Critical Ionization Velocity Effect Research Articles

Gaussian Decomposition of λ21 cm H i profiles, the Critical Ionization Velocity, and the Interstellar Helium Abundance

Following an established protocol of science—that results must be reproducible—we examine the Gaussian fits to Galactic λ21 cm (H i) emission profiles obtained by two seemingly complementary methods using data from the Leiden–Argentine–Bonn all-sky survey. One is based on the method used by Verschuur, the other by Nidever et al. (2008). The comparisons led to the identification of four problems that might arise when an algorithm is applied to huge databases without close monitoring: (1) different methods of calculating measuring the goodness of fit; (2) an ultra-broad component found to imperfectly bridge the gap between low- and intermediate-velocity gas; (3) the lack of an imposed spatial coherence allowing different components to appear and disappear in profiles separated by a fraction of a beamwidth; and (4) multiple, fundamentally different solutions for profiles at both the north and south Galactic poles. Confirming evidence emerges from this study of an underlying component with a line width of an order 34 km s−1. If this feature is the result of the critical ionization velocity effect acting on interstellar helium, it can be used to calculate its interstellar abundance. Analysis of H i profiles in an area in the southern Galactic hemisphere using multitelescope data gives a helium abundance of 0.094 ± 0.035, in excellent agreement with the accepted cosmic abundance of 0.085.

On Interstellar Neutral Hydrogen Associated With an Hα Filament, the Critical Ionization Velocity Effect and the Origin of Filaments

On Interstellar Neutral Hydrogen Associated With an Hα Filament, the Critical Ionization Velocity Effect and the Origin of Filaments

The Role of the Critical Ionization Velocity Effect in Interstellar Space and the Derived Abundance of Helium

Gaussian analysis of new, high-angular-resolution interstellar 21-cm neutral hydrogen emission profile structure more clearly reveals the presence of the previously reported signature of the critical ionization velocity (CIV) of helium (34 km s−1). The present analysis includes 1496 component line widths for 178 neutral hydrogen profiles in two areas of sky at galactic latitudes around −50°, well away from the galactic plane. The new data considered here allow the interstellar abundance of helium to be calculated, and the derived value of 0.095 ± 0.020 agrees extremely well with the value of 0.085 for the cosmic abundance based on solar data. Although the precise mechanisms that give rise to the CIV effect in interstellar space are not yet understood, our results may provide additional motivation for further theoretical study of how the mechanism operates.

Experiments With Plasmas Artificially Injected Into Near-Earth Space

Plasma injection experiments in space are being ordered according to five aspects: (1) Diagnostics of electric fields, (2) Coupling to the ionosphere, (3) Interactions with the solar wind, (4) Modification experiments, and (5) Special physical processes. Historically first were releases of neutral gases with the aim to measure atmospheric parameters. They were soon followed by plasma injections applied to the measurement of plasma flows and parallel electric fields. Long-range coupling to the environment was a most important aspect of the plasma releases. It concerned, on the one hand, the need for corrections of the derived diagnostic parameters and, on the other hand, the understanding of the formation of the ubiquitous striations and deformations of the plasma clouds. A special application was the investigation of cometary interactions by releases in the solar wind. Modification experiments in the ionosphere were done intentionally or occurred as byproducts of rocket launches or other activities. A particular goal was to trigger natural large-scale ionospheric instabilities like equatorial spread F in order to improve the understanding of the natural phenomena. Large-scale plasma injections in the magnetosphere have been performed in order to change the conditions of wave-particle interactions and potentially trigger observable effects. Special goals were so-called skidding experiments and testing Alfven’s critical ionization velocity effect. In this review we will emphasize the principle objectives and illustrate the results from selected experiments.

A PERVASIVE BROAD COMPONENT IN H I EMISSION LINE PROFILES: TEMPERATURE, TURBULENCE, OR A HELIUM SIGNATURE?

