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1.
We present a Monte Carlo (MC) model of [OI] 6300 Å and [SII] 6716 Å emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO2, O, SO, S, and O2 in Io’s atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300 Å emission. The [OI] radiation lifetime of ∼134 s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically <40 km) and in volcanic plumes (Pele, Prometheus, etc.) the number density is large enough (>4 × 109 cm−3) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io’s perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation’s viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake’s observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of ∼3, most likely because our model does not include direct dissociation-excitation of SO2 and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io’s changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731 Å emission is much weaker than that of [OI] even if the [SII] excitation cross section is 103 times larger than excitation to [OI]. This is because the density of S+ is much less than that of O and because the Einstein-A coefficient of the [SII] emission is a factor of ∼10 smaller than that of [OI].  相似文献   

2.
A rocket experiment was conducted which measured the infrared bands of the excited hydroxyl radical in the night airglow. The OH emission was found in a layer centered at 87 km having a half-width of 6 km and a total emission of 1.1 MR. The atomic oxygen altitude profile, ranging from 1.3 × 1010 atoms/cm3 at 83 km to 3 × 1011 atoms/cm3 at 90 km is determined from the hydroxyl airglow measurements. This derivation is based on the steady state balance between ozone formation from atomic oxygen and its destruction by hydrogen which produces the OH infrared emission.  相似文献   

3.
Simultaneous measurements of the nightglow profiles of the O2(b1Σg+?X3Σg?) A-band, the atomic oxygen green line and the OH (8?3) Meinel band are presented. The altitude profiles are used to determine both the excitation mechanisms for the oxygen emissions and the atomic oxygen altitude distribution. It is shown that the measurements are consistent with a green line excitation through the Barth mechanism and that the molecular oxygen emission is excited through oxygen recombination and the reaction between OH1 and atomic oxygen. The derived atomic oxygen concentrations,6.2 × 1011cm?3at 98km, are consistent with the Jacchia (1971) model.  相似文献   

4.
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(T s /100) cm−2. The HI absorption is redshifted by ∼400km s−1 compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster potential.  相似文献   

5.
This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the infrared atmospheric system of O2 at 1.27 μm. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO2 on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O2() may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO2 vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO2 density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O2 limb airglow. In other cases, an average CO2 profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1×1011 to about 5×1011 cm−3 with a global mean of 2.2×1011 cm−3. These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O2 infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models.  相似文献   

6.
The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1Δ) nightglow distribution at 1.27 μm in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 μm. Given all the O2(a1Δ) intensity profiles, O2(a1Δ) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O2(a1Δ) hemispheric average density is 2.1 × 109 cm?3, with a maximum value of 6.5 × 109 cm?3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO2 profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 1011 cm?3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 1011 cm?3 at 104 km and 2.0 × 1011 cm?3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17–27], respectively. Comparing the oxygen density map derived from the O2(a1Δ) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model.  相似文献   

7.
We investigate the dissociative recombination contribution to I(5577) and I(6300) of [OI] as a function of low energy cutoff for two measured solar proton spectra. The volume ionization rate profiles used in the calculation are obtained using a detailed atomic cross section approach in the continuous slowing down approximation. The ratio of the dissociative recombination contribution to the direct impact contribution for both the 5577 Å and 6300 Å [OI] emissions is found to be dependent upon the low energy cutoff. This ratio has a nominal value of ～2.0 for the 5577 Å [OI] emission and ～0.25 for the 6300 Å [OI] emission. The I(5577)/I(3914) and I(6300)/I(3914) ratios including the direct and dissociative recombination contributions are strongly dependent upon the low energy cutoff of the spectrum. We have also investigated F-layer enhancements resulting from the low energy spectrum component. For the Mizera et al. (1972) spectrum with a low energy cutoff of 12.4 keV, we find an NmF2 of ～4.5 × 103 electrons/cm3 or about 10 per cent of the ionization required to maintain the dip pole at a value of 5 × 104 electrons/cm3. Extension of the cutoff to 1 keV results in ～1 × 104 electrons/cm3, or about 20 per cent of the required maintenance ionization.  相似文献   

