首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Measured rates are presented for the reaction of He+ ions with H2 (and D2) molecules to form H+, H2+, and HeH+ ions, as well as for the subsequent reactions of H+ and HeH+ ions with H2 to form H3+. The neutralization of H3+ (and H5+) ions by dissociative recombination with electrons is shown to be fast. The reaction He+ + H2 is slow (k = 1.1 × 10?13 cm3/sec at300°K) and produces principally H+ by the dissociative charge transfer branch. It is concluded that there may be a serious bottleneck in the conversion of two of the primary ions of the upper Jovian ionosphere, H+ and He+ (which recombine slowly), to the rapidly recombining H3+ ion (α[H3+]?3.4 × 10?7 cm3/sec at 150°K).  相似文献   

2.
S.K. Atreya  T.M. Donahue 《Icarus》1975,24(3):358-362
Model ionospheres are calculated for Saturn, Uranus, and Neptune. Protons are the major ions above 150 km altitude measured from a reference level where the hydrogen density is 1 × 1016 molecules cm?3, while below 150 km quick conversion of protons to H3+ ions by a three-body association mechanism leads to a rapid removal of ionization in dissociative recombination of H3+. Electron density maxima are found at about 260 km for Saturn and Uranus and 200 km for Neptune. Present knowledge of the physical and chemical processes in the atmospheres of these planets suggests that their ionospheres probably will not be Jupiter-like.  相似文献   

3.
  相似文献   

4.
The dissociative recombination coefficients α for capture of electrons by H3+ and H5+ ions have been determined as a function of electron temperature Te using a microwave afterglow-mass spectrometer apparatus. At ion and neutral temperatures Tu+ = Tn = 240 K, the coefficient α (H3+) is found to vary slowly with Te at first, decreasing from 1.6 × 10?7 cm3/s at Te = 240 K to 1.2 × 10?7 cm3/s at Te = 500 K, thereafter falling as Te?1 over the range 500 K ? Te, ? 3000 K. These results, which have a ± 20% uncertainty, agree satisfactorily over the common energy range (0.03–0.36 eV) with the recombination cross sections determined in merged beam measurements by Auerbach et al. At T+ = Tn = 128 K, the coefficient α(H5+) is found to be (1.8 ± 0.3) × 10?6 [Te(K)/300]?0.69 cm3/s over the range 128 K ? Te ? 3000 K, with a more rapid decrease, as Te?1, between 3000 K and 5500 K. The implications of these results for modelling planetary atmospheres and interstellar clouds are briefly touched on.  相似文献   

5.
The evolution of the charged particles are followed during contraction of a model of an interstellar cloud, with initial density number of n = 10 cm–3. The contraction is followed up to density increase by five orders of magnitude. Special care is given to the details of the negative ions. In addition, we have tested the ambipolar diffusion according to the results of the ion density.The results predict the importance of atomic ions in the diffuse regions. H+ and C+ are distinctly enhanced in the beginning of contraction but decrease as contraction proceeds. Molecular ions enhance as contraction proceeds and becomes important in dense regions. The most enhanced molecular ions are HCO+, O2 +, C2H3 +, H3O+ and SO+, H3 + is less abundant. The atomic ions (except metalic ions) decrease noticeably as density increases. In general the negative ions are of negligible fractional abundances. It has also been found that the time of ambipolar diffusion is shorter than the dynamical time, hence the magnetic field should be weakened in the central core as the central density increases to n = 104 cm–3.  相似文献   

6.
The abundances of PH3, CH3D, and GeH4 are derived from the 2100- to 2250-cm?1 region of the Voyager 1 IRIS spectra. No evidence is seen for large-scale variations of the phosphine abundance over Jovian latitudes between ?30 and +30°. In the atmospheric regions corresponding to 170–200°K, the derived PH3/H2 value is (4.5 ± 1.5) × 10?7 or 0.75 ± 0.25 times the solar value. This result, compared with other PH3 determinations at 10 μm, suggests than the PH3/H2 ratio on Jupiter decreases with atmospheric pressure. In the 200–250°K region, we derive, within a factor of 2, CH3D/H2 and GeH4/H2 ratios of 2.0 × 10?7 and 1.0 × 10?9, respectively. Assuming a C/H value of 1.0 × 10?3, as derived from Voyager, our CH3D/H2 ratio implies a D/H ratio of 1.8 × 10?5, in reasonable agreement with the interstellar medium value.  相似文献   

7.
The temperature dependence of the binary recombination coefficient, α2, for the reaction NO++NO2? → products has been obtained over the range 185–530 K. It is found that the corresponding mean cross section σ is described by the power law σ ? A · T?0.9, and that α2 ? B · T?0.4. Data has also been obtained for two cluster ion recombination reactions which indicate that their recombination cross sections are only about 40% larger than for the parent ions at a given temperature, the cross sections for these reactions also apparently increasing with decreasing temperature. In the light of this data and by considering the most probable positive and negative ions existing at various altitudes up to 90km in the atmosphere, the most appropriate ionic recombination coefficients in various altitude ranges are deduced. Thus, between 30 and 90 km, where the recombination process is two-body, the coefficient varies over the narrow range 5–9 × 10?8 cm3s?1, while below 30 km the process is predominantly three-body with an effective two-body rate increasing rapidly to a maximum value ≈3 × 10?6 cm3s?1 in the troposphere, these deductions being based on published laboratory determinations of three-body recombination coefficients.  相似文献   

