recorded by a wide‐angle imaging system at Cajazeiras. Thermospheric neutral wind estimates are based upon common volume observations made by a bistatic Fabry‐Perot interferometer (FPI) experiment using FPIs located at Cajazeiras and Cariri in Brazil observing the Doppler shift of the O I 630.0 nm emission. The results illustrate a similar pattern of nighttime and night‐to‐night variations in the zonal neutral winds and EPB zonal drift velocities. In general, the geomagnetic zonal neutral winds and the EPB velocities show an excellent agreement illustrating that the F region dynamo is fully developed. However, in the early evening hours the EPB zonal speed is slower than that of the background winds on several occasions. We conclude that this indicates that during the bubble evolution period in the early evening the F region dynamo is not fully activated.

Modeling of ionospheric magnetic field perturbations induced by earthquakes

The objective of this study is to present a simple coupled seismo‐ionospheric model to account for magnetic field perturbations in response to geophysical phenomena such as Earthquakes or tsunamis. For this purpose, we include a simple gravity wave model in the two dimensional mid latitude ionosphere model SAMI2. Following an earthquake, the disturbances in the neutral atmosphere density and velocity associated with gravity waves propagate vertically upward up to ionospheric altitudes 150–350 km, where significant coupling between the neutral atmosphere and the ionosphere occurs. As a result, a rapid variation in the ionospheric plasma density and temperature is observed associated with the photoionization of the perturbed neutral density. This variation in the electron density leads in turn to a variation in the total electron content (TEC). Strong variations in the TEC are observed close to the epicenter which correspond to large plasma density perturbations in this region associated with the ion acoustic mode. The calculation of magnetic perturbations is based on a set of reduced MHD equations. These are solved numerically using the finite element code TOPO coupled with the mid latitude ionospheric model SAMI2. The coupling between the two codes accounts for the variations in neutral gas density and the collisional drag force between ions and neutrals. Two wave modes are considered: the shear Alfvén mode and the compressional mode. The results of the magnetic field perturbations and of ions' velocity perturbations associated with the shear Alfvén and the compressional modes are presented. Variations in the TEC values are also computed for the compressional mode by taking into account the density perturbations transverse to the magnetic field. The results representing TEC perturbations associated with the compressional mode show strong variations at high latitudes both as a function of time and latitude.

Characterization of high-current electron beam interaction with metal targets

The process of electron beam interaction with metal targets was characterized using electrical and optical diagnostics. Electron beams with current density of 5–10 A/cm2, electron energy up to 120 keV, pulse duration up to 200 μs, and cross-sectional area of 8–30 cm2 at the target surface were generated by GESA I and GESA II facilities. Streak imaging of the target surface specular reflectivity was used to determine the onset of melting and re-solidification of the target surface. Using time- and space-resolved schlieren imaging, the evolution of surface irregularities was studied. Experimental and numerical investigations of the neutral flow evaporated from the target surface showed a neutral density of ∼1019 cm−3 in the vicinity of the target and neutral velocities up to 2 × 105 cm/s. Framing and streak images of visible light emission were used to study the temporal evolution of the target surface plasma and vapors. Time- and space-resolved spectroscopy was applied to determine the surface plasma density and temperature, which were found to be ∼1014 cm−3 and ≤1 eV, respectively. Because of this small plasma density, electric fields in the plasma sheath are not sufficient to cause electrohydrodynamic instability of the liquid target surface. However, hydrodynamic instabilities due to the intense neutral flow observed in experimental and numerical studies are likely to be responsible for the growth of wavelike irregularities.

Global modeling of hot O distribution in the upper thermosphere

The existence of hot oxygen (hot O, Oh) in the upper thermosphere is mainly confirmed by optical observations of high-altitude airglow. In the experiments described here, a peak of Oh population was found at an altitude of approximately 550 km with a temperature of about 4000 K. Although it was shown that Oh concentration could reach a value of 1–2% with respect to ambient (cold) O, a realistic global distribution of Oh concentration and temperature has not been established. The presence of non-thermal atoms in the thermosphere leads to variations in the thermo-dynamical regime in the upper atmosphere. The major chemical processes involved in Oh production were taken into account in the time-dependent, Global Self-consistent Model of Thermosphere, Ionosphere and Protonosphere (GSM TIP) of the Earth in order to simulate global distribution of Oh concentration and temperature (Th). Calculations were carried out in the geomagnetic coordinate system for moderate solar, quiet geomagnetic conditions, and winter season. It was shown that the maximum Oh is located at −60° latitude, 300° longitude, and 24 UT. The Th maximum is about 2050 K. This temperature and Oh concentration cause an increase in neutral gas temperature at high thermosphere by ∼100 K during daytime and by ∼70 K during nighttime. Variations in the neutral gas velocity circulation were calculated. The maximum increase in neutral velocity was about 36 m/s, corresponding to Φ = 50°, Λ = 180° in the northern and Φ = −50°, Λ = 270° in the southern hemisphere.