Gaussian analysis of interstellar neutral hydrogen emission profiles has revealed a pervasive broad component with a width on the order of 34 km s–1. When present, this component can most readily be identified in high galactic latitude directions where the H I profiles are either intrinsically weak or simple. Examination of published data reveals that this characteristic line width has been found in a variety of other H I features including compact high-velocity clouds, very-high-velocity clouds, and the Magellanic Stream. When its presence is accounted for in the analysis of H I profiles, other families of line widths at 14 and 6 km s–1 are clearly revealed. Possible mechanisms for producing this broad background component are discussed, including temperature, turbulence, and the critical ionization velocity effect. A line width on the order of 34 km s–1 would imply a kinetic temperature of 24,000 K, too high to keep the gas neutral; hence it should not be observed in H I emission spectra. Turbulent motions could explain a pervasive broad component, but not why it always has the same numerical value in various classes of H I emission line features. The critical ionization velocity effect hypothesis is intriguing because 34 km s–1 is the value for helium. Clearly, this could be a coincidence but the other prominent distribution peaks correspond to two families of critical ionization velocities of abundant interstellar elements including C, N, and O (about 14 km s–1) and metals (about 6 km s–1). Unfortunately, the mechanism by which this effect operates, even in laboratory situations, is not clearly understood. It is suggested that further investigation of the distribution of H I component line widths by allowing for the existence of a pervasive broad underlying component may cast a clearer light on this intriguing phenomenon.

The evolution of electron overdensities in magnetic fields

When a neutral gas impinges on a stationary magnetized plasma an enhancement in the ionization rate occurs when the neutrals exceed a threshold velocity. This is commonly known as the critical ionization velocity effect. This process has two distinct timescales: an ion–neutral collision time and electron acceleration time. We investigate the energization of an ensemble of electrons by their self-electric field in an applied magnetic field. The evolution of the electrons is simulated under different magnetic field and density conditions. It is found that electrons can be accelerated to speeds capable of electron impact ionization for certain conditions. In the magnetically dominated case the energy distribution of the excited electrons shows that typically 1% of the electron population can exceed the initial electrostatic potential associated with the unbalanced ensemble of electrons.

Skeletal Structures in the Images of Cosmic Dust Clouds and Solar System Planets

Multilevel dynamical contrasting of cosmic dust cloud images reveals the presence of skeletallike structures that are similar to those found in various electrical discharges and in space plasmas. These results, which are concentric cylinders in interstellar space, corroborate the discovery of interstellar neutral hydrogen (HI) emission spectra that are recorded in radio astronomy from low- and high-velocity intergalactic clouds in which the critical ionization velocity effect plays a role in the formation of coaxial or twisted plasma filaments. The nanodust assembled ldquoskeletonsrdquo are similar to that observed by Voyager 1 images of solar system planets that show the filaments aligned perpendicular to the ecliptic plane.

Observations and modeling of neutral gas releases from the APEX satellite

Results from experiments of neutral xenon gas releases in the ionosphere are presented. The releases were made from the scientific satellite APEX, which had various plasma and field diagnostics. An enhancement of HF wave activity over a broad band (0.1–10 MHz) is shown to occur during releases made in the sunlight at high pitch angles to the magnetic field. No changes of plasma density were detected, but electron temperature measurements indicate electron energy enhancements during the releases. Theoretical calculations based on the conditions of the APEX releases show that charge‐exchange collisions and elastic scattering of the injected Xe neutrals by the ambient plasma can lead to an ion beam instability that may account for the observed wave and electron energy enhancements. The interaction is akin to the critical ionization velocity effect. We calculate that because of the low injection density, this turbulence‐fueled ionization should have a low yield (<1%), which may explain the lack of detectable plasma density enhancement.

Momentum coupling in the “CRIT II” critical ionization velocity experiment

A unified theory has been developed to explain the formation of a quasi‐dc electric pulse (ω ≤ Ωi) induced by an ionizing neutral barium beam across an ionospheric plasma. We obtained a generalized form for the dc electric field in the plane perpendicular to the magnetic field within a simplified slab barium cloud moving perpendicular to the geomagnetic fields. A current system associated with the quasi‐dc electric field was also proposed to provide a way to transfer the momentum between the streaming barium cloud and the ambient plasma. The characteristic time derived by using this model for momentum coupling between the streaming barium cloud and the ambient plasma is found to be in agreement with the previous result. The quasi‐dc electric field predicted by this model is reasonably consistent with the “CRIT II” critical ionization velocity measurements. On the basis of the constrains of the conservation of energy and momentum, we found that Alfvén's critical ionization velocity (CIV) effect is a self‐limiting ionization process in a finite extent neutral cloud. It may be the reason why the CIV effect took place in CRIT II but lasted only for a very short period, and it may have resulted in low ionization yields in most of space CIV experiments.

