Ion-neutral Collision Frequency Research Articles

Heating of ions by low-frequency Alfvén waves in partially ionized plasmas

In the solar atmosphere, the chromospheric and coronal plasmas are much hotter than the visible photosphere. The heating of the solar atmosphere, including the partially ionized chromosphere and corona, remains largely unknown. In this letter, we demonstrate that the ions can be substantially heated by Alfvén waves with very low frequencies in partially ionized low-beta plasmas. This differs from other Alfvén wave related heating mechanisms such as ion-neutral collisional damping of Alfvén waves and heating described by previous work on resonant Alfvén wave heating. We find that the nonresonant Alfvén wave heating is less efficient in partially ionized plasmas than when there are no ion-neutral collisions, and the heating efficiency depends on the ratio of the ion-neutral collision frequency to the ion gyrofrequency.

Variance of the vertical ion speed measured with the European Incoherent Scatter (EISCAT) UHF radar in the polar lower ionosphere at Tromsø, Norway

[1] The vertical component of the ion velocity measured with the European Incoherent Scatter (EISCAT) Tromso UHF radar (69.6°N, 19.2°E) in the lower ionosphere (from 95 to 130 km) was found to be characterized by notably large variances at oscillation periods of 2–8 min. Of particular interest was the geomagnetic activity dependence above ∼106 km, which showed larger variances (100–500 m2 s−2) during periods of geomagnetic disturbance than during quiet periods. Below 106 km, the variance was less sensitive to the geomagnetic activity. Height profiles of the variance during the disturbed period showed steep increases with heights above 106 km, then reaching a peak at 120 km, where the ion gyrofrequency is equal to the modeled ion-neutral collision frequency. A theoretical prediction well reproduced the height profile for the geomagnetically disturbed period by assuming oscillations in the meridional component of the electric field. The theoretical study suggested that the electric field oscillation is a possible cause of large variances of the vertical ion speed in the polar lower ionosphere. However, below 106 km, other mechanisms were necessary.

Selective formation of streamers in magnetized cylindrical plasmas

Turbulent plasmas form a variety of mesoscale structures, which affect the level of anomalous transport in fusion plasmas. A streamer is a poloidally localized, radially elongated vortex, which increases convective transport. We have carried out numerical simulations of resistive drift wave turbulence in a linear configuration, as a minimal model for analysing the structural formation mechanism in magnetized plasmas by mode coupling. A three-field reduced MHD model is extended to describe the resistive drift wave turbulence in cylindrical plasmas. A poloidally localized turbulent structure, which has the typical temporal scale of the streamer, is formed by the self-bunching of a couple of unstable modes in nonlinear saturation states. The formed structure depends on the magnitude of the ion–neutral collision frequency, which is the damping parameter of the zonal flow, and the streamer is formed when the damping of the zonal flow is stronger than that of the intermediate mode for the streamer formation. We found that the streamer is selectively formed to keep the particle balance as long as the drift wave is unstable.

The dynamics of ions entering the magnetized plasma sheath obliquely – collisional and collisionless situations

The characteristics of ions that enter the plasma sheath with an oblique incident angle have been investigated in the presence of an external magnetic field. The ion dynamics in a collisional and collisionless magnetized plasma sheath have been numerically calculated by using a fluid model. Several values for the ion velocity at the sheath edge, orientation and strength of the magnetic field and the ion-neutral collision frequency have been considered. The results show that in a collisionless magnetized plasma sheath, the behaviour of ions that obliquely enter the sheath with some specific velocities at the sheath edge and at some specific orientations and strengths of magnetic field, is more complicated than that of ions with normal entrance angles. For the oblique entrance of ions, the weak magnetic fields cause some fluctuations in ion velocity around its boundary value, i.e. the ion velocity does not accelerate. However, the numerical calculations show that the ion dynamics in a collisional magnetized plasma sheath are the same for both normal and inclined entrance of ions into the sheath.

