Critical Ionization Velocity Research Articles

Critical ionization velocity effects in the inner coma of comet Halley: Measurements by Vega‐2

Plasma density and plasma wave measurements aboard the "Vega‐2" spacecraft discovered two plasma density enhancements in the inner coma of comet Halley that are accompanied by bursts of plasma waves in the lower‐hybrid frequency range. These events are explained by the critical ionization velocity (CIV) effects of Alfvén. The persistent bursty plasma wave activity and the intermittent plasma density growth toward the cometary nucleus indicate that the CIV effect contributes to the ionization of the cometary gas.

Gas ionization induced by a high speed plasma injection in space

Gas ionization induced by a fast plasma injection has been observed with the Space Experiments with Particle Accelerators (SEPAC) Experiment on Spacelab‐1. When an impulsive high‐density plasma was injected from the orbiter, waves near the lower‐hybrid frequency were enhanced, and the surrounding gas drifting with the orbiter was ionized for several tens of milliseconds after the plasma injection. The long‐duration gas ionization was observed only when the plasma flux incoming to the orbiter cargo bay and the orbital velocity perpendicular to the magnetic field were relatively large. This effect has been explained by the concept of critical velocity ionization (CVI) for the gas drifting with the orbiter, although the gas velocity perpendicular to the magnetic field was less than the Alfvén critical velocity.

Plasma flow and critical velocity ionization in cometary comae

The plasma flow in the cometary coma is described by a simple relation which results from the balance of magnetic stresses and drag forces, the latter being mainly due to ion mass loading. Inertial and pressure effects are not explicitly considered, but lumped into a correcting factor of order unity. The relation is used to infer the ionization rate due to the critical velocity effect. The limits, rcrit and rlim, between which the effect dominates over photoionization are evaluated. Critical velocity ionization stabilizes the plasma flow speed at η−1/2 vcrit, where η is the overall efficiency of energy transfer to the ionizing electrons. This speed is estimated to be close to 20 km/s. At the outer limit, rlim, there is a strong rise of plasma density as seen from outside. Inside rcrit, recombination and ion‐neutral friction come into play. The contact surface is found to be largely determined by ion‐neutral collisions. A second neutral component of the order of 10² cm−3 is predicted to originate from recombination inside rlim. It is characterized by an antisunward flow with ∼ 20 km/s. Numerical values for the radial profile of ni and of the critical boundaries (except the bow shock) are given. The high plasma density found by ICE at the closest approach to comet Giacobini‐Zinner is interpreted as a result of the critical velocity effect.

An asymptotic state of the critical ionization velocity phenomenon

We consider the problem of how the momentum of ions created by electron impact ionization of neutrals moving at a speed υ0 perpendicular to the magnetic field through a background plasma is coupled to this plasma. We find that the plasma accelerates and the relative velocity between neutrals and plasma decreases. If this decrease is rapid and large enough the critical ionization velocity (CIV) phenomenon may turn off. We derive equations for the evolution of plasma density, electron and ion thermal energy and plasma velocity. We find that the CIV process reaches an asymptotic quasi‐steady state in which the ionization rate reaches a constant value which depends on the properties of the surrounding medium and the value of υ0.

Competing atomic processes in Ba and Sr injection critical velocity experiments

The critical ionization velocity effect requires a superthermal electron population to ionize through collisional impact. Such superthermal electrons can however lose energy to competing atomic processes, as well as to ionization, thus limiting the efficiency of the effect. Considering Ba and Sr magnetospheric injection experiments designed to test the CIV theory, we find that in both cases roughly 60% of the superthermal electron energy is lost on exciting line radiation. Moreover, energy loss to background neutral oxygen places a strict limit on the injected cloud densities for which critical velocity effects are possible; a finding which explains the consistently negative results in radial injection experiments.