CRIT II electric and magnetic observations inside and outside an ionizing neutral jet

The full electric and magnetic field data set from the subpayload of the CRIT II sounding rocket experiment is presented for the first time. CRIT II was an ionospheric injection experiment aimed at studying the critical ionization velocity (CIV) effect. It consisted of two payloads located on nearly the same magnetic field line. By using the data from both payloads, we are able to reach a good understanding of the momentum transfer between the injected ions and the ambient ionosphere. The data also make it possible to resolve the conflict between the two competing models for the energy transfer from the newly created ions to hot electrons in the CIV process. The results give a natural coupling between the energy and momentum transfer processes in CIV experiments.

On Ba+ production in the CRIT II Experiment

Analysis of particle data from the CRIT II experiment, studying Alfvén's critical ionization velocity (CIV) effect, shows that the density of newly created ions (presumably Ba+ from the shaped‐charge beam) is consistent with the increase in total plasma density measured by the independent RF plasma probe on board (Swenson et al., 1990) at the most active time period. We model this ion production using the measured electron flux data and the neutral barium model of Stenbaek‐Nielsen et al. (1990a). To identify the main source mechanisms which may contribute most to the barium ionization, a simple model for barium ion density at the payload location is developed based on Liouville's theorem. We estimate that the electron impact ionization is responsible for 90% of the barium ion production observed by CRIT II in the first release and up to 45% in the second release. By employing a “two‐state approximation” calculation (Rapp and Francis, 1962), the Ba‐O+ charge exchange cross section is found to range from about 2.0 × 10−17 cm2 at a velocity of 4 km/s to 2.0 × 10−15 cm2 at a velocity of 20 km/s. This result suggests that the Ba‐O+ charge exchange is probably dominant among all the non‐CIV ionization processes. By considering the charge exchange process in our density model, the barium ion densities are calculated for the two releases on CRIT II. The comparison between the model results and the observed data is found to be reasonably consistent if the cross sections, as calculated above, are multiplied by 0.3 for the first release and 1.0 for the second release. Our result suggests that the charge exchange process could be the most important non‐CIV ionization mechanism in the CRIT II experiment and it should be considered carefully case by case in CIV experiments.

Particle‐in‐cell simulations of the critical ionization velocity effect in finite size clouds

The critical ionization velocity (CIV) mechanism in a finite size cloud is studied with a series of electrostatic particle‐in‐cell simulations. It is observed that an initial seed ionization, produced by non‐CIV mechanisms, generates a cross‐field ion beam which excites a modified beam‐plasma instability (MBPI) with frequency in the range of the lower hybrid frequency. The excited waves accelerate electrons along the magnetic field up to the ion drift energy that exceeds the ionization energy of the neutral atoms. The heated electrons in turn enhance the ion beam by electron‐neutral impact ionization, which establishes a positive feedback loop in maintaining the CIV process. It is also found that the efficiency of the CIV mechanism depends on the finite size of the gas cloud in the following ways: (1) Along the ambient magnetic field the finite size of the cloud, L∥, restricts the growth of the fastest growing mode, with a wavelength λm∥ of the MBPI. The parallel electron heating at wave saturation scales approximately as (L∥/λm∥)1/2. (2) Momentum coupling between the cloud and the ambient plasma via the Alfvén waves occurs as a result of the finite size of the cloud in the direction perpendicular to both the ambient magnetic field and the neutral drift. This reduces exponentially with time the relative drift between the ambient plasma and the neutrals. The timescale is inversely proportional to the Alfvén velocity. (3) The transverse charge separation field across the cloud was found to result in the modulation of the beam velocity which reduces the parallel heating of electrons and increases the transverse acceleration of electrons. (4) Some energetic electrons are lost from the cloud along the magnetic field at a rate characterized by the acoustic velocity, instead of the electron thermal velocity. The loss of energetic electrons from the cloud seems to be larger in the direction of plasma drift relative to the neutrals, where the loss rate is characterized by the neutral drift velocity. It is also shown that a factor of 4 increase in the ambient plasma density, increases the CIV ionization yield by almost 2 orders of magnitude at the end of a typical run. It is concluded that a larger ambient plasma density can result in a larger CIV yield because of (1) larger seed ion production by non‐CIV mechanisms, (2) smaller Alfvén velocity and hence weak momentum coupling, and (3) smaller ratio of the ion beam density to the ambient ion density, and therefore a weaker modulation of the beam velocity. The simulation results are used to interpret various chemical release experiments in space.