8.
Observation of the hydrogen corona with SPICAM on Mars Express   总被引：1，自引：0，他引：1  
J.Y. Chaufray  E. Quémerais 《Icarus》2008,195(2):598-613
A series of seven dedicated Lyman-α observations of exospheric atomic hydrogen in the martian corona were performed in March 2005 with the ultraviolet spectrometer SPICAM on board Mars Express. Two types of observations are analyzed, observations at high illumination (for a solar zenith angle SZA equal to 30°) and observations at low illumination (for a solar zenith angle equal to 90° (terminator), and near the south pole). The measured Lyman-α emission is interpreted as purely resonant scattering of solar photons. Because the Lyman-α emission is optically thick, we use a forward model to analyze this data. Below the exobase, the hydrogen density is described by a diffusive model and above the exobase, it follows Chamberlain's approach without satellite particles. For different hydrogen density profiles between 80 and 50,000 km, the volume emission rates are computed by solving the radiative transfer equation. Such an approach has been used to analyze the Mariner 6, 7 exospheric Lyman-α data during the late 1960s. A reasonable fit of the set of observations is obtained assuming an exobase temperature between 200 and 250 K and an exobase density of ∼1-4 × 105 cm−3 in good agreement with photochemical models. However, when considering the average exospheric temperature of 200 K measured by other methods [Leblanc, F., Chaufray, J.Y., Witasse, O., Lilensten, J., Bertaux, J.-L., 2006a. J. Geophys. Res. 111 (E9), doi:10.1029/2005JE002664. E09S11; Leblanc, F., Chaufray, J.-Y., Bertaux, J.-L., 2007. Geophys. Res. Lett. 34, doi:10.1029/2006GL028437. L02206; Bougher, S.W., Engel, S., Roble, R.G., Foster, B., 2000. J. Geophys. Res. 105, 17669-17692; Mazarico, E., Zuber, M.T., Lemoine, F.G., Smith, D.E., 2007. J. Geophys. Res. 112, doi:10.1029/2006JE002734. E05014] a supplementary hot population is needed above the exobase to reconcile Lyman-α measurements with these previous measurements, particularly for the observations at low SZA. In this case, the densities and temperatures at the exobase for the two populations are 1.0±0.2×105 cm−3 and T=200 K and 1.9±0.5×104 cm−3 and T>500 K for the cold and hot populations respectively at low SZA. At high SZA, the densities and temperatures are equal to 2±0.4×105 cm−3 and T=200 K and n=1.2±0.5×104 cm−3 and T>500 K. These high values of the hot hydrogen component are not presently supported by the theory. Moreover, it is important to underline that the two population model remains relatively poorly constrained by the limited range of altitude covered by the present set of SPICAM measurements and cannot be unambiguously identified because of the global uncertainty of our model and of SPICAM measurements. For a one population solution, an average water escape rate of 7.5 × 10−4 precipitable μm/yr is estimated, yielding a lifetime of 13,000 years for the average present water vapor content of 10 precipitable microns.  相似文献   

9.
We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 1015 cm−2 in close agreement with the value provided by the VTS3 empirical atmospheric model.  相似文献   

10.
Observations of optical depth and scattering by instrumentation onboard the Huygens probe have been used by Tomasko et al. [Tomasko et al., 2005. Rain, winds and haze during Huygens probe's descent to Titan's surface. Nature 438 (8), 765-778] to deduce that the size and abundance of Titan aerosols could be nearly independent of altitude. Here we show that by assuming a constant mass flux with altitude and using the measured optical depth as a constraint, we obtain more realistic size and abundance distributions. In particular, the calculated abundance decreases from 3.5×107 m−3 at 100 km to 8×106 m−3 near the surface while the particle radius varies from 0.25 μm at 150 km to 1.1 μm at the surface. These distributions are consistent with the reported measurements for these quantities. Our results are then employed to compute electron and ion densities and conductivities for various solar UV photoelectron emission thresholds. Our model shows that to get agreement with the published (preliminary) conductivity measurements, photoemission cannot be an important source of electrons and ions. To get agreement with the electron and ion conductivity observations, both an additional population of aerosol embryos above 50 km and a trace amount of an electrophillic molecular species below 50 km are needed.  相似文献   

11.
Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan’s atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 108 cm2 s−1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H2. Production of both H and H2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 109 cm−2 s−1 and the latest retrieved value of the H2 mole fraction in the stratosphere: (1.1 ± 0.1) × 10−3. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C3H4, C4H2 and C6H6. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.  相似文献   