Limits on the magnetic field strength for critical ionization velocity interaction

A theoretical lower limit to the magnetic field strength required for critical ionization velocity (vc) interaction is found to correspond to the requirement v<vA(1+βe)1/2, which can also be expressed as ωpe/ωce<(c/v)[(1+βe) ×me/mi]1/2. This limit is found to agree, within the experimental uncertainties, with the results from all the vc experiments in the impact configuration. The different experiments have very different physical characteristics, and cover large ranges in plasma densities, stream velocities, and magnetic field strengths. A theoretical upper limit to the magnetic field strength is also found, corresponding to the requirement ωpe/ωce ≫(me/mn)1/2.

Review of the critical ionization velocity effect in space

Laboratory experiments have shown under a variety of conditions that when a neutral gas passes through a magnetized plasma with a relative velocity perpendicular to the magnetic field that is greater than a critical velocity, anomalously high ionization of the neutrals occurs. The conditions under which the same effect is to be expected in space plasmas is still unclear. The experimental evidence for the occurrence of the critical ionization velocity effect in space is summarized, and various areas in which it has been proposed that the effect should be significant are discussed.

Computer simulation of critical velocity ionization

The mechanism of critical velocity ionization is studied through electrostatic particle simulations which include a neutral-particle ionization process. It is shown that the rapid ionization of neutral particles takes place when the flow velocity of the neutral particles across a magnetic field exceeds a certain threshold. The mechanism is explained in terms of lower-hybrid wave excitation by a plasma instability, resultant electron energization, and neutral-particle ionization. It is found that the phase space relationship between the most unstable wave and the particles remains appropriate for the operation of the above process.

Theory for the critical ionization velocity phenomenon

A theoretical consideration is given to the critical ionization velocity. A macroscopic theory is constructed, assuming that (1) the newly born ions have an unstable velocity distribution and (2) the resultant fluctuating electric field heats the electrons. For the critical ionization velocity to operate in the case of m aV 2 2 > eφ ion , the initial values of high speed ion (or newly ionized ion) density and suprathermal electron energy density should be above a certain threshold. This threshold depends upon the velocity diffusion co-efficient and the collisional loss rate of the newly ionized ion.

On the shuttle glow (the plasma alternative)

It is shown that the anomalous glow observed in the recent space shuttle flights can be attributed to a combination of beam plasma discharge and critical ionization phenomena. While the shuttle velocity (∼8 km s−1) is below the critical ionization velocity, which for oxygen is 12.7 km s−1, specular reflection of a small fraction (2–3%) of ambient ions forms an ion beam with velocity 16 km s−1, making up for the energy deficit in the critical ionization velocity phenomenon. The wave activity observed is consistent with this model.

The role of the critical ionization velocity phenomena in the production of inner coma cometary plasma

The critical velocity triggering anomalous ionization of the neutral gas by plasma flow is calculated for the model based on the lower hybride instability. It depends strongly on the plasma and gas parameters, denning the instability development of the ionized atoms beam in the counter-streaming plasma. In particular, the possible role of the critical ionization mechanism for Halley's comet is examined. The fulfilment of both Townsend's condition for the selfustained beam plasma discharge and Alfven's condition for the critical velocity mechanism indicates that this mechanism may operate only within 10 4 km from the cometary nucleus and give an ion production rate close to that observed for Kohoutek's comet.

Experiments on the critical ionization velocity interaction in weak magnetic fields

The critical ionization velocity interaction is studied experimentally in a configuration with a magnetized plasma stream colliding with a stationary neutral gas cloud. In all previous experiments of this kind the magnetic field (1) has had a component transverse to the plasma flow and (2) has been strong in the sense that the electron gyro frequency, eB/mc, has exceeded or been approximately equal to the plasma frequency, wpc. Both these conditions play an important role in existing theories of the critical velocity interaction. The present experiments are performed to determine whether or not such interaction is possible when one of these conditions is not fulfilled, namely when the magnetic field is weak (eB/me>>wpc). Experiments have been performed both with a transverse and longitudinal (aligned with the plasma flow) magnetic field. It is found that in both cases the critical ionization velocity effect either disappears or becomes too small to be distinguishable among classical collisional processes.