Effect of subauroral polarization streams on the thermosphere: A statistical study

Using 2 years of coordinated CHAMP and DMSP observations we have investigated for the first time the relationship between subauroral polarization streams (SAPS), ionospheric Hall current (electrojet), upper thermospheric zonal wind, and mass density at subauroral regions in the dusk and premidnight sectors, separately for both hemispheres. For comparison, we have also analyzed the same parameters as a function of magnetic latitude (30 degrees-80 degrees magnetic latitude) during non-SAPS periods. During periods of non-SAPS, the neutral wind exhibits similar features as during SAPS events in the dusk to premidnight sector, streaming westward in the same direction as the plasma drift. Both neutral and plasma velocities peak at the same latitude regardless of SAPS occurrence. For higher geomagnetic activity both velocities are faster and the peaks shift equatorward. During non-SAPS periods, the ratio between plasma and neutral wind velocity is on average 2.75 +/- 0.4 in both hemispheres irrespective of geomagnetic activity. The neutral wind during SAPS events gets enhanced by a factor of 1.5/1.2 for Kp = 4 in the Northern/Southern Hemisphere, respectively, as compared to non-SAPS time. The velocity difference between SAPS and neutral wind is also larger during SAPS period than during non-SAPS period, and the difference tends to increase with increasing geomagnetic activity. The peak latitude of the eastward auroral electrojet appears 1.5 degrees poleward of the plasma drift during SAPS events, confirming the formation of SAPS equatorward of the high-conductivity channel. These SAPS-induced large winds can heat the upper thermosphere. As a result we observe a 10% enhanced mass density at 400 km altitude with respect to periods without SAPS. In addition a density anomaly peak occurs collocated with the SAPS, displaced from the electrojet peak. We regard this as an indication for efficient thermospheric heating by ion neutral friction.

A cold component and the complex velocity structure of DLA1331 + 170

[ABRIDGED] We examine the velocity structure in the gas associated with \ion{H}{1} in the damped Ly$\alpha$ absorption system at redshift $z=1.7764$ towards the QSO $1331+170$ using 21cm data, optical and STIS spectra. We find at least two, and possibly three, components showing \ion{C}{1} lines. One of these has Doppler parameter $b=0.55${\kms}, corresponding to a kinetic temperature of 220K if the broadening is thermal. We re-examine the H$_2$ analysis undertaken by \citet{Cui05} using the neutral carbon velocity structure, and find a model which is, consistent with a mixture of collisional and background radiation excitation of the observed H$_2$ rotational levels. For singly ionized heavy elements we find eight components covering a velocity range of $\sim 110$ {\kms}. The \ion{H}{1} structure is expected to follow some combination of the singly ionized and neutral gas, but the 21cm absorption profile is considerably different. This may be because of the different extent and brightness distributions of the radio and optical background sources, and so the spin temperature derived by comparing the Ly$\alpha$ and 21cm line strengths has little physical meaning. The neutral and singly ionized heavy element line profiles also show significant differences, and so the dominant components in each appear to be physically distinct. Attempts to use the range of atomic masses to separate thermal and turbulent components of their Doppler widths were not generally successful. The velocity structure in all ionization stages up to $+3$, apart from the neutral heavy elements, is sufficiently complex that it is difficult to separate out the corresponding velocity components for different ionization levels and determine their column densities.

Neutral gas free molecular flow algorithm including ionization and walls for use in plasma simulations

Several plasma devices, including Hall and ion thrusters, operate by ionizing a low density neutral gas for which the mean free path between collisions of gas molecules is greater than typical device dimensions. In general, the discrete-particle algorithms used to calculate the neutral gas ignore velocity changes due to collisions between gas molecules. However, particle algorithms are a source of unphysical statistical noise that may detract from the study of the plasma physics, the prime purpose of most simulations. In this paper we present a new neutral gas algorithm for use in plasma simulation codes that exploits the fact that very few collisions change the velocity of neutral gas molecules. The algorithm assumes that the particle velocity distribution function for neutrals emitted from a given surface remains unchanged except for a scale factor that reflects the loss of neutrals to ionization. The sources of neutrals may be gas inlets, and isotropic, thermally accommodated, gas molecules coming off chamber surfaces including recombined ions. The algorithm is implemented in two dimensions (R–Z) with emitting surfaces represented as surfaces of revolution. The advantage of this algorithm over the conventional particle approach is the absence of statistical noise.