Neutral-ion interaction in the auroralEregion obtained from coordinated Fabry-Perot imager and VHF radar observations

[1] It is important to investigate the small-scale dynamics of thermospheric neutrals and ionospheric plasmas at high latitudes since there are local and temporal variations in energy input and transfer processes associated with auroral activities. To clarify these ionosphere-thermosphere interactions in the auroral E region, we examined neutral winds and plasma motions obtained by coordinated Fabry-Perot imager and VHF radar observations at Syowa Station, Antarctica. From case studies on 11 and 12 September 1996, we found a good correlation between temporal variation (period < 1 hr) of E region neutral winds measured by the Fabry-Perot imager and the Doppler velocities of plasmas obtained from the VHF radar echoes. On the other hand, the absolute mean values of the neutral winds were around zero whereas the plasma Doppler velocities were 350–400 m/s, showing a significant discrepancy. Considering that the neutral-ion and ion-neutral momentum transfer collision frequencies in the E region are ∼10−6 Hz and ∼102 Hz, respectively, these good correlations are probably due to ion motions driven by neutral drag force, suggesting a strong coupling between neutrals and ions in the E region. The observed correlation and discrepancy between the radar Doppler velocity and the neutral wind velocity are consistent with the linear theory of E region irregularities that shows the radar Doppler velocity is the sum of two components; one is the E × B drift speed, and another is proportional to the neutral wind velocity.

Collisional effects in magnetized plasma sheath with two species of positive ions

The effects of ion–neutral collision on the characteristics of a magnetized plasma sheath which consists of two species of positive ions are investigated. It is assumed that the ions have different masses. In contrast to our previous work, the effects of ion–neutral collision frequency are added to the magnetized plasma sheath. Using a three-fluid hydrodynamic model and some dimensionless variables, the dimensionless equations are obtained and solved numerically. By taking into account the ion–neutral collision effects on a magnetized three-component plasma sheath, it is shown that apart from the presence of the second ion species, by increasing the collision frequency of two ions with neutrals, the amplitude of fluctuations of ion species density distributions increases and the position of these fluctuations is shifted towards the plasma sheath edge. Also, by increasing the ion–neutral collision frequency these fluctuations turn off faster than those in a collisionless case. It is shown that in the collisional magnetized plasma sheath, the effects of the presence of the heavier ion species on the lighter ion density turn off much faster in comparison with what happens in a collisionless magnetized plasma sheath. Furthermore, it is found that in a collisionless plasma sheath by increasing the density of the heavier ion species, the normalized electrostatic potential decreases while in a collisional plasma sheath the presence of the heavier ion species does not have any considerable effect on the normalized electrostatic potential. In addition, it is shown that when the distance of each ion species from the plasma sheath boundary becomes larger than five times the electron Debye length (x > 5λDe) the fluctuations of the ion species velocities disappear by increasing the ion–neutral collision frequency. Also, it is found that the electron density distribution decreases by increasing the ion–neutral collision frequency.

Selection rule for turbulent structural formation: study of magnetized cylindrical plasmas

The three-field reduced fluid model was extended to describe the resistive drift wave turbulence in magnetized cylindrical plasmas. There include multiple energy exchange paths, and two kinds of nonlinear steady states were obtained in simulations with a fixed particle source: one with a zonal flow generated by modulational coupling of various unstable modes, which suppress transport, and the other with a streamer, which is a azimuthally-localized vortex structure and induces convective transport, formed by parametric coupling. These structures are formed selectively, depending on the ion-neutral collision frequency, which is a damping force of a zonal flow and has a stabilizing effect of linear instability in this model. This is a minimal model for analyzing the turbulent structural formation mechanism by mode coupling in cylindrical plasmas, and competitive nature of structural formation was revealed.

