PARTICLE-IN-CELL SIMULATIONS OF THE CRITICAL IONIZATION VELOCITY EFFECT IN FINITE-SIZE CLOUDS

The critical ionization velocity (CIV) mechanism in a finite size clou d is studied with a series of electrostatic particle-in-cell simulatio ns. It is observed that an initial seed ionization, produced by non-CI V mechanisms, generates a cross-field ion beam which excites a modifie d beam-plasma instability (MBPI) with frequency in the range of the lo wer 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 p ositive feedback loop in maintaining the CIV process. It is also found that the efficiency of the CIV mechanism depends on the finite size o f the gas cloud in the following ways: (1) Along the ambient magnetic field the finite size of the cloud, L(parallel-to), restricts the grow th of the fastest growing mode, with a wavelength lambda(mparallel-to) , of the MBPI. The parallel electron heating at wave saturation scales approximately as (L(parallel-to)/lambda(mparallel-to))1/2. (2) Moment um coupling between the cloud and the ambient plasma via the Alfven wa ves occurs as a result of the finite size of the cloud in the directio n perpendicular to both the ambient magnetic field and the neutral dri ft. This reduces exponentially with time the relative drift between th e ambient plasma and the neutrals. The timescale is inversely proporti onal to the Alfven velocity. (3) The transverse charge separation fiel d across the cloud was found to result in the modulation of the beam v elocity 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 characteri zed 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 los s rate is characterized by the neutral drift velocity. It is also show n 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 o f 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 produc tion by non-CIV mechanisms, (2) smaller Alfven velocity and hence weak momentum coupling, and (3) smaller ratio of the ion beam density to t he ambient ion density, and therefore a weaker modulation of the beam velocity. The simulation results are used to interpret various chemica l release experiments in space.