Determination of magnetic field direction in a comet's head

The Stokes parameters of resonance radiation scattered by a Na atom with the angular momentum F aligned by directed unpolarized radiation in a magnetic field H ～ 10^?5?10^?1 Oe are presented. An influence of the orientation of the magnetic field on these parameters are studied; the intensity ratio $\text{I(D}{}_2\text{)}\text{I(D}{}_1\text{)}$ changes within ±5%, and the polarization degree P(D₂) within ±25%. Measurements of $\text{I(D}{}_2\text{)}\text{I(D}{}_1\text{)}$ and P(D₂), if the geometry of scattering is known, may give information on the direction of the magnetic field in the sodium atmospheres of comets, as well as Io's sodium cloud or man-made cosmic clouds.     

The DH ratio in the atmosphere of Uranus: Detection of the R5(1) line of HD

Observations of Uranus during the 1975, 1976, and 1978 apparitions reveal a weak absorption at the wavelength of the R₅(1) line of HD with equivalent width $\text{1.0 ± 0.4}\text{m}\text{A}\text{?}$. The $\text{D}\text{H}$ ratio in Uranus' atmosphere implied by this line and other published spectra is (4.8 ± 1.5) × 10^?5, and may not be significantly different from that in the atmospheres of Jupiter and Saturn. In addition, the spectra exhibit two weak absorption at $\text{6044.76 ± 0.02}\text{and}\text{6045.54 ± 0.02}\text{A}\text{?}$ which we were unable to identify. No trace of absorption is visible near these wavelengths or near the HD wavelength in a laboratory spectrum of 4.92 km-am CH₄ which we obtained in an attempt to identify these absorption features and to verify that the HD feature does not arise from CH₄.     

Diameter to depth dependence of impact craters

Laboratory impact experiments in the micron to millimeter projectile size range in silicate and metal targets have been performed in order to clarify the still ambigously interpreted velocity dependence of the crater diameter to depth ratios $\text{(}\text{D}\text{T}\text{)}$. The experimental results clearly show the independence of the $\text{D}\text{T}$ ratio of velocities above a threshold velocity of 3–4 km s^?1. The $\text{D}\text{T}$ ratio is a function of target properties and of projectile density ?. For a given target, the resulting approximate relation is $\text{D}\text{T}\text{～ ?}{}^{\mathrm{?\alpha }}$ with α varying between $\text{1}\text{2}\text{and}\text{1}\text{5}$.     

The abundance of water on Jupiter from the voyager IRIS data at 5 μm

The abundance of H₂O is derived from the 1900- to 2100-cm^?1 region of the Voyager 1 IRIS spectra. Scale variations of about a factor of 2 are seen in the water abundance between the North and South Equatorial Belts. Averaged over the full disk, the mixing ratio is $\text{H}{}_2\text{O}\text{H}{}_2\text{=(4.0±1.0) × 10}{}^{\mathrm{?6}}$, if H₂O is uniformly mixed in the atmospheric region having temperatures of 230 to 270°K; this result implies a solar depletion by a factor of 100 in this region. In the belts, the best agreement is obtained for a H₂O/H₂ mixing ratio of 4.0 × 10^?6 in the NEB and 7.2 × 10^?6 in the SEB, assuming a constant mixing ratio.     

Detection of [Ol] 1S-1D in Comet IRAS-Araki-Alcock

Spectra of the $\text{[}\text{OI}\text{]}{}^1\text{D-}{}^3\text{P}$ “red” doublet and the ¹S-¹D “green” line in Comet IRAS-Araki-Alcock (1983d) were obtained during its close approach to the Earth. This is the first unequivocal photoelectric detection of the green line in a cometary spectrum. The population ratio of the O (¹S) state to the O (¹D) state in the inner coma is ≦0.03. This ratio eliminates CO or CO₂ and points strongly to H₂O as the primary parent for excited oxygen atoms.     

Aeronomical determination of the efficiency of O1D) production by dissociative recombination of O2+

Simultaneous measurements of the 6300 Å airglow intensity, the electron density profile, and F-region ion temperatures and vertical ion velocities taken at the Arecibo Observatory in March 1971 are utilized in the height integrated continuity equation to extract the number of photons'of 6300 Å emitted per recombination. After accounting for quenching of $\text{O}\text{(}{}^1\text{D}\text{)}$ and the electrons lost via NO⁺ recombination, the efficiency of $\text{O}\text{(}{}^1\text{D}\text{)}$ production by the dissociative recombination of O₂⁺ is determined to be 0.6 ± 0.2 including cascading from the $\text{O}\text{(}{}^1\text{S}\text{)}$ state. The uncertainty includes both random measurement errors and estimates of possible systematic errors.     