The formation of sporadic E layers by a vortical perturbation excited in a horizontal wind shear flow

Abstract. The formation of the mid-latitude sporadic E layers (Es layers) by an atmospheric vortical perturbation excited in a horizontal shear flow (horizontal wind with a horizontal linear shear) is investigated. A three-dimensional atmospheric vortical perturbation (atmospheric shear waves), whose velocity vector is in the horizontal plane and has a vertical wavenumber kz≠0, can provide a vertical shear of the horizontal wind. The shear waves influence the vertical transport of heavy metallic ions and their convergence into thin and dense horizontal layers. The proposed mechanism takes into account the dynamical influence of the shear wave velocity in the horizontal wind on the vertical drift velocity of the ions. It also can explain the multi-layer structure of Es layers. The pattern of the multi-layer structure depends on the value of the shear-wave vertical wavelength, the ion-neutral collision frequency and the direction of the background horizontal wind. The modelling of formation of sporadic E layers with a single and a double peak is presented. Also, the importance of shear wave coupling with short-period atmospheric gravity waves (AGWs) on the variations of sporadic E layer ion density is examined and discussed.

Chromospheric magnetic reconnection: two-fluid simulations of coalescing current loops

Aims: To investigate magnetic reconnection rates during the coalescence of two current loops in the solar chromosphere, by altering the neutral-hydrogen to proton density ratio, ioniziation/recombination coefficients, collision frequency and relative helicity of the loops. Methods: 2.5D numerical simulations of the chromosphere were conducted using a newly developed two-fluid (ion-neutral) numerical code. Developed from the Artificial Wind scheme, the numerical code includes the effects of ion-neutral collisions, ionization/recombination, thermal/resistive diffusivity and collisional/resistive heating. Results: It was found that the rates of magnetic reconnection strongly depend on the neutral-hydrogen to proton density ratio; increasing the density ratio by a thousand-fold decreased the rate of magnetic reconnection by twenty-fold. This result implies that magnetic reconnection proceeds significantly faster in the upper chromosphere, where the density of ions (protons) and neutral-hydrogen is comparable, than in the lower chromosphere where the density of neutral-hydrogen is over a thousand times the ion density. The inclusion of ionization/recombination, an important physics effect in the chromosphere, increases the total reconnected magnetic flux, but does not alter the rate of magnetic reconnection. Reductions in the ion-neutral collision frequency, result in small increases to the rates of magnetic reconnection. The relative helicity of the two current loops was not observed to have any significant effect on the rates of magnetic reconnection. Conclusions: The magnetic reconnection rates of coalescing current loops are strongly affected by the inclusion of neutral-hydrogen particles.

Hall magnetohydrodynamics of partially ionized plasmas

The Hall effect arises in a plasma when electrons are able to drift with the magnetic field but ions cannot. In a fully ionized plasma this occurs for frequencies between the ion and electron cyclotron frequencies because of the larger ion inertia. Typically this frequency range lies well above the frequencies of interest (such as the dynamical frequency of the system under consideration) and can be ignored. In a weakly ionized medium, however, the Hall effect arises through a different mechanism – neutral collisions preferentially decouple ions from the magnetic field. This typically occurs at much lower frequencies and the Hall effect may play an important role in the dynamics of weakly ionized systems such as the Earth's ionosphere and protoplanetary discs. To clarify the relationship between these mechanisms we develop an approximate single-fluid description of a partially ionized plasma that becomes exact in the fully ionized and weakly ionized limits. Our treatment includes the effects of ohmic, ambipolar and Hall diffusion. We show that the Hall effect is relevant to the dynamics of a partially ionized medium when the dynamical frequency exceeds the ratio of ion to bulk mass density times the ion-cyclotron frequency, i.e. the Hall frequency. The corresponding length-scale is inversely proportional to the ion to bulk mass density ratio as well as to the ion-Hall beta parameter. In a weakly ionized medium, the critical frequency becomes small enough that Hall magnetohydrodynamics (MHD) is an accurate representation of the dynamics. More generally, ohmic and ambipolar diffusion may also be important. We show that both ambipolar and Hall diffusion depend upon the fractional ionization of the medium. However, unlike ambipolar diffusion, Hall diffusion may also be important in the high fractional ionization limit. The wave properties of a partially ionized medium are investigated in the ambipolar and Hall limits. We show that in the ambipolar regime wave damping is dependent on both fractional ionization and ion–neutral collision frequencies. In the Hall regime, since the frequency of a whistler wave is inversely proportional to the fractional ionization, and bounded by the ion–neutral collision frequency it will play an important role in the Earth's ionosphere, solar photosphere and astrophysical discs.

