In an experiment where a high velocity plasma stream penetrates a stationary neutral gas cloud in a magnetic field, it is found that the interaction between the plasma and the gas is far more violent than what can be expected from simple atomic interaction theories. The parameters are such that the character of the interaction is collisionless. The observations demonstrate that the plasma is very efficiently retarded as it penetrates into the gas cloud and that the electron energy distribution changes drastically towards higher energies, at least for a part of the population, resulting in partial ionization of the gas. In order to account for the observations it is necessary to assume some kind of collective interaction between the plasma and the neutral gas. The results are analyzed from the point of view of Alfvén's critical velocity hypothesis.

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An interaction between a plasma and a neutral gas in relative motion is demonstrated. If the relative velocity is high enough, the interaction leads to heating of the electrons, partial ionization of the gas, and braking of the motion. The limiting velocity is found to be very close to Alfvén’s critical velocity. Two new means of plasma diagnostics are tried for the first time in this experiment. Measurements of the polarization of emitted spectral lines have yielded information on the velocity distribution of the electrons. An ion beam interaction technique has been employed to determine the density of energetic electrons. New evidence is also presented of electron heating and of the necessity of a magnetic field for the interaction to take place. The previously observed minimum gas density is also discussed.

A model experiment simulating the interaction between the solar wind and comets is described. The aim is to study the ionization and acceleration of molecules in the atmosphere of a comet. A plasma stream (ne = 1013 cm−3, Te = 3 ev, υ = 6 × 106 cm/sec) in a vacuum tunnel interacts with a cloud of CO2 gas sublimated from a dry ice core of 1‐cm radius. The gas density at the surface of the ice is 5 × 1013 cm−3. Diagnostic methods include image converter photography and spectrography. The interaction, which is characterized as collisionless, gives rise to a well‐defined tail developing to a length of 20 cm within 10 μsec after the first impact of the stream against the cloud. A visible coma extends about 3 cm in front of the ice. The CO2 gas is found to be substantially ionized and dragged along with the stream. The ionization is believed to be explainable in terms of an increased average electron energy caused by the interaction. The drag on the ions is probaly due to magnetic coupling to the plasma stream.

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