8.
The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H2O+with ground-based observations of H2O+from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H2O+and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.  相似文献   

9.
Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH3CN is a viable candidate molecule for the species X in the observed stratospheric ion series H+ (H2On (X)m and (ii) to determine the binary mutual neutralization rate coefficients αi for the reactions ofH+ (H2O4 and H+(H2O)(CH3CN)3 with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH3CN is indeed a viable candidate for X and from (ii) that the αi for stratospheric ions are within the limited range (5–6) × 10?8 cm3 s?1.  相似文献   

10.
S.K. Atreya  T.M. Donahue 《Icarus》1975,25(2):335-338
The role of hydrocarbons as a possible sink for H+ and H3+ ions in the lower ionosphere of the outer planets is examined. Calculations indicate that H+ and H3+ are efficiently converted to hydrocarbon ions on reaction with methane. The terminal ions, CH5+ and C2H5+ are rapidly neutralized in dissociative recombination with electrons. Extreme ultraviolet photolysis of hydrocarbons as a potential additional source of lower elevation ions in investigated.  相似文献   

11.
A radiative seasonal model which incorporates a multilayer radiative transfer treatment at wave-lengths longward of 7 μm is presented and applied to Saturn's stratosphere. Opacities due to H2-He, CH4, C2H2, and C2H6 are included. Season-dependent insolation is shown to produce a strong hemispheric asymmetry decreasing with depth at the Voyager encounter times, and seasonal amplitudes of 30°K at the poles are predicted in the high stratosphere. The ring-modulated dependence of the insolation and the orbital eccentricity are shown to have a significant effect. Calculations agree closely with the Voyager 1 and 2 radio occultation ingress profiles recorded at 76°S and 36.5°S for CH4/H2 = 3.5 + 1.4/? 1.0 × 10?3;the estimated errors include modeling systematic errors and uncertainties in the occultations profiles. The possible role of aerosols in the stratospheric heating is analyzed. The Voyager 2 egress profile recorded at 31°S cannot be reproduced by calculations. Some constraints on the C2H2 and C2H6 abundances are derived. The upper portion of the occultation profiles (p < 3mbar) can be matched for C2H2/H2 = 1.0 + 1.3/?0.6 × 10?7, C2H6/H2 = 1.5 + 1.8/?0.9 × 10?6 at 76°S and C2H2/H2 = 4 + 6/?4 × 10?8, C2H6/H2 = 6 + 9/?6 × 10?7 at 36.5°N. At the northern occultation latitude, the discrepancy with the concentrations derived from analysis of IRIS spectra by R. Courtin, D. Gautier, A. Marten, B. Bézard, and R. Hanel (1984, Astrophys. J.287) can be explained by a sharp variation of the mixing ratios of these gases with altitude in the upper stratosphere. Other interpretations are discussed.  相似文献   

12.
Mass spectrometric measurements of the neutrals and positive ions in the Space Shuttle environment have indicated the presence of large amounts of contaminants, particularly CO2+ and H2O+. The ionic abundances are analyzed in terms of known ion-neutral kinetics and from this analysis absolute abundances of CO2 and H2O are calculated. The implication of these results for optical measurements is considered.  相似文献   

13.
A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D+ (and not H2+). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (～ 130 km) for the two orbits. The H2+ contribution to the mass-2 ion density is less than 10%, and the H2 mixing ratio must be <0.1 ppm at 130 km altitude. The He+ data require a downward He+ flux of ～2 × 107 cm?2 sec?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O+ ion flow.  相似文献   

14.
RX J1856.5–3754 is one of the brightest nearby isolated neutron stars, and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5–3754, within the uncertainties. In our simplest model, the best-fit parameters are an interstellar column density N H≈1×1020 cm?2 and an emitting area with R ∞≈17 km (assuming a distance to RX J1856.5–3754 of 140 pc), temperature T ∞≈4.3×105 K, gravitational redshift z g ～0.22, atmospheric hydrogen column y H≈1 g cm?2, and magnetic field B≈(3–4)×1012 G; the values for the temperature and magnetic field indicate an effective average over the surface.  相似文献   

15.
We have constructed a chemical reaction model in a contracting interstellar cloud including 104 species which are involved in a network of 557 reactions. The chemical kinetic equations were integrated as a function of time by using gear package. The evolution of the system was followed in the density range 10–107 particles cm-3.The calculated fractional abundances of the charged species are in good agreement with those given by other investigators. The charge density has been followed in diffuse, intermediate and dense regions. The most dominant ionic species are metallic ions, HCO+ and H 3 + in the shielded regions and atomic ions H+, C+, Si+, He+, S+ and metal ions in the diffuse and intermediate regions. The abundances of negatively charged ions were found to be negligible. The results of the calculations on the different metallic ions are interpreted.  相似文献   

