Experimental Research
The Royal Institute of Technology in Stockholm carried out the first laboratory tests, and found that (a) the relative velocity between a plasma and neutral gas could be increased to the critical velocity, but then additional energy put into the system then went into ionizing the neutral gas, rather than into increasing the relative velocity, (b) the critical velocity is roughly independent of the pressure and magnetic field.
In 1973, Lars Danielsson published a review of critical ionization velocity, and concluded that the existence of the phenomenon "is proved by sufficient experimental evidence". In 1976, Alfvén reported that "The first observation of the critical velocity effect under cosmic conditions was reported by Manka et al. (1972) from the Moon. When an abandoned lunar excursion module was made to impact on the dark side of the Moon not very far from the terminator, a gas cloud was produced which when it had expanded so that it was hit by the solar wind gave rise to superthermal electrons."
In the laboratory, critical ionization velocity has been recognised for some time, and is seen in the penumbra produced by a dense plasma focus device (or plasma gun). Its existence in cosmic plasmas has not been confirmed.
In 1986, Gerhard Haerendel, suggested that critical velocity ionization may stabilize the plasma flow in a cometary coma,. In 1992, E. Golbraikh and M. Filippov argued that critical ionization velocity could play a role in coronal mass ejections and solar flares, and in 1992, Anthony Peratt and Gerrit Verschuur suggested that interstellar neutral hydrogen emissions bore the signature of critical velocity ionization,.
A 2001 review of the phenomenon by Shu T. Lai reports that ".. laboratory experiments, and computer simulations have all shown CIV as feasible and reasonably understood, although all CIV experiments in space have yielded negative results with perhaps three exceptions".
Also in 2001, C. Konz, et al., ".. discuss the critical velocity effect as a possible explanation for the observed Hα emission in the Galactic halo near the edges of cold gas clouds of the Magellanic Stream"
Read more about this topic: Critical Ionization Velocity
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