The scattering mean free path in the Uranian atmosphere

New measurements of the equivalent widths of the 4-0 S(0) and S(1) H₂ quadrupole lines in the Uranian spectrum have been obtained using high dispersion (4.12 Å/mm) image-tube spectrography. The measured equivalent widths are $\text{62 ± 19}\text{m}\text{A}\text{?}\text{and}\text{58 ± 13}\text{m}\text{A}\text{?}$ for the S(0) and S(1) lines, respectively. Curve-of-growth analysis in terms of a reflecting layer model yields an H₂ column-density of 780_?330⁺⁹⁴⁰km amagat and a temperature of 78_?24⁺⁸⁰°K. Interpretation using a semi-infinite, homogeneous, isotropically scattering model for line formation yields a scattering mean free path at λ6400 Å of 550 ± 250 km amagat. Quoted errors for both the H₂ column-density and the scattering mean free path include the effect of uncertainty in the choice of atmospheric temperature. The results are discussed in terms of current models for the Uranian atmosphere.     

Production and condensation of organic gases in the atmosphere of Titan

The rates and altitudes for the dissociation of atmospheric constituents of Titan are calculated for solar UV, solar wind protons, interplanetary electrons, Saturn magnetospheric particles, and cosmic rays. The resulting integrated synthesis rates of organic products range from 10²–10³ g cm^?2 over 4.5 × 10⁹ years for high-energy particle sources to 1.3 × 10⁴ g cm^?2 for UV at $\text{λ < 1550}\text{A}\text{?}$, and to 5.0 × 10⁵ g cm^?2 if $\text{λ > 1550}\text{A}\text{?}$ (acting primarily on C₂H₂, C₂H₄, and C₄H₂) is included. The production rate curves show no localized maxima corresponding to observed altitudes of Titan's hazes and clouds. For simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at 2825 km. Such condensates comprise the principal cloud mass, with molecules of greater complexity condensing at higher altitudes. The scattering optical depths of the condensates of molecules produced in the Titanian mesosphere are as great as ～ 10²/(particulate radius, μm) if column densities of condensed and gas phases are comparable. Visible condensation hazes of more complex organic compounds may occur at altitudes up to ～ 3060 km provided only that the abundance of organic products declines with molecular mass no faster than laboratory experiments indicate. Typical organics condensing at 2900 km have molecular masses = 100–150 Da. At current rates of production the integrated depth of precipitated organic liquids, ices, and tholins produced over 4.5 × 10⁹ years ranges from a minimum ～ 100 m to kilometers if UV at $\text{λ > 1550}\text{A}\text{?}$ is important. The organic nitrogen content of this layer is expected to be ～ 10^?1?10^?3 by mass.     

Cross-section for the dissociative photoionization of hydrogen by 584 Å radiation: The formation of protons in the Jovian ionosphere

The cross-section for dissociative photoionization of hydrogen by 584 Å radiation has been measured, yielding a value of 5 × 10^?20 cm². The process can be explained as a transition from the $\text{X}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ ground state to a continuum level of the $\text{X}{}^2\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ ionized state of H₂ The branching ratio for proton (H⁺) vs molecular ion (H₂⁺) production at this energy is 8 × 10^?3. This process is quite likely an important source of protons in the Jovian ionosphere near altitudes where peak ionization rates are found.     

Auroral implications of recent measurements on O(1S) and O(1S) formation in the reaction of N+ with O2

Recent flowing afterglow measurements have shown that the reaction of N⁺ with O₂ produces 70 ± 30% of the oxygen atom product as $\text{O}\text{(}{}^1\text{D}\text{)}$ and < 0.1% as $\text{O}\text{(}{}^1\text{S}\text{)}$. These results indicate that this reaction does not contribute to the auroral green line emission (5577 Å), but can account for ～10% of the observed red line (6300 Å) auroral emission.     

