Critical ionization velocity
Encyclopedia
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
and plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
(an ionized gas), at which the neutral gas will start to ionize. If more energy is supplied, the velocity of the atoms or molecules will not exceed the critical ionization velocity until the gas becomes almost fully ionized.
The phenomenon was predicted by Swedish engineer and plasma scientist, Hannes Alfvén
Hannes Alfvén
Hannes Olof Gösta Alfvén was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics . He described the class of MHD waves now known as Alfvén waves...
, in connection with his model on the origin of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
(1942). At the time, no known mechanism was available to explain the phenomenon, but the theory was subsequently demonstrated in the laboratory,. Subsequent research by Brenning and Axnäs (1988) have suggested that a lower hybrid
Lower hybrid oscillation
A lower hybrid oscillation is a longitudinal oscillation of ions and electrons in a magnetized plasma. The direction of propagation must be very nearly perpendicular to the stationary magnetic field, within about √ radians...
plasma instability
Instability
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...
is involved in transferring energy from the larger ions to electrons so that they have sufficient energy to ionize. Application of the theory to astronomy though a number of experiments have produced mixed results .
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 intoincreasing 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 [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."
In the laboratory, critical ionization velocity has been recognised for some time, and is seen in the penumbra produced by a dense plasma focus
Dense plasma focus
A dense plasma focus is a machine that produces, by electromagnetic acceleration and compression, a short-lived plasma that is so hot and dense that it can cause nuclear fusion and emit X-rays. The electromagnetic compression of the plasma is called a pinch. It was invented in the early 1960s by...
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 ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
s and solar flare
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
s , and in 1992, Anthony Peratt and Gerrit Verschuur
Gerrit Verschuur
Gerrit L. Verschuur, PhD, born in 1937 in Cape Town, South Africa, is a naturalized American scientist who is best known for his work in radio astronomy...
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"
Theory development
| Typical Critical Ionization Velocities (After Alfvén (1976)) | |||
| Element | lonization potential Vion (V) | Average atomic mass | Critical Velocity Vcrit (105 cm/sec) |
| Hydrogen | 13.5 | 1.0 | 50.9 |
| Helium | 24.5 | 4.0 | 34.3 |
| Neon | 21.5 | 20.2 | 14.3 |
| Nitrogen | 14.5 | 14.0 | 14.1 |
| Carbon | 11.2 | 12.0 | 13.4 |
| Oxygen | 13.5 | 16.0 | 12.7 |
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:
where eVion 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,, and also Critical velocity effect,.
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, Vion, occurs when:
that is, at a distance of:
(where ri is the ion distance from the Sun of mass M, m is the atom weight, Vion 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 ri where it will accumulate, and perhaps condense into planets.
Alfvén found that by taking a gas cloud with an average ionisation voltage of 12V, and average atomic weight of 7, then the distance ri is found to coincide with the orbit of Jupiter.
The critical ionization velocity of hydrogen 50.9 x 105cm/s (50.9 km/s), and helium is 34.3 x 105cm/s (34.3 km/s), .
Background
Alfvén discusses his thoughts behind critical velocity, in his NASA publications Evolution of the Solar System, .. After criticising the "Inadequacy of the Homogeneous Disc Theory", he writes:".. it is more attractive to turn to the alternative that the secondary bodies derive from matter falling in from "infinity" (a distance large compared to. the satellite orbit). This matter (after being stopped and given sufficient angular momentum) accumulates at specific distances from the central body. Such a process may take place when atoms or molecules in free fall reach a kinetic energy equal to their ionization energy. At this stage, the gas can become ionized by the process discussed in sec. 21.4; the ionized gas can then be stopped by the magnetic field of the central body and receive angular momentum by transfer from the central body as described in sec. 16.3.".
Other references
- Brenning, N .
- A comparison between laboratory and space experiments on Alfven's CIV effect, in IEEE Transactions on Plasma Science (ISSN 0093-3813), vol. 20, no. 6, p. 778-786. (1996)
- Review of the CIV phenomenon, in Space Science Reviews (ISSN 0038-6308), vol. 59, Feb. 1992, p. 209-314. (1992)
- Limits on the magnetic field strength for critical ionization velocity interaction, Physics of Fluids -- November 1985 -- Volume 28, Issue 11, pp. 3424-3426