Chemistry of the nightside ionosphere of Venus

We have constructed a one-dimensional model of the nightside ionosphere of Venus in which it is assumed that the ionization is maintained by day-to-night transport of atomic ions. Downward fluxes of O⁺, C⁺ and N⁺ in the ratios measured on the dayside at high altitudes are imposed at the upper boundary of the model (about 235 km). We discuss the resulting sources and sinks of the molecular ions NO⁺,CO⁺,N₂⁺,CO₂⁺ and O₂⁺. As the O⁺ flux is increased, the peak density of O⁺ increases proportionally and the altitude of the peak decreases. The O₂⁺ peak density is approximately proportional to the square root of the O⁺ flux and the peak rises as the O⁺ flux increases. NO⁺ densities near the peak are relatively unaffected by changes in the O⁺ flux. If the ionosphere is maintained mostly by transport, the ratio of the peak densities of O⁺ and O₂⁺ indicates the downward flux ofO⁺, independent of the absolute magnitudes of the densities. The densities of mass-28 ions are, however, still considered to be the most sensitive indicator of the importance of electron precipitation. We examine here the inbound and outbound portions of six early nightside orbits with low periapsis and use data from the Pioneer Venus orbiter ion mass spectrometer, the retarding potential analyzer and the electron temperature probe to determine the relative importance of ion transport and electron precipitation. For most of the orbits, precipitation is inferred to be of low to moderate importance. Only for orbit 65, which was the first nightside orbit published by Taylor et al. [J. geophys. Res. 85, 7765 (1980)] and for the inbound portion of orbit 73 does the ionization structure appear to be greatly affected by electron precipitation.     

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O₂⁺, NO⁺ and N₂⁺ using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [NO⁺]/[O₂⁺] ratio. In the region of most intense electron fluxes (20 erg cm⁻² sec⁻¹) at 280 km, the N density is found to be between 2 and 7 × 10⁷ cm⁻³. The resulting N densities are found to account for approx. 60% of the production of N⁺ through electron impact on N and the resonant charge exchange of O⁺(²P) with N(⁴S). This reaction also provides a significant source of O(¹S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O⁺(²P).     

Hydrogen atoms and ions in the thermosphere and exosphere

The distribution of atomic hydrogen in the thermosphere and exosphere is computed taking into account the upward flow which balances the escape flux. Because of the upward flow the number-density gradient is much steeper than it would be in a static atmosphere. Attention is drawn to the fact that the ratio of the amount of hydrogen above the 100 or 110km levels to the amount of hydrogen above the 200 or 300 km levels is a sensitive measure of the temperature of the exosphere. The evidence on the absolute abundance of atomic hydrogen is examined. It is concluded that the number density at the 120km level is probably about 5 × 10⁵/cm³. The Ly. absorption line at this level is beyond the linear part of the curve of growth.

Consideration is also given to the steady-state distributions of O⁺ and H⁺ ions. In the lower part of the exosphere the number density of O⁺ ions falls with increase in altitude (the associated scale height being twice that of the O atoms) and the number density of H⁺ ions rises at the same rate (as was first pointed out by Dungey). The altitude at which the number densities of O⁺ and H⁺ ions become equal is calculated on various assumptions regarding the temperature and hydrogen content of the exosphere. It is found to be about 1200 km when the temperature is 1250° K and the hydrogen content corresponds to the number density cited near the end of the preceding paragraph. The gradient of the predicted electrondensity distribution at several Earth radii is much less than that deduced from whistler studies.

The passage from charge transfer to diffusive equilibrium is discussed in an Appendix.     

A study of F2 region night-time vertical ionization fluxes at millstone hill

Vertical fluxes of ionization in the F2 region have been measured by the incoherent scatter technique over Millstone Hill in 1969. The results obtained near midnight for the region above h_maxF2 have been examined to determine whether there is a significant flux of ionization from the magnetosphere to the ionosphere that serves to maintain the F-layer. It is found that H⁺ ions are a minor constituent over the altitude range in which useful measurements can be made, so that any conclusion must rest upon properly interpreting the observed O⁺ fluxes. By selecting periods when the layer did not appear to be decaying rapidly it was hoped to find cases where the O⁺ flux did not vary with altitude in the range 500 h 800 km (i.e. where losses are unimportant), since this would imply that the flux is of magnetospheric origin.