Some necessary conditions for a critical velocity interaction between the ionospheric plasma and a xenon cloud

The conditions for an experiment to study the critical ionization velocity effect in the interaction between a Xenon cloud, released from a satellite, and the ionospheric plasma are investigated. The model used is based on the assumption that there exists an effective process that transfers the energy, that is available in the relative motion, to the electrons. Some necessary conditions to obtain significant heating or deceleration of the plasma penetrating the cloud are calculated. The conditions are mainly given by the energy available in the relative motion and the rates of the different binary collision processes involved. As the released gas cloud expands the possibilities for a critical velocity interaction will exist only within a certain range of cloud radii. It is shown that the charge transfer collision cross section between the ionospheric ions and the cloud atoms is an important parameter and that Xenon is a very suitable gas in that respect.

The influence of the plasma inhomogeneity on the critical velocity phenomenon

The phenomenon of a critical ionization velocity is discussed under the aspect of experimentally measured inhomogeneities. The difficulties inherent in homogeneous plasma models on the phenomenon are shown. A simple sheath model is presented which can be compared with the local plasma parameters measured in a rotating plasma device. The origin of the observed turbulent heating is attributed to a modified two-stream instability occurring in the sheath under discussion.

Model Calculations of Solar Wind Expansion Including an Enhanced Fraction of Ionizing Electrons

Collective interactions of the solar wind and newly ionized interstellar gas cause turbulent electron heating to ionizing energies analogous to laboratory experiments on the critical ionization velocity effect. Implications for solar wind and interstellar gas dynamics are calculated by simultaneously solving continuity equations for solar wind protons, interstellar hydrogen atoms, and energetic electrons. Electron impact ionization is shown to be practically as important as photoionization, giving rise to a stronger deceleration and heating of the distant solar wind, a weaker terminating shock, a smaller stand-off distance of the helio pause, and implying higher densities of the outer solar wind and the interstellar neutral gas.

A Spacelab experiment on the critical ionization velocity

Although the existence of a critical ionization velocity has been shown in laboratory experiments certain difficulties remain when one tries to scale the parameters to cosmic dimensions. These problems are discussed in the light of recent theoretical developments, and a possible experiment is described which makes use of the high orbital velocity of spacelab and the large uniform plasma through which it travels.

Experimental investigation of the critical ionization velocity in gas mixtures

The critical ionization velocity which is of cosmogonic and astrophysical interest has hitherto mainly been investigated for pure gases. Since in space we always have gas mixtures, it is of interest also to study gas mixtures. The present report, which is a summary of a more detailed report (Axnas, 1976), summarizes the results of systematic experiments on the critical ionization velocity as a function of the mixing ratio for binary gas mixtures of H2, He, N2, O2, Ne and Ar. The apparatus used is a coaxial plasma gun with an azimuthal magnetic field. The discharge parameters are chosen so that the plasma is weakly ionized. In some of the mixtures it is found that one of the components tends to dominate in the sense that only a small amount (regarding volume) of that component is needed for the discharge to adopt a limiting velocity close to that for the pure component. Thus in a mixture between a heavy and a light component having nearly equal ionization potentials, the heavy component dominates. Also, if there is a considerable difference in ionization potential between the components, the component with the lowest ionization potential tends to dominate.

Critical ionization velocity and the dynamics of a coaxial plasma gun

Critical ionization velocity and the dynamics of a coaxial plasma gun

The role of electrostatic instabilities in the critical ionization velocity mechanism

The role of electrostatic instabilities in the critical ionization velocity mechanism is investigated. The analysis is based on the theory developed by Sherman, which interprets Alfven's critical velocity in terms of a circular process. This process involves the acceleration of electrons by a two-stream instability modified by the presence of a magnetic field. A general expression for the energy and momentum of ions and electrons associated with an electrostatic mode is derived in terms of the plasma dielectric constant. This is used in the case of the modified two-stream instability to determine the distribution of energy between ions and electrons. An extrapolation from the linear phase then gives an estimate of the energy delivered to the electrons which is compared to that required to ionize the neutral gas.