The combined effects of electrojet strength and the geomagnetic activity (&amp;lt;I&amp;gt;K&amp;lt;sub&amp;gt;p&amp;lt;/sub&amp;gt;&amp;lt;/I&amp;gt;-index) on the post sunset height rise of the F-layer and its role in the generation of ESF during high and low solar activity periods

Abstract. Several investigations have been carried out to identify the factors that are responsible for the day-to-day variability in the occurrence of equatorial spread-F (ESF). But the precise forecasting of ESF on a day-to-day basis is still far from reality. The nonlinear development and the sustenance of ESF/plasma bubbles is decided by the background ionospheric conditions, such as the base height of the F-layer (h'F), the electron density gradient (dN/dz), maximum ionization density (Nmax), geomagnetic activity and the neutral dynamics. There is increasing evidence in the literature during the recent past that shows a well developed Equatorial Ionization Anomaly (EIA) during the afternoon hours contributes significantly to the initiation of ESF during the post-sunset hours. Also, there exists a good correlation between the Equatorial Ionization Anomaly (EIA) and the Integrated Equatorial ElectroJet (IEEJ) strength, as the driving force for both is the same, namely, the zonal electric field at the equator. In this paper, we present a linear relationship that exists between the daytime integrated equatorial electrojet (IEEJ) strength and the maximum elevated height of the F-layer during post-sunset hours (denoted as peak h'F). An inverse relationship that exists between the 6-h average Kp-index prior to the local sunset and the peak h'F of the F-layer is also presented. A systematic study on the combined effects of the IEEJ and the average Kp-index on the post-sunset, peak height of the F-layer (peak h'F), which controls the development of ESF/plasma bubbles, is carried out using the ionosonde data from an equatorial station, Trivandrum (8.47° N, 76.91° E, dip.lat. 0.5° N), an off-equatorial station, SHAR (13.6° N, 79.8° E, dip.lat. 10.8° N) and VHF scintillations (244 MHz) observed over a nearby low-latitude station, Waltair (17.7° N, 83.3° E, dip.lat. 20° N). From this study, it has been found that the threshold base height of the F-layer at the equator for the development of plasma bubbles is reduced from 405 km to 317 km as the solar activity decreases from March 2001 (mean Rz=113.5) to March 2005 (mean Rz=24.5). This decrease in threshold height with the decreasing solar activity is explained on the basis of changes in the local linear growth rate of the collisional Rayleigh-Taylor instability, due to the variability of various terms such as inverse density gradient scale length (L−1), ion-neutral collision frequency (νin) and recombination rate (R) with the changes in the solar activity.

Reduction effect of neutral density on the excitation of turbulent drift waves in a linear magnetized plasma with flow

The importance of reducing the neutral density to reach strong drift wave turbulence is clarified from the results of the extended magnetohydrodynamics and Monte Carlo simulations in a linear magnetized plasma. An upper bound of the neutral density relating to the ion-neutral collision frequency for the excitation of drift wave instability is shown, and the necessary flow velocity to excite this instability is also estimated from the neutral distributions. Measurements of the Mach number and the electron density distributions using Mach probe in the large mirror device (LMD) of Kyushu University [S. Shinohara et al., Plasma Phys. Control. Fusion 37, 1015 (1995)] are reported as well. The obtained results show a controllability of the neutral density and provide the basis for neutral density reduction and a possibility to excite strong drift wave turbulence in the LMD.