Revisiting the Definition of the Drag Coefficient in the Marine Atmospheric Boundary Layer

Abstract A new functional form of the neutral drag coefficient for moderate to high wind speeds in the marine atmospheric boundary layer for a range of field measurements as reported in the literature is proposed. This new form is found to describe a wide variety of measurements recorded in the open ocean, coast, fetch-limited seas, and lakes, with almost one and the same set of parameters. This is the result of a reanalysis of the definition of the drag coefficient in the marine boundary layer, which finds that a constant is missing from the traditional definition of the drag coefficient. The constant arises because the neutral friction velocity over water surfaces is not directly proportional to the 10-m wind speed, a consequence of the transition to rough flow at low wind speeds. Within the rough flow regime, the neutral friction velocity is linearly dependent on the 10-m wind speed; consequently, within this rough regime, the new definition of the drag coefficient is not a function of the wind speed. The magnitude of the new definition of the neutral drag coefficient represents an upper limit to the magnitude of the traditional definition.

TRACING TURBULENT AMBIPOLAR DIFFUSION IN MOLECULAR CLOUDS

Though flux freezing is a good approximation frequently assumed for molecular clouds, ambipolar diffusion (AD) is inevitable at certain scales. The scale at which AD sets in can be a crucial parameter for turbulence and the star formation process. However, both observation and simulation of AD are very challenging and our knowledge of it is very limited. We proposed earlier (Li and Houde 2008) that the difference between ion and neutral velocity spectra is a signature of turbulent AD and can be used to estimate the AD scales and magnetic field strength. Here we present observational evidence showing that this difference between the velocity dispersions from coexistent ions and neutrals is indeed correlated with magnetic field strength.

On the effect of oxygen flooding on the detection of noble gas ions in a SIMS instrument

We have investigated the report by Desgranges and Pasquet (2004) [1] that O2 gas flooding in a secondary ion mass spectrometer could enhance by a factor of about ∼5 the Xe+ ion yield for a Xe implant in UO2 sputtered by O2+ primary ions. For a Xe implant in Si sputtered by O2+ primary ions and for Xe+ sputtering of silicon in steady state, O2 gas flooding reduced the Xe+ ion signal by a factor of about 2, presumably due to loss of Xe+ by resonant charge exchange with gas-phase oxygen molecules. The yield of a Kr co-implant in Si was unaffected by oxygen flooding. However, we demonstrate that for steady-state Ar+ sputtering of uranium, the Ar+ ion yield can be increased by a factor of ∼1.7 by oxygen flooding. This enhancement, predicted by a simple model to be a factor of √3, is attributed to a lowering of the neutral Ar emission velocity and hence increased post-ionization efficiency, caused by the suppression of argon bubble formation in uranium by oxidation of the near-surface region, and re-emission of argon by thermal effusion rather than in supersonic expansions from bursting high-pressure gas bubbles. We show that bubble formation, evidenced by the observation of Xe2+ clusters, also occurs for Xe+-bombarded uranium when the surface is sufficiently sputter-cleaned, and that the bubbles are similarly suppressed by oxygen flooding producing a factor of ∼√3 enhancement in the Xe+ signal.

Measurement of neutral flow velocity in an ECR plasma using tunable diode laser LIF spectroscopy combined with saturated absorption spectroscopy

The radial flow of neutral particles in an electron-cyclotron-resonance (ECR) argon plasma has been measured by using a newly developed high resolution laser induced fluorescence (LIF) measurement system. The flow velocity is determined by the Doppler shift of the LIF spectrum. A very high accuracy calibration of an excitation laser has been achieved by installing a saturated absorption spectroscopy unit into the LIF system. We utilized the Lamb dip, which is obtained by the saturated absorption spectroscopy, as a frequency standard for the determination of the flow velocity. The use of Lamb dip is particularly appropriate for the flow velocity measurement, since the position of Lamb dip in the frequency scale is Doppler-shift free and is not disturbed by the motion of reference medium. By utilizing the Lamb dip as the frequency standard, the reliability and stability of the laser frequency calibration is increased. From the radial measurements of LIF spectra of metastable argon atoms at the microwave power of 250 W and 5 kW, it is found that there exists an inward flow of neutral particles in the both plasmas. Both of the radial flow velocity profiles peak around 4 cm from the center, which is comparable with the radius of the boundary of the E×B rotation and anti-E×B rotation in the 5 kW discharge. The maximum flow velocity increases with the microwave power.

