Cosmic rays ionization

Once a significant amount of molecular hydrogen is produced, a rich gas-phase chemistry ensues.24 Ion-molecule processes are initiated in the interiors of dense clouds mainly via cosmic ray ionization, the most important reaction being,... 

Once formed, molecular nitrogen can be broken apart by reaction with He+ (generated from He via cosmic ray ionization),... 

At polar latitudes in winter, massive discharges of electricity in the upper atmosphere can be seen as the very beautiful aurora borealis. The solar wind and cosmic rays ionize O and N atoms about 80 km above the earth s surface the ionized N and O atoms emit faint light as they regain an electron and return to the ground state. 

Fig. 8a. Equilibrium temperature of a gas heated by cosmic rays, ionization rate % =...

The primary energy source behind the ion-molecule scheme in interstellar molecular clouds is the cosmic ray ionization of H, H2 and He, which can be transferred efficiently to less abundant atoms or species, notably C, N and O. Thus their effective time scale for ionization is reduced by a factor proportional to their abundance. In addition, exothermic feactions between positive ions and neutrals occur with no... 

Cosmic ray ionization of H leads to the formation of HD through a sequence of reactions. The resonant charge-exchange reaction, whose rate constant has been given (Watson et al. 1978)... 

According to the ion-molecule scheme, the chemistry in dense molecular clouds is driven by the assumed cosmic ray ionization of the most abundant species i.e. H, H2 and He. The cosmic ray ionization rate should be 10 sec tmsed upon the cosmic ray flux measured at earth. 

So far we have summarized some basic reactions starting with the cosmic ray ionization of Hj. However cosmic ray ionization of He, which is considerably less abundant in dense clouds than H2 (about V4) seems to be important for two reasons firstly, an activation energy barrier (Johnsen and Biondi, 1974) is likely to keep the reaction rates of H with H and Hj anomalously small (reaction rate 8 x 10 cm sec Sando et al. 1972), and therefore He remains available for the ionization of neutral molecules. Secondly, in most cases, the charge transfer from He to diatomic molecules dissociates them, producing essentially ionized heavy elements, such as C, N , O". The reaction sequence has the general form (see note added in proof). 

Interstellar molecules are detected at the position where they are formed. Their formation mechanism is usually modelled for a steady-state situation, although their abundances are not in thermodynamic equilibrium. Cosmic rays and ultraviolet radiation prevent equilibrium from being reached. Cosmic ray ionization is seen as the driving force for a large number of chemical reactions. 

The only important source of ionization in the stratosphere, under most conditions, is galactic cosmic rays [32]. These are mostly energetic protons having average kinetic energies of about 100-1000 MeV corresponding to an atmospheric penetration depth of about 10-15 km. Thus, the galactic cosmic ray ionization rate, Q, reaches a maximum of about 10-100 cm" s around this altitude. 

Primary charged species formed by galactic cosmic ray ionization are N 2, 0 2, O, N, and free electrons. The latter are rapidly attached to gas molecules, giving rise to simple negative ions, mostly 0"2. Subsequent ion molecule reactions of primary positive and negative ions lead to complex positive and negative cluster ions. Ultimately these are removed by ion-ion recombination involving either a binary or a ternary mechanism [33]. 

In the troposphere the most important sources of ionization are radioactivity and galactic cosmic rays [60]. The former, which is due to , P, and Y radiations from thoron and radon, is dominant up to about one kilometer altitude. Above this height, as in the stratosphere, galactic cosmic ray ionization is most important. 

The Hj molecular ion plays the pivotal role in the ion-neutral reactions scheme now generally believed to be the major mechanism for the chemical evolution of dense molecular clouds. Hj is produced through cosmic ray ionization of H followed by the ion-neutral reaction (1). Since the latter reaction is extremely fast, the rate-determining process for the production of HJ is the cosmic ray ionization, whose rate is generally taken to be f a 10"g-i 63.68 main destruction mechanism of Hj is the proton-hop reaction (2). 

So far, unsuccessful searches for Hj in BN, GL 2591, LkH 101, NGC 2024/IRS, W33IR, NGC 2264 and AFGL 2591 have been reported. Black et alP observed spectral lines of CO simultaneously with their search for Hj. The abundance of CO thus obtained together with the upper limit of the Hj column density set a limit on the rate of the cosmic ray ionization C through eqn. (7). van Dishoeck and Black have proposed on chemical grounds that the abundance of Hj may be equally high in diffuse interstellar clouds. ... 

The carbon ion is responsible for the rich organic chemistry observed. The production of C " by the route indicated (17.6) is at least 500 times (the ratio of He/C cosmic abundance) more efficient than direct cosmic-ray ionization of CO, the predominant interstellar carbon source - hence, the rich organic chemistry observed in the interstellar medium. The efficient production of C+ has as origin the lack of... 

The cosmic ray ionization driving dark cloud chemistry could be different. The production and energy distribution of cosmic rays are thought to depend on a high-energy event, such as a supernova or accretion onto a black hole. How fortuitous... 