Fluid modeling of the electron flow driven ion acoustic mode in a collisional plasma with magnetized electrons

A fluid analysis is presented of the ion sound mode in a weakly ionized collisional plasma. The ion-neutral collision frequency exceeds the ion gyrofrequency while the electrons remain magnetized. Under these conditions, an ion sound wave can propagate at arbitrary angles with respect to the direction of the magnetic field. In the presence of an electron flow along the magnetic lines the sound mode can grow. Due to the electron collisions the mode is unstable while ion collisions cause an angle dependent instability threshold which is such that the mode is most easily excited at very large angles. Hot ion effects are included in the study by means of an effective viscosity which effectively describes the ion Landau damping effect. In the presence of an additional light ion specie, the mode frequency and increment in a certain parameter range are increased.

First E- and D-region incoherent scatter spectra observed over Jicamarca

Abstract. We present here the first Jicamarca observations of incoherent scatter radar (ISR) spectra detected from E- and D-region altitudes. In the past such observations have not been possible at Jicamarca due a combined effect of strong equatorial electrojet (EEJ) clutter and hardware limitations in the receiving system. The observations presented here were made during weak EEJ conditions (i.e., almost zero zonal electric field) using an improved digital receiving system with a wide dynamic range and a high data throughput. The observed ISR spectra from E- and D-region altitudes are, as expected, narrow and get even narrower with decreasing altitude due to increasing ion-neutral collision frequencies. Therefore, it was possible to obtain accurate spectral measurements using a pulse-to-pulse data analysis. At lower altitudes in the D-region where signal correlation times are relatively long we used coherent integration to improve the signal-to-noise ratio of the collected data samples. The spectral estimates were fitted using a standard incoherent scatter (IS) spectral model between 87 and 120 km, and a Lorentzian function below 110 km. Our preliminary estimates of temperature and ion-neutral collisions frequencies above 87 km are in good agreement with the MSISE-90 model. Below 87 km, the measured spectral widths are larger than expected, causing an overestimation of the temperatures, most likely due to spectral distortions caused by atmospheric turbulence.

Unstable ion sound in plasmas with drifting electrons

The properties of a weakly ionized plasma with a high concentration of flowing neutrals are discussed as well as the excitation of ion sound oscillations propagating obliquely to the magnetic field lines. The model used in the present study includes collisions of plasma species with neutrals in the limit when electrons are magnetized but ions are not. The electron collision frequency is higher than the shifted wave frequency, which allows for the fluid description of the electrons. The ion-neutral collision frequency remains arbitrary and, therefore, the ion species is described by a collisional Boltzman kinetic equation. The electron drift in the perpendicular direction results in the instability of accidentally excited ion sound oscillations, which turn out to be highly unstable for practically all physically acceptable values of the electron drift. In addition, the presence of a population of hotter electrons is shown to reduce the perpendicular electron drift and to increase the instability threshold.

A self-consistent derivation of ion drag and Joule heating for atmospheric dynamics in the thermosphere

Abstract. The thermosphere is subject to additional electric and magnetic forces, not important in the middle and lower atmosphere, due to its partially ionized atmosphere. The effects of charged particles on the neutral atmospheric dynamics are often parameterized by ion drag in the momentum equations and Joule heating in the energy equation. Presented in this paper are a set of more accurate parameterizations for the ion drag and Joule heating for the neutral atmosphere that are functions of the difference between bulk ion velocity and neutral wind. The parameterized expressions also depend on the magnetic field, the Pedersen and Hall conductivities, and the ratio of the ion cyclotron frequency to the ion-neutral collision frequency. The formal relationship between the electromagnetic energy, atmospheric kinetic energy, and Joule heating is illustrated through the conversion terms between these three types of energy. It is shown that there will always be an accompanying conversion of kinetic energy into Joule heating when electromagnetic energy is generated through the dynamo mechanism of the atmospheric neutral wind. Likewise, electromagnetic energy cannot be fully converted into kinetic energy without producing Joule heating in the thermosphere.

