Critical ionization velocity

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 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.[4]

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".[8] In 1976, Alfvén reported that "The first observation of the critical velocity effect under cosmic conditions was reported by Manka et al. (1972)[9] from the Moon. When an abandoned lunar [391] 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."[10]

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,.[11] In 1992, E. Golbraikh and M. Filippov argued that critical ionization velocity could play a role in coronal mass ejections and solar flares,[12] and in 1992, Anthony Peratt and Gerrit Verschuur suggested that interstellar neutral hydrogen emissions bore the signature of critical velocity ionization,.[13]

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".[7]

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"[14]

The CIV phenomenon has been also demonstrated in different Low Temperature Plasma (LTP) laboratory experiments where a plasma and a neutral gas are in relative motion across a magnetic field such as magnetrons. Simulations involving plasma accelerated in a gas similar to the cross-field LTP experiments shows rotating instabilities moving at a velocity close to the critical ionization velocity[15]

Mathematically, the critical ionization velocity of a neutral cloud, that is, when the cloud begins to become ionized, is when the relative kinetic energy is equal to the ionization energy, that is:

${\displaystyle {\frac {1}{2}}mv^{2}=eV_{ion}}$

where eV_ion is the ionization potential of the atoms or molecules in the gas cloud, m is the mass, v is the velocity. The phenomenon is also called the Critical velocity ionization,[11] and also Critical velocity effect,.[16]

Alfvén considered a neutral gas cloud entering the Solar System, and noted that a neutral atom will fall towards the Sun under the influence of gravity, and its kinetic energy will increase. If their motion is random, collisions will cause the gas temperature to rise, so that at a certain distance from the Sun, the gas will ionize. Alfvén writes that the ionization potential of the gas, V_ion, occurs when:

${\displaystyle eV_{ion}=E_{g}={\frac {kMm}{r}}}$

that is, at a distance of:

${\displaystyle r_{i}=E_{g}={\frac {kMm}{eV_{ion}}}=6.9\cdot 10^{-20}M{\frac {m'}{V_{ion}}}=13.5\cdot 10^{13}{\frac {m'}{V_{ion}}}cm}$

(where r_i is the ion distance from the Sun of mass M, m' is the atom weight, V_ion is in volts, k is the gravitational constant). Then when the gas becomes ionized, electromagnetic forces come into effect, of which the most important is the magnetic force which is usually greater than the gravitational force which gives rise to a magnetic repulsion from the Sun. In other words, a neutral gas falling from infinity toward the Sun is stopped at a distance r_i where it will accumulate, and perhaps condense into planets.

Alfvén found that by taking a gas cloud with an average ionisation voltage of 12 V, and average atomic weight of 7, then the distance r_i is found to coincide with the orbit of Jupiter.

The critical ionization velocity of hydrogen 50.9 x 10⁵ cm/s (50.9 km/s), and helium is 34.3 x 10 ⁵cm/s (34.3 km/s).[17]