The critical velocity ionization, a concept first proposed by Hannes Alfven in 1954, holds that if neutral atoms move through a background-magnetized plasma with kinetic energy equal their ionization energy, then they will ionize through collective plasma processes in excess of that predicted by collisions and charge exchange. This theory has profound implications for astrophysics and models of solar system formation because it addresses the dynamics of gases falling toward a central star interacting with stellar winds and magnetic fields.
This hypothesis has been verified recently by chemical releases performed by the CRRES satellite at an altitude of approximately 500 km over the South Pacific. In the experiments, gas clouds of barium, calcium, and strontium were released beneath the solar ultraviolet terminator when the spacecraft velocity was greater than the critical velocities of these elements. Ions produced in the release cloud were observed when they were transported along the magnetic field line upward into sunlight. The interpretation of the data has put definite limits on the range of parameters over which the critical velocity process can operate.
Source: Christensen and Reasoner: Satellite Probes Plasma Processes in Earth Orbit in EOS, 73, Number 2, 1992