While three cases exhibited this behaviour, the majority exhibited a decrease in the O⁺ flux with height, indicating that the layer was descending. Attempts to correct for this were made, and the average flux from the magnetosphere was estimated as 3 × 10⁷ el/cm²/sec. This is in fair agreement with other recent estimates, and implies that at this latitude the ionosphere is not maintained solely by the magnetospheric flux. Moreover, large increases in flux that could give rise to nocturnal increases in the total content of the layer do not appear to have been seen.     

Twilight effects of solar ionizing radiation

The absorption of solar ionizing radiation during twilight is investigated. Ion production rates are obtained as a function of altitude and twilight intensities and altitude profiles of emissions arising from the fluorescence of solar ionizing radiation are calculated for various solar depression angles. For an atmosphere with an exospheric temperature of 750°K, the predicted overhead intensity from fluorescence of the O⁺(²P — ²D) lines at 7319–7330 diminishes from 175 R at dusk to 10 R at a solar depression angle of 10°. The predicted overhead intensities from fluorescence of the N₂⁺ Meinel and first negative systems are respectively about 175 R and 20 R at dusk diminishing to respectively 1.5 R and 0.1 R at a solar depression angle of 10°.

It is suggested that a charge transfer reaction of O⁺²D in N₂ is a significant source of N₂⁺ ions. This reaction offers a possible explanation for the high apparent rotational temperatures in the first negative system observed by Broadfoot and Hunten. Other excitation and ionization mechanisms are briefly discussed.     

Dynamics of the O H2O charge-transfer reaction and its implications for space-borne measurements

O⁺(⁴S) + H₂O charge-transfer cross-section and product-ion time-of-flight measurements are presented over a center-of-mass collision energy range of 0.2–15 eV. The results are obtained with a newly constructed guided-ion beam experiment. The measured energy dependence of the charge-transfer crosssection agrees well with previous measurements and ADO model predictions of Bates [Chem. Phys. Lett. 82, 396 (1981) Proc. R. Soc. Lond. A384, 289 (1982) Chem. Phys. Lett. 111, 428 (1984)] at low collision energies, but exhibits significant differences at collision energies above 2 eV. The product kinetic energy analysis shows that most of the product ions are produced with near-thermal kinetic energies. At low collision energies, a significant backscattered channel is observed that is associated with complex formation. This channel exhibits a cross-section of approximately 1.4 Ųat a collision energy of 2.65 eV, corresponding to a laboratory ion energy of 5 eV.     

Some consequences of turbulent dissipation on diurnal thermal tide

It is shown that the phase difference between the horizontal components of the wind velocity induced by the first diurnal propagating tidal mode between 90 and 100 km altitude is modified by turbulent diffusion. Experimental data are compared with calculations made by two different methods. The calculated results agree well with the data for a coefficient of eddy diffusion K = 5 × 10⁶ cm² s⁻¹ which is a realistic one at this altitude. Incidentally, it is shown that some measurements which have been interpreted as a presence of evanescent modes are probably due to an increase in vertical wavelength of the first propagating diurnal mode due to turbulent diffusion.     

N2 emission in the twilight

About a year's observations of the N₂⁺ band (3914 Å) at Kitt Peak (latitude 32°) are reported. Morning intensities are the same throughout the year, but there is a strong winter maximum in the evening. It is suggested that the additional ionization is produced by photoelectrons from the magnetic conjugate point. Heights are estimated by the zenith-horizon method, which gives 235 km for the constant component and 350 km during the evening enhancement. The intensity variation through twilight is therefore entirely due to changes of the N₂⁺ concentration; each ion scatters light at a constant rate. The rotational distribution resembles that for a temperature of 1600°K, much higher than the temperature of the atmosphere. It is suggested that part of the ions may be produced by charge transfer from metastable O⁺(²D). N₂⁺ concentrations resulting from photoionization are calculated; they give a fair account of the observed horizon intensities, but not the zenith. Non-local electrons from higher in the atmosphere are suggested as a possible extra source; alternatively, the zenith measurements may be perturbed by scattered horizon light. The band intensity in the nightglow cannot be measured; the upper limit is 1 R.     