Ion transition heights from topside electron density profiles

Theoretical electron density profiles are calculated for the topside ionosphere to determine the major factors controlling the profile shape. Only the mean temperature, the vertical temperature gradient and the $\text{O}{}^{\mathrm{+}}\text{H}{}^{\mathrm{+}}$ ion transition height are important. Vertical proton fluxes alter the ion transition height but have no other effect on the profile shape. Diffusive equilibrium profiles including only these three effects fit observed profiles, at all latitudes, to within experimental accuracy.Values of plasma temperature, temperature gradient and ion transition height h_tT were determined by fitting theoretical models to 60,000 experimental profiles obtained from Alouette l ionograms, at latitudes of 75°S–85°N near solar minimum. Inside the plasmasphere h_T varies from about 500 km on winter nights to 850 km on summer days. Diurnal variations are caused primarily by the production and loss of O⁺ in the ionosphere. The approximately constant winter night value of h_T is close to the level for chemical equilibrium. In summer h_T is always above the equilibrium level, giving a continual production of protons which travel along lines of force to aid in maintaining the conjugate winter night ionosphere. Outside the plasmasphere h_T is 300–600 km above the equilibrium level at all times. This implies a continual near-limiting upwards flux of protons which persists down to latitudes of about 60° at night and 50° during the day.     

Estimates of quasipolar absorption by methane in the atmosphere of Titan

The basis for “quasipolar” absorption (QPA) by CH₄ is the existence of a small electric dipole moment in its ground state. The integrated intensity α_QPA at a temperature of 90K is calculated to be between 4.8 × 10^?5 and 1.9 × 10^?2 cm^?2 atm^?1. With an assumed mean pressure of 0.1 atm and a relative abundance of $\text{[}\text{CH}{}_4\text{]}\text{[}\text{H}{}_2\text{]}\text{= 1}$, it is estimated that the ratio of quasipolar to pressure-induced absorption (PIA) is 0.05 ? α_QPA/α_PIA ? 18 for the spectral range from 0 to 300 cm^?1. This result suggests that quasipolar absorption may contribute to a weak, CH₄-induced greenhouse in the atmosphere of Titan.     

Laboratory measurements of the microwave opacity and vapor pressure of sulfuric acid vapor under simulated conditions for the middle atmosphere of Venus

Microwave absorption observed in the 35- to 48-km-altitude region of the Venus atmosphere has been attributed to the presence of gaseous sulfuric acid (H₂SO₄) in that region. This has motivated the laboratory measurement of the microwave absorption at 13.4- and 3.6-cm wavelengths from gaseous H₂SO₄ in a CO₂ atmosphere under simulated conditions for that region. As part of the same experiments, upper limits on the saturation vapor pressure of gaseous H₂SO₄ have also been determined. The measurements for microwave absorption have been made in the 1- to 6-atm pressure range, with temperatures in the 500 to 575°K range. Using a theoretically derived temperature dependence, the best-fit expression for absorption from gaseous H₂SO₄ in a CO₂ atmosphere at the 13.4-cm wavelength is $\text{9.0 × 10}{}^9\text{q(P)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{T}{}^{\mathrm{?3}}\text{(}\text{dB km}{}^{\mathrm{?1}}\text{),}\text{where}\text{q}$ is the H₂SO₄ number mixing ratio, P is the pressure in atmospheres, and T is the temperature in degrees Kelvins. The best-fit expression for absorption at the 3.6-cm wavelength is 4.52 × 10¹⁰q(P)^0.85T^?3 (dB km^?1). The inferred H₂SO₄ vapor pressure above liquid H₂SO₄ corresponds to ln p = 8.84 ? 7220/t where p is the H₂SO₄ vapor pressure (in atm) and T is the temperature in degrees Kelvins. These results suggest that abundances of gaseous H₂SO₄ on the order of 15 to 30 ppm could account for the microwave absorption observed by radio occultation experiments at 13.3- and 3.6-cm wavelengths. They also suggest that such abundances would correspond to saturation vapor pressure existing at or above the 46- to 48-km range, which correlates with the observed cloud base. It is suggested that future measurements of absorption in the 1- to 3-cm wavelength range will provide additional tools for monitoring variations in H₂SO₄ abundance via radio occultation and radio astronomical observations.     