Momentum coupling in ionospheric critical ionization velocity experiments*

The critical ionization velocity (CIV) effect is a process that can rapidly ionize a neutral gas which moves through a magnetized plasma. The process has been studied for several decades in laboratory experiments, but presently the emphasis has moved to ionospheric injection experiments. In these experiments, the neutral gas component is released at high velocity, with respect to the ionosphere, from a rocket or a satellite. Efficient momentum coupling between the injected cloud and the ambient ionosphere is achieved by means of Alfvén waves that are launched along the magnetic field. A computer model is presented for the momentum exchange between a cloud of injected ions and the ionosphere, and the model electric fields and particle spectra are shown to agree in detail with measurements from the Critical Ionization Test II (CRIT II); [Swenson et al., Geophys. Res. Lett. 17, 2337 (1990)] ionospheric injection experiment.

Prompt ionization in the CRIT II barium releases

Observations of electron and ion distributions inside a fast neutral barium jet in the ionosphere show significant fluxes within 4 km of release, presumably related to beam plasma instability processes involved in the Critical Ionisation Velocity (CIV) effect. Electron fluxes exceeding 5 × 1012/cm2‐str‐sec‐keV were responsible for ionizing both the streaming barium and ambient oxygen. Resulting ion fluxes seem to be consistent with 1–2% ionization of the fast barium, as reported by optical observations, although the extended spatial distribution of the optically observed ions is difficult to reconcile with the in‐situ observations. When the perpendicular velocity of the neutrals falls below critical values, these processes shut off. Although these observations resemble the earlier Porcupine experimental results [Haerendel, 1982], theoretical understanding of the differences between these data and that of earlier negative experiments is still lacking.

Review of the CIV phenomenon

Alfven's Critical Ionization Velocity (CIV) phenomenon is reviewed, with the main emphasis on comparisons between experimental and theoretical results. The review covers (1) the velocity measurements in laboratory experiments, (2) the effect of wall interaction, (3) the experimental and theoretical limits to the magnetic field strength and the neutral density, (4) ionospheric release experiments, (5) theoretical models for electron energization in comparison to experimental results, and (6) CIV models. All laboratory investigations of the CIV are found to obey the three following simple rules of thumb: (1) if the magnetic field is so strong that V A > 3V 0, and if there is enough neutral gas that the Townsend condition is fulfilled, then the CIV effect occurs, (2) when it occurs, the threshold velocity (or E/B value) is within ± 50% of Alfven's proposed value V c , and (3) for weaker magnetic fields, the effect gradually becomes irreproducible or weak and disappears altogether for V A < V 0. The theoretical understanding of the process has grown rapidly during the last decade, mainly due to the introduction of computer simulation models which have to a large degree confirmed and extended earlier analytical theories. The CIV mechanism is not due to one single plasma process, but to several different mechanisms which are applicable in different parameter regimes and geometries. The computer simulations have shown that in order to understand the mechanism properly it is necessary to consider a large number of interlocking collisional and plasma processes. The theoretical development has reached the stage where it should be possible to adapt computer simulation models to specific experiments and predict ionization rates, plasma flow velocities, E/B values, particle distributions, and wave spectra. Such models should for the first time provide a really firm basis for extrapolations of the CIV process to space applications.