Properties of the infrared dark cloud G79.2+0.38

The MSX infrared dark cloud G79.2+0.38 has been observed over a 11′×′ region simultaneously in the J=1-0 rotational transition lines of the ¹²CO and its isotopic molecules ¹³CO and ¹⁸CO. The dense molecular cores defined by the C¹⁸O line are found to be associated with the two high-extinction patches shown in the MSX A-band image. The two dense cores have the column density N (H₂) (5 – 12) × 10²² cm⁻² and the mean number density n (3 ± 1) × 10⁴ cm⁻³. Their sizes are 1.7 and 1.2 pc in ¹³CO(1-0) line, 1.2 and 0.6 pc in C¹⁸O(1-0) line, respectively. The masses of these cloud cores are estimated to be in the range from 2 × 10² to 2 × 10³ M. The profile of radial mean density of the cloud core can be described by the exponential function ¯n(p) p^{−0.34±0.02}. Compared with the cases of typical optical dark clouds, the abundances of the CO isotopic molecules ¹³CO and C¹⁸O in this MSX infrared dark cloud appear to be depleted by a factor of 4–11, but at present there is no evidence for any obvious variation of the relative abundance ratio X_13/18 between ¹³CO and C¹⁸O with the column density.     

Effects of greatly increased O loss in the ionospheric F-region

A model of the ionosphere and plasmasphere is used to study some of the signatures of an idealized SAID (subauroral ion drift) event in the nightside ionosphere. A closed subauroral tube of plasma is considered under solar maximum atmospheric conditions and the westward velocity of 3 km s⁻¹ persists for 30 min. By pursuing, in turn, calculations in which plasma diffusion is suppressed and in which chemical loss of O⁺ is suppressed, the signatures of O⁺ chemistry alone and of field-aligned diffusion alone during the SAID event can be elucidated. Both chemical loss and transport contribute to the decay of the F-layer. Results from full calculations (including both chemistry and transport) demonstrate strong chemical-transport interaction.     

Ionospheric scale height from the refraction of satellite signals

Accurate observations of the elevation angle of arrival of 20 MHz signals from the polar orbiting satellite Beacon-B for a 20 month period have provided transmission ionograms which may be reduced to give H_p, the scale height at the peak of the ionosphere. Noon seasonal averages of H_p are 1.35 (in winter) to 1.55 (in summer) times greater than the scale height obtained from bottom-side ionograms. A comparison of scale height at the peak with routine measurements of total content and peak electron density indicates that the O⁺/H⁺ transition level is above 1000 km during the day but comes down to about 630 km on winter nights. A predawn peak in the overall scale height (∝ total content/peak density) is caused by a lowering of the layer to a region of increased recombination and is magnified in winter by low O⁺/H⁺ transition levels. After sunrise in winter and equinoxes the overall scale height is less than the scale height at the peak, implying an outwards flux of ionisation which lasts for about three hours. The summer evening increase in ƒ₀F2 requires both a cooling and a raising of the layer for its occurrence.     

Observation and Study of the CO, CO, HCO and N2H Molecular Lines Towards IRAS 02232+6138

Using the 13.7 m millimeter-wave telescope at the Qinghai Station of Purple Mountain Observatory, we have made observations of ¹³CO, C¹⁸O, HCO⁺ and N₂H⁺ molecular lines towards IRAS 02232+6138. As the excitation density of the probe molecule increases from ¹³CO to HCO⁺, the size of the cloud core associated with IRAS 02232+6138 decreases from 2.40 pc to 0.54 pc, and the virial mass of the cloud core decreases from 2.2 × 10³M to 5.1 × 10²M. A bipolar molecular outflow is found towards IRAS 02232+6138. Using the power function n(r) ∝ r⁻ to fit the spatial density structure of the cloud core, we obtain the power-law index  = 2.3 − 1.2; and we find that, as the probed density increases, the power function becomes more flat. The abundance ratio of ¹³CO to C¹⁸O is 12.4 ± 6.9, comparable with the values 11.8 ± 5.9 for dark clouds and the values 9.0–15.6 for massive cores. The abundance of N₂H⁺ molecules is 3.5 ± 2.5 × 10⁻¹⁰, consistent with the value 1.0 − 5.0 × 10⁻¹⁰ for dark cloud cores and the value 1.2 − 12.8 × 10⁻¹⁰ for massive cores. The abundance of HCO⁺ molecules is 0.9 ± 0.5 × 10⁻⁹, close to the value 1.6 − 2.4 × 10⁻⁹ for massive cores. An increase of HCO⁺ abundance in the outflow region was not found. Combining with the IRAS data, the luminosity-mass ratio of the cloud core is obtained in the range 37–163(L/M). Based on the IRAS luminosity, it is estimated that a main-sequence O7.5 star is probably embedded in the IRAS 02232+6138 cloud core.     