Spectral analysis of plasma drift measurements from the AE-E satellite: evidence of an inertial subrange in equatorial spread F

Plasma drift data from the AE-E satellite are spectrally analyzed to investigate the characteristics of the flow in the topside equatorial F region ionosphere during strong spread F conditions. Plasma flow around rapidly rising depletions is thought to exhibit behavior similar to two-dimensional Kolmogorov turbulence, but only on flux tubes with sufficiently small integrated ion–neutral collision frequencies. We find that one-dimensional spectra computed from vertical plasma drift measurements made in such depletions on such flux tubes tend to display a −5/3 spectral index over scale sizes from about 1 to 100 km , suggesting the operation of an inverse energy cascade. This universal spectral form is evidence of an inertial regime of the underlying ionospheric interchange instability.

Effect of ion-neutral charge exchange on radial electric fields in the edge plasma

A fluid treatment of the perpendicular plasma conductivity is extended to include the effects of arbitrary ion-neutral collision frequency ν and large perpendicular drifts. It is shown that ion-neutral friction (charge-exchange) can give a nonmonotonic relationship between the nonambipolar radial current and the radial electric field, leading to a bifurcated plasma equilibrium. This bifurcation may influence the properties of the low-to-high-confinement (L-H) transition in the presence of neutrals. The physical origin of this effect is an interplay between the surface-averaged equilibrium drifts and the small poloidal variations in the pressure and parallel flow around the flux surface generated by the geodesic curvature.

Zonal neutral winds at equatorial and low latitudes

Fabry–Perot interferometric (FPI) measurements of thermospheric zonal neutral winds at Arequipa, Peru ( 16.7° S, 71.5° W, −2.7° dip ), and Carmen Alto, Chile ( 23.1° S, 69.4° W, −10.2° dip ), were collected during the solar minimum periods of September–October 1996 and 1997. The data set included 39 nights from Arequipa and 14 nights for Carmen Alto, with 8 nights of simultaneous observations. Analysis of averaged results found the peak evening zonal neutral wind speed of ∼127±15 m/ s eastward for the Arequipa observatory, which is located near the magnetic equator, to occur between 21:30 and 22:30 LT. In contrast, the peak evening zonal winds of ∼100±10 m/ s eastward observed from Carmen Alto, which is located near the crest of the equatorial ionization anomaly (EIA), occurred ∼0.5– 1 h later. These measurements represent the first case of groundbased FPI observations of the so called equatorial temperature and wind anomaly (ETWA) over such a small latitude range in the same longitude sector. This reduction in speed of ∼20–25% at Carmen Alto relative to Arequipa is attributed to increased ion drag at Carmen Alto caused by the higher electron density within the EIA region at altitudes of 220– 300 km . Model studies were conducted using electron density and neutral atmosphere parameters form the parameterized ionospheric model (PIM) and the mass spectrometer incoherent scatter (MSIS) models, respectively, to calculate the ratio of ion–neutral collision frequencies at the two sites. We found that the increase in electron density within the EIA was sufficient to account for the observed reduction in the zonal wind. Thus, this analysis confirms the dominant role of ion drag in modulating thermospheric dynamics at equatorial latitudes. A comparison of the FPI results with the predictions by two current neutral wind models, the Horizontal Wind Model-90 and the NCAR Thermospheric Ionosphere Electrodynamics General Circulation Model (TIEGCM), reveals that neither is able to reproduce accurately the latitude dependence reported here. Model refinements for electrodynamics and improved resolution are suggested.

Nonlinear wake potential in a dusty plasma

The linear response of a dust grain to a flowing plasma has been well studied, both through analytic theory as well as by plasma simulation. The principal feature of the plasma response is a quasi-periodic wavetrain downstream of the grain, whose characteristics have been investigated as a function of the plasma conditions and grain properties. In linear theory, the magnitude of the wake potential scales as the grain charge Q. For large enough charge, one expects that nonlinear behavior will occur. Using one- and two-dimensional simulations with particle ions flowing past a fixed charged grain, we show that for sufficiently large Q, the wake potential becomes essentially independent of the grain charge. The potential is large enough to reflect some of the incident ions, which become trapped in the vicinity of the grain. Compared to our previous calculations, we also assume a larger value for the ion-neutral collision frequency, and show that the resulting wake is significantly shorter in length and steadier in time.