12.
New measurements of the Herzberg I emission height profile in the night airglow are reported and indicate a peak emission height near 96 km in agreement with previous measurements. Using an atomic oxygen concentration profile determined from the oxygen green line profile measured on the same rocket it is concluded that the O2(A3Σu+) state is not excited in the direct three body recombination of atomic oxygen. It is suggested that the excitation mechanism is a two step process, similar to the Barth mechanism for the atomic oxygen green lineand that the excited intermediate state is C3Δu.  相似文献   

13.
Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10−9 g cm−2 for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm−3) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10−10 g cm−2), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm−3 vs. a few times 0.1 cm−3 for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).  相似文献   

14.
Thirteen lines of the CO band near 4.7 μm have been observed on a jovian hot spot at a resolution of 0.045 cm−1. The measured line profiles indicate that the CO mole fraction is 1.0±0.2 ppb around the 6-bar level and is larger in the upper troposphere and/or stratosphere. An external source of CO providing an abundance of 4+3−2×1016 molecules cm−2 is implied by the observations in addition to the amount deposited at high altitude by the Shoemaker-Levy 9 collision. From a simple diffusion model, we estimate that the CO production rate is (1.5-10)×106 molecules cm−2 s−1 assuming an eddy diffusion coefficient around the tropopause between 300 and 1500 cm2 s−1. Precipitation of oxygen atoms from the jovian magnetosphere or photochemistry of water vapor from meteoroidal material can only provide a negligible contribution to this amount. A significant fraction of the CO in Jupiter's upper atmosphere may be formed by shock chemistry due to the infall of kilometer- to subkilometer-size Jupiter family comets. Using the impact rate from Levison et al. (2000, Icarus143, 415-420) rescaled by Bottke et al. (2002, Icarus156, 399-433), this source can provide the observed stratospheric CO only if the eddy diffusion coefficient around the tropopause is 100-300 cm2 s−1. Higher values, ∼700 cm2 s−1, would require an impact rate larger by a factor of 5-10, which cannot be excluded considering uncertainties in the distribution of Jupiter family comets. Such a large rate is indeed consistent with the observed cratering record of the Galilean satellites (Zahnle et al. 1998, Icarus136, 202-222). On the other hand, the ∼1 ppb concentration in the lower troposphere requires an internal source. Revisiting the disequilibrium chemistry of CO in Jupiter, we conclude that rapid vertical mixing can provide the required amount of CO at ∼6 bar for a global oxygen abundance of 0.2-9 times the solar value considering the uncertainties in the convective mixing rate and in the chemical constants.  相似文献   

15.
Night airglow of oxygen 130.4 and 135.6 nm emissions was measured by a spectrophotometer aborad an S520 sounding rocket, launched at 19:50 JST (10:50 UT) on 14 February, 1982 from Kagoshima, Japan. The altitude variation of the emissions was obtained from 110 to 266 km at zenith angles of 35.5°±4°. The emission intensity around 260 km was about 160R and is roughly compatible with model calculations taking account of O++e – radiative recombination as well as O+–O– mutual neutralization. Some excess of about 50R, compared to the model calculation, was observed around 200 km. Possible explanations of the excess are: (i) remnant oxygen ions during the transition period from day to night and (ii) diffuse radiation from the background sky. Model calculations taking account of remnant oxygen ions were also performed by adding an excess electron density to the original density profile. However, it was found that an unreasonably large electron density is required around 200 km (5×105 cm–3) to produce the observed intensity. It is also probable that some contribution from the background sky is present in the observed intensity.  相似文献   

16.
R. de Kok  P.G.J. Irwin  N.A. Teanby 《Icarus》2008,197(2):572-578
In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C4N2 gas was determined to be 9×10−9, which is less than required to make the C4N2 cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm−1 and is located around an altitude of 140 km. The upper limit for propionitrile (C2H5CN) gas shows that the feature around 220 cm−1 is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm−1 is not consistent with that of pure HCN ice at 172 cm−1 and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.  相似文献   

17.
Doyle  J. G.  Banerjee  D.  Perez  M. E. 《Solar physics》1998,181(1):91-101
Line-width measurements of the coronal ion Siviii confirm earlier observations which show an increase in the non-thermal velocity above the solar limb. The present data, taken at the equatorial limb, show an increase from 24 km s-1 at the limb to 28 km s-1 some 25000 km above the limb. The electron density as measured from the Siviii line pair shows a decrease from 3.5 × 108 cm-3 to 1.8 × 108 cm-3 over the same distance. These data imply that the non-thermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with that predicted for undamped radially propagating Alfvén waves.  相似文献   