Eddy mixing and circulation at ionospheric levels

The rates of heat input into the mesosphere and lower thermosphere are calculated and compared with the heat losses. The worldwide average eddy diffusion coefficient required to maintain continuity in the heat budget is calculated and found to vary from about 10⁷ cm²/sec at 120 km down to about 10⁵ cm²/sec at 60 km. From the global asymmetry in heating at the solstice, it is concluded that a systematic pattern of vertical velocities prevails ranging from less than 1 cm/sec in the mesosphere up to 10 cm/sec near 120 km, upward over the summer polar region and downward over the winter polar region. This can be balanced by a wind system towards the winter polar region with velocities near 1 m/sec at 60 km increasing to 30 m/sec at 120 km. Such a wind system provides an explanation for the helium bulge in the upper thermosphere over the winter polar region.     

Calculation of the structure and transition probabilities of low energy levels of Fe and Fe

We use analytical wave functions in intermediate coupling to make a complete relativistic calculation of the structure and transition probabilities for the n = 1, 2, 3 states of Fe⁺²⁴. Data for Fe⁺²⁵ are also given for comparison. The results have a high accuracy and those relating to the n = 3 state are given in detail for the first time here.     

Satellite beacon measurements of protonospheric fluxes

The ionospheric and protonospheric regions of the plasmasphere, which are dominated by the O⁺ and H⁺ ionic species, respectively, interact by means of proton fluxes within tubes of magnetic force. The present study is concerned with the determination of these fluxes by the beacon satellite technique as used in the ATS-6 experiment in relation to three observing sites: Boulder, Colorado; Lancaster, U.K.; and Fairbanks, Alaska. From plasmasphere models based on solutions of the time dependent O⁺ and H⁺ momentum and continuity equations, it is shown that the time differential of the “residual content” as measured at Lancaster, provides a good estimate of the protonospheric flux at 4000km altitude in the L = 1.8 magnetic shell under quite geomagnetic conditions. The effect of the neutral thermospheric wind on the protonospheric flux is also investigated. Fluxes determined by the beacon technique for the period from September 1975 to July 1976 are shown, and these are compared with typical results derived from other techniques.     

The winter anomaly in ionospheric absorption and the D-region ion chemistry

A detailed analysis of the D-region ion composition measurements performed by Zbinden et al. (1975), during a winter day of high ionospheric absorption, has been carried out. The study examines the interactive mesosphere-D-region processes which occur in such anomalous conditions and their implication for water cluster ion chemistry. Two clustering regimes for NO⁺ have been observed in the data. Association with N₂ is identified as the dominant process below 76 km. Between 76 and 78 km altitude the effective loss rate of NO⁺ drops by two orders of magnitude. Above 77 km, the three-body reaction NO⁺ + CO₂+M→NO⁺CO₂+M seems to be the main NO⁺ loss. A mesospheric temperature profile could be derived from the ion composition data. This indicates the presence of a strong inversion above 76 km altitude. The wavelike structure obtained, is shown to be consistent with in situ winter temperature measurements. The sharp suppression of the N₂ association reaction could, thus, be explained by an increase in the collisional break-up of the NO⁺N₂ ion because of the enhanced temperature. In conclusion, our study indicates that, besides the increase in the production of NO⁺ and O₂⁺, due to an enhancement in the minor ionizable constituents, an additional thermal mesosphere-D-region interaction seems necessary to explain this winter anomalous ion composition data.     

