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

Journal Article · Fri Apr 01 04:00:00 EST 1994 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/93JA02280· OSTI ID:121278

Moghaddam-Taaheri, E; Lu, G; Nishikawa, K I ^[1]

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1. Univ. of Iowa, Iowa City, IA (United States); and others

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 established 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{parallel}, restricts the growth of the fastest growing mode, with a wavelength {lambda}{sub m{parallel}}, of the MBPI. (2) Momentum coupling between the cloud and the ambient plasma via the Alfven 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. (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. 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. The simulation results are used to interpret various chemical release experiments in space. 68 refs., 22 figs., 4 tabs.

• OSTI ID: 121278
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A4 Vol. 99; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
ATOMS
CLOUDS
COULOMB IONIZATION
CRITICAL VELOCITY
GAS INJECTION
IONOSPHERE