18.
P. Hedelt  Y. Ito  L. Esposito 《Icarus》2010,210(1):424-435
Based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini orbiter, Titan’s atomic hydrogen exosphere is investigated. Data obtained during the T9 encounter are used to infer the distribution of atomic hydrogen throughout Titan’s exosphere, as well as the exospheric temperature.The measurements performed during the flyby are modeled by performing Monte Carlo radiative transfer calculations of solar Lyman-α radiation, which is resonantly scattered on atomic hydrogen in Titan’s exosphere. Two different atomic hydrogen distribution models are applied to determine the best fitting density profile. One model is a static model that uses the Chamberlain formalism to calculate the distribution of atomic hydrogen throughout the exosphere, whereas the second model is a Particle model, which can also be applied to non-Maxwellian velocity distributions.The density distributions provided by both models are able to fit the measurements although both models differ at the exobase: best fitting exobase atomic hydrogen densities of nH = (1.5 ± 0.5) × 104 cm−3 and nH = (7 ± 1) × 104 cm−3 were found using the density distribution provided by both models, respectively. This is based on the fact that during the encounter, HDAC was sensitive to altitudes above about 3000 km, hence well above the exobase at about 1500 km. Above 3000 km, both models produce densities which are comparable, when taking into account the measurement uncertainty.The inferred exobase density using the Chamberlain profile is a factor of about 2.6 lower than the density obtained from Voyager 1 measurements and much lower than the values inferred from current photochemical models. However, when taking into account the higher solar activity during the Voyager flyby, this is consistent with the Voyager measurements. When using the density profile provided by the particle model, the best fitting exobase density is in perfect agreement with the densities inferred by current photochemical models.Furthermore, a best fitting exospheric temperature of atomic hydrogen in the range of TH = (150-175) ± 25 K was obtained when assuming an isothermal exosphere for the calculations. The required exospheric temperature depends on the density distribution chosen. This result is within the temperature range determined by different instruments aboard Cassini. The inferred temperature is close to the critical temperature for atomic hydrogen, above which it can escape hydrodynamically after it diffused through the heavier background gas.  相似文献   

19.
Measurements from the 1225 to 1340 Å region by the ultraviolet detectors on Mars-3 are presented. Model calculations of the intensity of the OI triplet lines at 1304 Å are compared with the measurements made on December 27, 1971, and February 17, 1972. Agreement is found between experimental data and a model in which the neutral oxygen density at 100 km is 2–8 × 109 cm?3.  相似文献   

20.
Photoelectron peaks in the atmosphere of Mars caused by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons have been observed by the Electron Spectrometer (ELS), a component of the Mars Express (MEx) Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment. Ionization mostly occurs at the Mars exobase with the majority of the photoionized electron flux trapped in the remanent and induced magnetic field, with a portion of that flux escaping the planet down its tail. Since Mars is overall charge neutral, the number of electrons must be identical to the number of ion charges which escape the planet. An estimate of the fraction of the total number of escaping electrons is obtained for the year 2004, specifically those produced by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons. In achieving this process, an illustrative example pass is used to show how the electron spectrum is adjusted for the potential on the spacecraft; then the region of the electron spectrum which shows photoelectron peaks is integrated over energy, yielding a flux of 5.74 × 106 electrons/(cm2 s sr). This technique is then applied to a subset of 22 sample averaged spectra from the 2004 data (5 January 2004 through 25 January 2005), yielding an average result of 4.15 × 106 electrons/(cm2 s sr) for the 22 cases. The observation cone of 33.75° is used to integrate over solid angle (assuming the flux is constant), giving 4.39 × 106 electrons/(cm2 s). This average value was taken as representative of the full data interval. Frequency of occurrence statistics showing about a 6.2% occurrence rate for the 2004 data is applied to give an average escape flux from Mars of 2.72 × 105 electrons/(cm2 s) during 2004. By estimating the outflow area as 1.16 × 1018 cm2 at X = −1.5 RMars the electron escape rate of 3.14 × 1023 electrons/s is obtained. Thus about 9.92 × 1030 electrons or 16.5 Mmole of electrons escaped Mars during 2004 due to the ionization of carbon dioxide and atomic oxygen by the He 30.4 nm line. Due to the caveats of the analysis, these derived escape rates should be considered lower limits on the total electron escape rate from Mars.  相似文献   

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