An asymptotic state of the critical ionization velocity phenomenon

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We studied the critical ionization velocity (CIV) process by first investigating a coupled system of equations describing the production of several ion species and electrons by impact ionization, their collisions with neutrals, and the heating of electrons. Analytic relations derived from this were tested with the help of a particle simulation including collisonal processes between neutrals and plasma particles. It was found that resistive heating of electrons plays an important role when the density of the neutrals is high and that electron heating due to lower hybrid waves is significant when the neutral density is low. In both cases we verified the control of the plasma production rate by the ratio of the beam velocity to the critical velocity. Himreel et al., 1976; Axniis, 1978; Mb'bius, 1979; Venkatararnani and Mattoo, 1980;Brenning, 1981], and related phenomena were reported in a rocket experiment [Haerendel, 1982] in relation with the space shuttle [Papadopoulos, 1984; Sasaki et al., 1986] and in some other phenomena in space [Lindeman et al., 1974; Cloutier et al., 1978; Petelski, 1981; Forrnisano et al., 1982; Galeev and Chabibrachmanov, 1983-]. However, the critical ionization velocity (CIV) process itself is not easily explained, since the neutral particles cannot be ionized by collisions with either background ions or electrons when the relative velocity is only slightly larger than V• [Danielsson, 1973].Early attempts to understand this phenomena focused on the interpretation of laboratory experiments and did not specify the basic mechanism underlying the phenomena, although the importance of some anomalous electron heating was recognized. A review of these theoretical considerations is given by Sherman [1973]. Relevance of the lower hybrid wave to the CIV process was postulated and studied by several authors [Raadu, 1978;Galeev, 1981;Abe, 1984;Papadopoulos, 1985]. According to them, it is possible to heat the electrons to energies roughly equal to the kinetic energy of the flowing neutrals. These energized electrons ionize the neutrals, producing new plasma to act as an additional generator of waves and thus forming a positive feedback loop.Investigations of the electron heating in the nonlinear stage of the lower hybrid wave instability were performed by computer simulations [McBride et al., 1972; Tanaka and Papadopoulos, 1983;Abe and Machida, 1985]. Moreover, in order to study the feedback mechanism directly, Machida et al. [1984] In this study we are concerned with the CIV process in both high and low neutral density cases. It is necessary to include several collisional processes between plasma and neutral particles in a high neutral density case in addition to the impact ionization collisions treated previously by several researchers [Raadu, 1978;Galeev, 1981;Papadopoulos, 1985]. In general, elastic collisions between electrons and neutrals occur most frequently when the electron energy is in a range below several tens of electron volts, which is the energy consideration in this work. It shou...

Section: Zr½s•a•'a•-me\xvn/ Vels Ementioning

confidence: 94%

Section: Basic Equations For the CIV Processmentioning

confidence: 94%

A simulation study of the critical ionization velocity process

Machida¹,

Goertz²

1986

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“…The basic mechanism for electron heating through their interaction with lower hybrid (LH) electrostatic waves is well understood. As mentioned above, the LH instability, driven by an ion beam produced by the interaction of a neutral gas with the ambient plasma, is considered in almost all of the CIV theories [Galeev, 1981;Formisano et al, 1982;Papadopoulos, 1983Papadopoulos, , 1984Papadopoulos, , 1992Abe and Machida, 1985;Goertz et al, 1985;Goertz, 1986, 1988;Mdibius et al, 1987]. The equations for the particles and waves in a homogeneous infinite medium have been solved in the quasi-linear approximation [Galeev, 1981;Formisano et al, 1982;Papadopoulos, 1983Papadopoulos, , 1984 and have been successfully modeled by particle simulation [McBride et al, 1972;Tanaka and Papadopoulos, 1983;Abe and Machida, 1985;Goertz, 1986, 1988;Goertz et al, 1990;McNeil et al, 1990].…”

Section: Nba(x) = Nbaolnc(x) [1 -•T(x)] (2)mentioning

confidence: 99%

Numerical quasi‐linear study of the critical ionization velocity phenomenon

Moghaddam-Taaheri¹,

Goertz²

1993

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The critical ionization velocity (CIV) for a neutral barium (Ba) gas cloud moving across the static magnetic field is studied numerically using quasi‐linear equations and a parameter range which is typical for the shaped‐charge Ba gas release experiments in space. For consistency the charge exchange between the background oxygen ions and neutral atoms and its reverse process, as well as the excitation of the neutral Ba atoms, are included. The numerical results indicate that when the ionization rate due to CIV becomes comparable to the charge exchange rate the energy lost to the ionization and excitation collisions by the superthermal electrons exceeds the energy gain from the waves that are excited by the ion beam. This results in a CIV yield less than the yield by the charge exchange process.

“…The efficiency of the process is denoted by the energy transfer factor r/, which is the fraction of the new ion's kinetic energy mnV2/2 that goes to the electrons. Different theoretical estimates of r/ [Raadu, 1978;Galeev and Sagdeer, 1983 The ve]ocity of the plasma within the stream is determined by the pickup of new ions and by the momentum coupling to the ambient plasma; this coupling is, in the homogeneous model [Haerendel, 1982;Goertz and Machida, 1985 The conservation of momentum along the neutral flow requires that a transverse current (the electron Hall current of the model) flows across the stream. In the laboratory experiments where this model was first proposed this current is driven by an external generator.…”

mentioning

confidence: 99%

Interpretation of the electric fields measured in an ionospheric critical ionization velocity experiment

Brenning¹,

Fälthammar²,

Haerendel³

et al. 1991

This paper deals with the quasi‐dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV⋕ mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three‐dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic‐field‐aligned component of the electric field; the quasi‐dc electric field observed within the streams had a large magnetic‐field‐aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic‐field‐aligned electric fields and probably current limitation as important processes. One interpretation of the quasi‐dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic‐field‐aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence‐free. It is argued that this interpretation can explain both a reversal of the strong explosion‐directed electric field in burst 1 and the absence of such a reversal in burst 2.