Summer high‐latitude mesospheric observations of supersonic bursts and O(1S) emission rate with the UARS WINDII instrument and the association with sprites, meteors, and lightning

Supersonic neutral velocities, called supersonic bursts, are observed in localized regions lasting for over 8 s of enhanced O(1S) emission rate for tangent altitudes of 73–80 km during summer daytime by the Wind Imaging Interferometer (WINDII) on board the UARS satellite; the occurrence rate is 40% (≥500 m s−1) for latitudes above 65°N. These bursts are proposed to be produced by accelerated electrons and ions in a strong electric field. Theory suggests that an electric field of 45 V m−1 is required to generate a typical supersonic velocity of 750 m s−1 forced by ion‐neutral collisions at 82 km, the altitude at which polar mesospheric clouds occur. Coincidences in space and time between supersonic velocity profiles and space‐based lightning detection observed at a wavelength of 777.4 nm were found in a limited number of cases in high‐latitude summer, a region for which the lightning detection has the highest occurrence frequency (2–3.5%) and optical energy (1 J m−2 sr−1μm−1) as a function of month and latitude. The electric field must have sufficient energy to generate atomic emission features, more than for sprites and comparable with tropospheric lightning. Support for the existence of supersonic bursts at high latitude is provided through the observation of “chirps” in infrasound emissions which frequently occur in the absence of tropospheric lightning.

Direct evidence for two-fluid effects in molecular clouds

We present a combination of theoretical and simulation-based examinations of the role of two-fluid ambipolar drift on molecular line widths. The dissipation provided by ion-neutral interactions can produce a significant difference between the widths of neutral molecules and the widths of ionic species, comparable to the sound speed. We demonstrate that Alfven waves and certain families of magnetosonic waves become strongly damped on scales comparable to the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show that the wave damping causes the power spectrum of the ion velocity to drop below that of the neutral velocity when measured on a relative basis. Following a set of motivational observations by Li & Houde (2008), we produce synthetic line width-size relations that shows a difference between the ion and neutral line widths, illustrating that two-fluid effects can have an observationally detectable role in modifying the MHD turbulence in the clouds.

DAMPING OF MAGNETOHYDRODYNAMIC TURBULENCE IN PARTIALLY IONIZED GAS AND THE OBSERVED DIFFERENCE OF VELOCITIES OF NEUTRALS AND IONS

Theoretical and observational studies on the turbulence of the interstellar medium developed fast in the past decades. The theory of supersonic magnetized turbulence, as well as the understanding of projection effects of observed quantities, are still in progress. In this work we explore the characterization of the turbulent cascade and its damping from observational spectral line profiles. We address the difference of ion and neutral velocities by clarifying the nature of the turbulence damping in the partially ionized. We provide theoretical arguments in favor of the explanation of the larger Doppler broadening of lines arising from neutral species compared to ions as arising from the turbulence damping of ions at larger scales. Also, we compute a number of MHD numerical simulations for different turbulent regimes and explicit turbulent damping, and compare both the 3-dimensional distributions of velocity and the synthetic line profile distributions. From the numerical simulations, we place constraints on the precision with which one can measure the 3D dispersion depending on the turbulence sonic Mach number. We show that no universal correspondence between the 3D velocity dispersions measured in the turbulent volume and minima of the 2D velocity dispersions available through observations exist. For instance, for subsonic turbulence the correspondence is poor at scales much smaller than the turbulence injection scale, while for supersonic turbulence the correspondence is poor for the scales comparable with the injection scale. We provide a physical explanation of the existence of such a 2D-3D correspondence and discuss the uncertainties in evaluating the damping scale of ions that can be obtained from observations. However, we show that the statistics of velocity dispersion from observed line profiles can provide the spectral index and the energy transfer rate of turbulence. Also, comparing two similar simulations with different viscous coefficients it was possible to constrain the turbulent cut-off scale. This may especially prove useful since it is believed that ambipolar diffusion may be one of the dominant dissipative mechanism in star-forming regions. In this case, the determination of the ambipolar diffusion scale may be used as a complementary method for the determination of magnetic field intensity in collapsing cores. We discuss the implications of our findings in terms of a new approach to magnetic field measurement proposed by Li & Houde (2008).

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization

Molecular dynamics simulations of matrix-assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix.

DAMPED LYα ABSORBER KINEMATICS AND OUTFLOWS FROM STARBURST GALAXIES

We present results from a numerical study of the multiphase interstellar medium in sub-Lyman-break galaxy protogalactic clumps. Such clumps are abundant at z=3 and are thought to be a major contributor to damped Ly-alpha absorption. We model the formation of winds from these clumps and show that during star formation episodes they feature outflows with neutral gas velocity widths up to several hundred km/s. Such outflows are consistent with the observed high-velocity dispersion in DLAs. In our models thermal energy feedback from winds and supernovae results in efficient outflows only when cold (~ 300 K), dense (> 100 msun/pc^3) clouds are resolved at grid resolution of 12 pc. At lower 24 pc resolution the first signs of the multiphase medium are spotted; however, at this low resolution thermal injection of feedback energy cannot yet create hot expanding bubbles around star-forming regions -- instead feedback tends to erase high-density peaks and suppress star formation. At 12 pc resolution feedback compresses cold clouds, often without disrupting the ongoing star formation; at the same time a larger fraction of feedback energy is channeled into low-density bubbles and winds. These winds often entrain compact neutral clumps which produce multi-component metal absorption lines.