The picture of a gas-phase chemistry in the dense molecular clouds, initiated by cosmic ray ionization of H2 and He, has been greatly strengthened by the direct observation of the very early reactant 11 (Oka, 1992). The identification and determination of the spectrum of H have been a great triumph of laboratory studies (Oka, 1980). This ion is now observed in absorption in many sources by means of its vibrational spectrum (McCall et al., 1998), as well as in planetary atmospheres in emission (Oka, 1992). Its abundance allows critical exploration of ionizing fluxes, primarily cosmic rays, and further provides an exceptional sight into dense molecular clouds. 

Photoionizaton of neutral C to C and C2H2 to C2H2 by UV photons and cosmic ray ionization of H2 to H3 are the major ion sources for subsequent ion molecule reactions and dissociative recombination reactions 59). So far only two ions (HCO, CeH fdO)) have been observed in IRC+10216. Ion-molecule reactions involving C2H2 can account for the abundances of several molecules produced in the outer CSE (e.g., C H, C S, C3O, CH3CN, HNC, C N, C3H2HC2n+iN, etc.), but other reactions may also be required 59-64). 

In 1752 T. D Alibard and B. Franklin independently confirmed the electrical nature of thrmderclouds in the same year L. Lemomiier observed that electrical effects oc-cmred in fine weather. Not rmtil the late nineteenth cen-tiuy was the electrical conductivity of air established by several workers, and the discovery of cosmic ray ionization by V Hess and the postulation of the global electrical circuit by C. T. R. Wilson came in the early twentieth century. Now known as the classical global cir-... 

Most models of interstellar clouds assume that the ultraviolet photons of the interstellar radiation field cannot penetrate the inner parts of dense clouds. However, Prasad and Tarafdar (1983) have suggested that a dilute flux of ultraviolet photons can be produced following excitation of Hj by energetic secondary electrons resulting from cosmic ray ionization of H and H2. The cosmic ray induced spectrum has been calculated by Sternberg... 

Thus the abundance of HD is also sensitive to the cosmic ray ionization rate (o which produces H" ". 

Another measure of the cosmic ray ionization rate would be provided by the observed HD abundances, if the overall deuterium abundance D]/[HJ in interstellar clouds were known. Alternatively, the values of (o derived from the oxygen chemistry can be used to infer DJ/ H from the measured HD column densities. The derived deuterium abundances in the models of vDB are in the range (0.5-2.0)xl0 for the various clouds, consistent with other estimates of the deuterium abundance in the interstellar medium, [D]/[H]=( 1.5 1.0) X 10" (Vidal-Madjar and Gry 1984). The models of the f Per cloud favor the upper part of this range, whereas those of the ( Oph cloud give somewhat lower values, due to the order of magnitude lower HD column density. A similar deuterium abundance for the ( Oph cloud has been obtained by VRA. 

The presence of LMs with abundances of a few times 10 has little effect on the abundances of small molecules such as CH, OH, CO and CN. However, as Table 6 shows, the LMs strongly affect the amounts of HD, and consequently the inferred deuterium abundances in diffuse clouds. The decrease in the HD abundance is caused mainly by the large reduction of both the H" " and D" concentrations through reactions (12), which initiate the formation of HD through reactions (5) and (6). As discussed above, the HD abundance depends on two disposable parameters the deuterium abundance Sp, where Sp= refers to [Dj/ H] = 1.5 X 10 , and the cosmic ray ionization rate Jo- If (o Is chosen to be constrained by the observed OH column densities, then the order of magnitude smaller HD column density observed toward f Oph compared with ( Per would imply a four times smaller deuterium abundance for the former cloud, in the absence of LMs. However, if LMs are included at the levels suggested by the atomic ionization balances, the inferred deuterium abundances for the two clouds are virtually the same, D]/[Hj=(1.5 0.5)xI0 . 

Indriolo N, McCall BJ (2012) Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observations of Hs. Astrophys J 745 91... 

Wakelam V, Herbst E, Selsis F, Massacrier G (2006) Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds. Astron Astrophys 459 813-820. doi 10.1051/0004-6361 20065472... 

Leach S (2012) Why COBE and CN spectroscopy cosmic background radiation temperature measurements differ, and a remedy. Mon Not R Astron Soc 421 1325-1330 Indriolo N, McCall BJ (2012) Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observation of Ha" ". Astrophys J 745 91-1-17... 

The chemistry of gas-phase reactions, either in the interstellar medium or in stellar atmospheres, is mediated by the abundance of ions. These can be formed in several ways by cosmic-ray ionization, or by the direct photoionization of the atoms involved in the reactions with subsequent charge transfer to the molecules. Ionic reactions are generally exothermic and so occur efficiently at low temperature. In the presence of an ion, a neutral molecule or atom develops an induced dipole which increases its capture cross section. Thus the reactions occur quickly and lead to stable states, in addition to allowing the molecule to form in a radiatively unstable excited state which, upon decaying, radiates the energy of formation away from the site of the reaction. 

The most important reaction network is initiated by the ionization of nitrogen by charge exchange with He+ or by cosmic-ray ionization of N ... 

Another ion process which has a role in polar-night stratospheric chemistry is hydroxyl radical formation by ion chemistry driven by galactic cosmic rays. The rate of this ion-assisted OH formation is one to two OH molecules per ionization event or 40-80 OH molecules cm s at high geomagnetic latitudes. Because the galactic cosmic ray ionization rate varies with solar activity, an 11-year modulation will be induced. A more detailed discussion of ion and aerosol processes will be given in Ref. [10]. 

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