A comparison between laboratory and space experiments on Alfven's CIV effect

Laboratory experiments on Alfven's critical ionization velocity (CIV) effect have established the reality of the CIV effect and determined the parameter limits within which the effect can be expected in the laboratory for different combinations of plasma and neutral gas species: velocity, magnetic field strength, and neutral gas and plasma density. However, in the laboratory experiments there are always walls and usually electrodes present, and the available parameter regime is such that the details inside the CIV process are difficult to study. Such studies can better be made in ionospheric release experiments, where waves and particle spectra can be directly measured inside the interaction region. Injection experiments are also better suited to study the momentum exchange process between the CIV region and a larger surrounding plasma. >

Electric and magnetic field measurements inside a high‐velocity neutral beam undergoing ionization

Vector electric field measurements have been made inside two ionizing, high‐velocity streams of barium atoms in the Earth's ionosphere. A variety of electrical phenomena were observed across the frequency spectrum and are presented in this paper, which emphasizes the experimental results. Comparisons with a theoretical model for the interactions of the stream with the magnetic field and the ionosphere are presented in a companion paper (Brenning et al., this issue (a)). A most startling result is that a very large quasi‐dc electric field was detected antiparallel to the beam velocity. This by itself is not unreasonable since newly ionized barium ions with their large gyroradii are expected to create such a field. But since the beam had roughly a 45° angle with the magnetic field, Bo, we find a very large (≳500 mV/m) component of E parallel to Bo. The fluctuating electric fields were also quite large, in fact, of the same order of magnitude as the quasi‐dc pulse. The wave energy was found to maximize at frequencies below the barium lower hybrid frequency and included strong signatures of the oxygen cyclotron frequency. Measurements made on a subpayload separated across Bo by several hundred meters and along Bo by several kilometers do not show the large pulse, although a variety of wave emissions were seen. In addition, very large amplitude magnetic field fluctuations were detected in both bursts. Although we have no clear explanation, they appear to be a real phenomenon and worthy of future study. Finally, we note that even though the critical ionization velocity effect did not go into a discharge mode in this experiment, remarkable electromagnetic effects were seen in the neutral beam‐plasma interaction.

SR90, Strontium Shaped‐Charge Critical Ionization Velocity Experiment

In May 1986 we carried out an experiment to test Alfvén's critical ionization velocity (CIV) effect in free space, using the first high‐explosive shaped charge with a conical liner of strontium metal. The release, made at 540 km altitude at dawn twilight, was aimed at 48° to B. The background electron density was 1.5×104 cm−3. A faint field‐aligned Sr+ ion streak with tip velocity of 2.6 km s−1 was observed from two optical sites. Using two calibration methods, we calculate that between 4.5×1020 and 2×1021 ions were visible. We have calculated an ionization time constant of 1920 s for Sr from the solar UV spectrum and ionization cross section, which combined with a computer simulation of the injection predicts 1.7×1021 solar UV ions in the low‐velocity part of the ion streak. Thus all the observed ions are from solar UV ionization of the slow (less than critical) velocity portion of the neutral jet. The observed neutral Sr velocity distribution and computer simulations indicate that 2×1021 solar UV ions would have been created from the fast (greater than critical) part of the jet. They would have been more diffuse and were not observed. Using this fact, we estimate that any CIV ions created were less than 1021. We conclude that future Sr CIV free space experiments should be conducted below the UV shadow height and in much larger background plasma density.

Laboratory simulation of cometary neutral gas ionization

The laboratory simulation of the interaction of the solar wind with a comet is used to study the cometary neutral gas ionization. The experiment is carried out in the UCR T‐1 facility with an ice ball as the comet model. Photographs and data are taken with a variety of values of the solar wind velocity, interplanetary magnetic field (IMF), and comet configurations. The results show that the cometary neutral gas ionization depends on both the velocity of the solar wind and the interplanetary magnetic field. The plasma cloud surrounding the comet is visible only when the solar wind velocity and IMF are both above certain minimum values. This velocity dependent phenomena is explained by Alfvén's critical ionization velocity effect. The critical magnetic field may be explained by assuming two stream lower hybrid instability as a triggering mechanism for the ionization of the neutral gas by plasma flow. Critical upper and lower limits for the magnetic field, required by anomalous ionization, are also derived that satisfy the experimental observations.

Lidar technique for measuring ionospheric barium release ion density

An evaluation of the resonant fluorescence lidar technique as a method of measuring ion density during ionospheric barium releases is presented. We find that it is technically feasible to measure ion densities of 103 cm−3 at an altitude of 200 km with a time resolution of 0.5 s by using a ground‐based laser and telescope. One application of the lidar technique would be observation of the critical ionization velocity effect at nighttime, without the complication of solar illumination. Criteria useful for evaluating the feasibility of other experiments are presented.