16.
The University of Wisconsin–Madison and NASA–Goddard conducted acomprehensive multi-wavelength observing campaign of coma emissionsfrom comet Hale–Bopp, including OH 3080 Å, [O I] 6300 Å H2O+ 6158 Å, H Balmer-α 6563 Å, NH2 6330 Å, [C I] 9850 ÅCN 3879 Å, C2 5141 Å, C3 4062 Å,C I 1657 Å, and the UV and optical continua. In thiswork, we concentrate on the results of the H2O daughter studies.Our wide-field OH 3080 Å measured flux agrees with other, similarobservations and the expected value calculated from published waterproduction rates using standard H2O and OH photochemistry.However, the total [O I] 6300 Å flux determined spectroscopically overa similar field-of-view was a factor of 3-4 higher than expected.Narrow-band [O I] images show this excess came from beyond theH2O scale length, suggesting either a previously unknown source of[O I] or an error in the standard OH + ν→ O(1 D) + H branching ratio. The Hale–Bopp OH and[O I] distributions, both of which were imaged tocometocentric distances >1 × 106 km, were more spatiallyextended than those of comet Halley (after correcting for brightnessdifferences), suggesting a higher bulk outflow velocity. Evidence ofthe driving mechanism for this outflow is found in the Hα lineprofile, which was narrower than in comet Halley (though likelybecause of opacity effects, not as narrow as predicted by Monte-Carlomodels). This is consistent with greater collisional coupling betweenthe suprathermal H photodissociation products and Hale–Bopp's densecoma. Presumably because of mass loading of the solar wind by ionsand ions by the neutrals, the measured acceleration of H2O+ downthe ion tail was much smaller than in comet Halley. Tailwardextensions in the azimuthal distributions of OH 3080 Å,[O I], and [C I] , as well as a Doppler asymmetry in the[O I] line profile, suggest ion-neutral coupling. While thetailward extension in the OH can be explained by increased neutralacceleration, the [O I] 6300 Å and [C I] 9850 Å emissions show 13%and >200% excesses in this direction (respectively), suggesting anon-negligible contribution from dissociative recombination of CO+and/or electron collisional excitation. Thus, models including theeffects of photo- and collisional chemistry are necessary for the fullinterpretation of these data.  相似文献   

17.
Lis  D. C.  Mehringer  D. M.  Benford  D.  Gardner  M.  Phillips  T. G.  Bockelée-Morvan  D.  Biver  N.  Colom  P.  Crovisier  J.  Despois  D.  Rauer  H. 《Earth, Moon, and Planets》1997,78(1-3):13-20
We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H13CN, and H3O+ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) × 10-4. HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 ± 0.2) × 10-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) × 10-4. H3O is detected for the first time from the ground. The H13CN (3-2)transition is also detected and the derived HCN/H13CN abundance ratio is 90 ± 15, consistent with the terrestrial12C/13C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO+, CO+, and CN(the last two are first detections in a comet at radio wavelengths). This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

18.
Loss processes which remove Si+ ions selectively relative to other meteor-derived atomic ions in the E- and D-regions of the ionosphere have been identified and measured in the laboratory. The major Si+ loss in the E-region is the reaction Si+ + H2O → HSiO+ + H (1) with a rate constant 2.3 ± 0.9 × 10?10 cm3s?1 at 300 K. The corresponding reactions with Fe+, Mg+ and other metallic meteor ions are endothermic. Presumably (1) is followed by a fast dissociative-recombination with electrons to produce neutral SiO or Si. At lower altitudes Si+ ions associate in a three-body reaction with O2 with a much larger rate constant than the corresponding associations of Fe+ and Mg+ with O2.  相似文献   

19.
We investigated the chemical evolution in IC 63 nebula, a photo-dominated region (PDR). The chemical structure and the ionization state depend directly on the intensity of the incident UV radiation. The electron density is also affected by the incident UV radiation. It decreases gradually with the increase of the depth in the cloud varying from 5.9×10-5 at the surface to 9.6×10-9 in the core. Ionic carbon(C+) dominates the electron density in the outer region while ionic metals and other ions (H+, CH2D+, and HCO+) are the most dominant in the deepest region. Our results at A V = 6.7 mag are in good agreement with observations except in the case of H2S, where the calculated value is lower than the observed value by about two orders of magnitude.  相似文献   

20.
Measured fractional abundances for stratospheric positive ions are reported for the first time. The measurements which were obtained from balloon-borne ion mass spectrometer experiments relied on recent simulation studies of electric field induced cluster ion dissociation conducted at our laboratory.The ion abundance data provide strong support for identifications of the observed ions as H+(H2O)n and Hx+xL(H2O)m proposed previously. Moreover, it is found that x most likely cannot be identified as NaOH or MgOH which implies that gaseous metal compounds do not exist in the middle stratosphere in significant abundances.Implications of the present findings for the composition and chemistry of stratospheric ions as well as for stratospheric aerosols are discussed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号