Venus mesosphere and thermosphere temperature structure: II. Day-night variations

A two-dimensional nonlinear hydrodynamic model has been developed for studying the global scale winds, temperature, and compositional structure of the mesosphere and thermosphere of Venus. The model is driven by absorption of solar radiation. Ultraviolet radiation produces both heating and photodissociation. Infrared solar heating and thermal cooling are also included with an accurate NLTE treatment. The most crucial uncertainty in determining the solar drive is the efficiency by which $\text{λ < 1080}\text{A}\text{?}$ solar radiation is converted to heat. This question was analyzed in Part I, where it was concluded that essentially all hot atom and O(¹D) energy may be transferred to vibrational-rotational energy of CO₂ molecules. If this is so, the minimum possible euv heating occurs and is determined by the quenching of the resulting excess rotational energy. The hydrodynamic model is integrated with this minimum heating and neglecting any small-scale vertical eddy mixing. The results are compared with predictions of another model with the same physics except that it assumes that 30% of $\text{λ < 1080}\text{A}\text{?}$ radiation goes into heat and that the heating from longer-wavelength radiation includes the O(¹D) energy. For the low-efficiency model, exospheric temperatures are ?300°K on the dayside and drop to < 180°K at the antisolar point. For the higher-efficiency model, the day-to-night temperature variation is from ?600°K to ?250°K. Both versions of the model predict a wind of several hundred meters per second blowing across the terminator and abruptly weakening to small values on the nightside with the mass flow consequently going into a strong tongue of downward motion on the nightside of the terminator. The presence of this circulation could be tested observationally by seeing if its signature can be found in temperature measurements. Both versions of the model indicate that a self-consistent large-scale circulation would maintain oxygen concentrations with ?5% mixing ratios near the dayside F-1 ionospheric peak but ?40% at the antisolar point at the same pressure level.     

Airborne spectroscopy and spacecraft radiometry of Venus in the far infrared

In March 1979, the spectrum of Venus was recorded in the far infrared from the G.P. Kuiper Airborne Observatory when the planet subtended a phase angle of 62°. The brightness temperature was observed to be 275°K near 110 cm^?1, dropping to 230°K near 270 cm^?1. Radiance calculations, using temperature and cloud structure formation from the Pioneer Venus mission and including gaseous absorption by the collision-induced dipole of CO₂, yield results consistently brighter than the observations. Supplementing the spectral data, Pioneer Venus OIR data at similar phase angles provide the constraint that any additional infrared opacity must be contained in the upper cloud, H₂SO₄ to the Pioneer-measured upper cloud structure serves to reconcile the model spectrum and the observations, but cloud microphysics strongly indicates that such a high particle density haze $\text{(N ? 1.6 × 10}{}^7\text{cm}{}^{\mathrm{?3}}\text{)}$ is implausible. The atmospheric environment is reviewed with regard to the far infrared opacity and possible particle distribution modifications are discussed. We conclude that the most likely possibility for supplementing the far-infrared opacity is a population of large particles $\text{(}\text{r}\text{? 1 μm)}$ in the upper cloud with number densities less than 1 particle cm^?3 which has remained undetected by in situ measurements.     

Zodiacal light gathered along the line of sight: The vicinity of the terrestrial orbit studied with photopolarimetry and with Doppler spectrometry

The expression for the zodiacal brightness integral is especially simple if the integrand contains the ‘directional scattering coefficient’, $\text{D}$, (a.u.^?1), or equivalently the scattering cross-section per unit-volume. The two intersections of the terrestrial orbit with a line of sight lying in the ecliptic offer the possibility of isolating the contribution of the chord, with a conservative assumption of steadiness, but without the controversial assumption of a homogeneous zodiacal cloud. The zodiacal brightnesses between 60 and 120° elongation can be used to derive $\text{D}$₀ and $\text{D}$, the value of $\text{D}$ and its heliocentric radial derivative, both at 1 a.u. and at a scattering angle of 90°. A polarimetric treatment leads to the local polarization degree, $\text{P}$₀, and to its heliocentric derivative, $\text{P}$. Applied to all three available observational sources, this method invalidates the assumption of homogeneity, leading to a rather high relative gradient $\text{P}\text{P}{}_0$ near 1 a.u. (? 12, ? 16 or ? 24%, according to the source, as the Sun's distance decreases from 1.0 to 0.9 a.u.).The method is extended to Doppler spectrometry, taking advantage of the two equal projections on the line of sight of the Earth's velocity vector. The brightness Z₀ and the Dopplershift Δλ₀ observed at 90° elongation, together with the derivatives w.r.t. elongation ε, of the brightness, $\text{Z}\text{?}$ and of the Dopplershift, Δλ, can be used to retrieve the mean orbital velocity, v, of the interplanetary scatterers in the region of the terrestrial orbit. The two most reliable observational sources lead, with fair agreement, to a relative excess $\text{(}\text{v ? V)}\text{V}$, over the terrestrial velocity, of the order of + 25%.     

