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Critical ionization velocity (CIV), or critical velocity (CV), is the relative velocity between a neutral gas and plasma (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, in connection with his model on the origin of the Solar System (1942).〔Hannes Alfvén "On the cosmogony of the Solar System", in ''Stockholms Observatoriums Annaler'' (1942) (Part I ) (Part II ) (Part III )〕〔Hannes Alfvén, ''(On the Origin of the Solar System )''. Oxford: Clarendon Press, 1954.〕〔Hannes Alfvén, (Collision between a nonionized gas and a magnetized plasma ), ''Rev. Mod. Phys''., vol. 32, p. 710, 1960.〕 At the time, no known mechanism was available to explain the phenomenon, but the theory was subsequently demonstrated in the laboratory.〔U.V. Fahleson, "Experiments with plasma moving through neutral gas", Phys. Fluids, 4 123 (1961)〕 Subsequent research by Brenning and Axnäs (1988)〔Brenning, N ., Axnas, I: "(Critical ionization velocity interactions : Some unsolved problems )", (1988) ''Astrophys. Space Sci''. 144 15〕 have suggested that a ''lower hybrid'' plasma instability 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.〔R. Torbert,: "(Review of ionospheric CIV experiments )", XXVIIth COSPAR Meet., (1988) Helsinki, Finland, paper XIII.2. 1〕〔Lai, Shu T., (A review of critical ionization velocity ) (2001) ''Reviews of Geophysics'', Volume 39, Issue 4, p. 471-506〕 ==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 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".〔Lars Danielsson, "(Review of the Critical Velocity of Gas-Plasma Interaction. I: Experimental Observations )", ''Astrophysics and Space Science'' (1973) 〕 In 1976, Alfvén reported that "The first observation of the critical velocity effect under cosmic conditions was reported by Manka et al. (1972)〔Manka, R. H., et al., "Evidence for acceleration of lunar ions", in ''Lunar Science III'', C. Watkins, ed., (The Lunar Science Institute, Houston, Tx.): 504. (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."〔Hannes Alfvén, "(Mass Distribution and the Critical Velocity )", ''Evolution of the Solar System'' (1976) 〕 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,.〔G. Haerendel: "(Plasma flow and critical velocity ionization in cometary comae )", (1986) ''Geophys. Res. Lett''. 13 25 5〕 In 1992, E. Golbraikh and M. Filippov argued that critical ionization velocity could play a role in coronal mass ejections and solar flares,〔Golbraikh, E. I.; Filippov, M. A., (Possible manifestation of the critical ionization velocity phenomenon in the solar corona ) (1992), ''ESA, Study of the Solar-Terrestrial System''. 〕 and in 1992, Anthony Peratt and Gerrit Verschuur suggested that interstellar neutral hydrogen emissions bore the signature of critical velocity ionization,.〔Peratt, Anthony; Verschuur, Gerrit, (The Critical Ionization Velocity Signature Manifested in Interstellar Neutral Hydrogen Emission Profile Structure ), (1992), ''Bulletin of the American Astronomical Society'', Vol. 34, p.766 〕 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"〔Konz, C.; Lesch, H.; Birk, G. T.; Wiechen, H., "(The Critical Velocity Effect as a Cause for the Hα Emission from the Magellanic Stream )" (2001) in ''The Astrophysical Journal'', Volume 548, Issue 1, pp. 249-252〕 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〔Boeuf, J. P.; Chaudhury, B., "(Rotating Instability in Low-Temperature Magnetized Plasmas )" (2013) in ''Physical Review Letter'', Volume 111, 155005〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Critical ionization velocity」の詳細全文を読む スポンサード リンク
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