Cosmic ray ionization rate

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

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. 

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. 

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... 

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]. 

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)... 

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). 

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. ... 

Latitude distribution of cosmic ray dose rates outdoors at sea level. Population-weighted average in the world ionization 30.9 nSv h neutron 5.5 nSv h (UNSCEAR 2000c)... 

Figure 7.7. Rate of ionization provided by cosmic rays at different geomagnetic latitudes (0, 50, and 70°) and for minimum and maximum levels of solar activity. These values are compared to the ion pair production producted by direct and diffuse Lyman a. From Rosenberg and Lanzerotti (1979).

Figure 7.8. Rate of ionization by cosmic rays in the lower stratosphere and troposphere. From Brasseur and Nicolet (1973).

Figure 7.11. Ionization rates associated with some solar proton events, compared to that due to cosmic rays. Adapted from Solomon et al. (1983).

In the stratosphere, ionization is produced by cosmic rays. Many of the reaction processes are similar to those occurring in the D-region. The rate of formation of NO+, however, becomes negligible the precursor ions are O J and (which immediately forms by charge exchange with O2). The reaction chain leading to water vapor clusters is, however, modified by the presence of certain stratospheric neutral species. The reaction of O4" with 03 must also be mentioned ... 

In the dense clouds which we are most interested in, the star radiation is completely shielded and the ionization occurs through cosmic ray bombardment. The universal cosmic ray flux is fairly well established 3 to be C = 10 7/sec. This should be compared with 0.2/sec given earlier for the laboratory plasma. The degree of ionization in dense clouds can be estimated by equating the production rate and the destruction rate by electron recombination. 

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