Photoionization of barium clouds via the 3D metastable levels

The photoionization of optically thin barium clouds is analyzed and shown to occur primarily by a two-step process involving the ${}^3\text{D}$ metastable term as the ntermediate state. The equilibrium populations of the ${}^1\text{D}$ and ${}^3\text{D}$ metastable levels are calculated and found to differ significantly from the values now in the literature. These populations are combined with our newly available photoionization data to calculate the resulting photoionization rate in the upper atmosphere.     

A self-consistent evaluation of the rate constants for the production of the OI 6300 Å airglow

The quenching rate k_N2 of $\text{O(}{}^1\text{D)}$ by N₂ and the specific recombination rate α_1D of O₂⁺ leading to $\text{O(}{}^1\text{D)}$ are re-examined in light of available laboratory and satellite data. Use of recent experimental values for the $\text{O(}{}^1\text{D)}$ transition probabilities in a re-analysis of AE-C satellite 6300 Å airglow data results in a value for k_N2 of 2.3 × 10^?11 cm³s^?1 at thermospheric temperatures, in excellent agreement with the laboratory measurements. This implies a value of J_O2 = 1.5 × 10^?6s^?1 for the O₂ photodissociation rate in the Schumann-Runge continuum. The specific recombination coefficient α_1D = 2.1 × 10^?7cm³s^?1 is also in agreement with the laboratory value. Implications for the suggested $\text{N}\text{(}{}^2\text{D) + O}{}_2\text{→ O(}{}^1\text{D) + NO}$ reaction are discussed.     

Contamination of ground-based measurements of OI (6300 Å) and NI (5200 Å) airglow by OH emissions

High resolution spectra of the 6300 Å and 5200 Å regions of the night sky have been obtained using a 1 m spectrometer. Typical errors in measurements of $\text{O}\text{(}{}^1\text{D}\text{)}$ 6300 Å and $\text{N}\text{(}{}^2\text{D}\text{)}$ 5200 Å intensities due to contanimation by overlapping OH emissions have been calculated for a fixed-filter photometer, a tilting-filter photometer and a spectrophotometer. The importance of careful selection of certain instrumental parameters in order to minimize measurement errors is emphasized.     

Ring current simulation in connection with interplanetary space conditions

A ring current model has been obtained which permits calculations ofD_st variations on the Earth's surface during magnetic storms. The changes in D_st are described by the equation

$\text{d}\text{d}\text{t}\text{D}{}_{\mathrm{sto}}\text{= F(EM)?}\text{D}{}_{\mathrm{sto}}\text{tau;}$

where $\text{D}{}_{\mathrm{sto}}\text{= D}{}_{\mathrm{st}}\text{-bp}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+~tc}$; p = mnv² is solar wind pressure; F(EM) is the function, controlled by the electromagnetic parameters of interplanetary medium, of injection into ring current ; τ is the constant of ring current decay. $\text{C}\text{= C}\text{u}\text{τ?=18}\text{nT}$, where C is the level of the D_st-variation field measurements; ? is the injection function characterizing the quasisteady-state injection of energy into the ring-current region. The constant Ç is determined from the condition that the change of the ring current energy from magnetic storm commencement to end should equal the difference between the injected and dissipated energy throughout the storm. The values of the factors b and τ were found by the method of minimizing the sum of the quadratic deviations of the calculated D_st from the values observed throughout the storm : b = 0.23 nT/(eV cm^?3)^{$\text{1}\text{2}$}, τ = 8.2 h at D_st? ? 55 nT and τ = 5.8 h at -120 ? D_st ? — 55 nT. The injection function F(EM) is of the form F(EM) = d(E_y? A) at the values of the azimuthal component of the solar wind electric field E_y ? A, and F(EM) =0 at A?E_y.d = ? 1.2 × 10^?3 Ts^?1 (mV/m)^?1 and A = ? 0.9 mV m^?1 . Thus, the injection to ring current is possible at the northward B_z component of the IMF.