Chemical models for the contracting W3 IRS4

We have studied the chemical evolution in the central core of a contracting cloud representing W3 IRS4. We modified the equation of temperature to satisfy the physical conditions of W3 IRS4. The chemical rate equations and the hydrodynamics are integrated simultaneously. The contraction is followed from a very low density of n = 10 cm⁻³ to high core density of n = 10⁶ cm⁻³. The chemical evolution is studied for various chemical conditions, including both the effects of varying cosmic ray ionization rate and the effect of ion–dipole molecule collisions.

The main results by using the more extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation at intermediate and enhanced cosmic ray ionization rate at high depletion of elemental atoms.     

Ionospheric conditions

A general analysis of ionospheric conditions has been made in the light of possible ionic reactions occurring in the upper atmosphere. Data obtained on various parameters, such as ionic production and recombination, show that precise knowledge of the spectral distribution of solar radiation is needed and that other experimental determinations on dissociative recombinations are required.

The ionic complexity of the ionosphere is underlined by describing how the atomic ions O⁺ and N⁺ react with N₂, O₂ and NO molecules. The behavior of the molecular ions N⁺₂, O⁺₂and NO⁺depends on a group of simultaneous processes involving charge transfers and ionatom interchanges which are more important than dissociative recombinations. The altitude distribution of ions is exemplified by discussing the relative importance of various loss coefficients in the D-, E- and F-regions. It is seen that molecular nitrogen ions are subject to important charge transfer processes, that nitric oxide ions are always final products destroyed only by dissociative recombination. Additionally, the entire production of atomic oxygen ions is related to the photoionization of molecular nitrogen. Some information is also given on possible anomalies in the ratio of O⁺₂ and NO⁺ densities in the lower ionosphere. From the lack of sufficient experimental information on ionic processes it is shown that a precise analysis of ionospheric behavior remains highly speculative.     

Deactivation of N2A Σumolecules in the aurora

Recent rocket observations of the N₂ V-K (Vegard-Kaplan) system in the aurora have been reinterpreted using an atmospheric model based on mass spectrometer measurements in an aurora of similar intensity at the same time of year. In contrast to the original interpretation, we find that population by cascade from the C³Π_u and B³Π_g states in the A³Σ_u⁺v=0,1 levels, as calculated using recently measured electron excitation cross sections, accurately accounts for the observed relative emission rates (IV-K/12PG_0.0). In addition there is no need to change the production rate of A ³ Σ _u⁺ molecules relative to that of C³Π_uv=0 as a function of altitude in order to fit the profile of the deactivation probability to the atmospheric model. Quenching of A ³ Σ _u⁺ molecules at high altitudes is dominated by atomic oxygen. The rate constants for the v=0 and v=1 levels are 8 × 10⁻¹¹ cm³ sec⁻¹ and 1.7 × 10⁻¹⁰ cm³ sec⁻¹ respectively, as determined using the model atmosphere mentioned above. Recent observations with a helium cooled mass spectrometer suggest that conventional mass spectrometer measurements tend to underestimate the atomic oxygen relative concentration. The rate coefficients may therefore be too large by as much as a factor of 3. Below 130 Km we find that it is possible to account for the deactivation in bright auroras by invoking large nitric oxide concentrations, similar to those recently observed mass spectrometrically and using a rate constant of 8 × 10⁻¹¹ cm³ sec⁻¹ for both the v=1 levels. This rate constant is very nearly the same as that measured in the laboratory (7 × 10⁻¹¹ cm³ sec⁻¹). Molecular oxygen appears not to play a significant role in deactivating the lower A ³ Σ _u⁺ levels.     

The spectrum of electron content fluctuations in the ionosphere

Continuous records of the electron content of the ionosphere, from 1965 to 1970, are used to obtain power spectra covering periods from 30 sec to 2 yr at latitudes of 34°S and 42°S. At periods up to 5 min, amplitudes were less than 0.2 per cent of the total electron content. Variations produced by gravity waves were very common in the range 20–80 min, with no preferred periods. The r.m.s. amplitude per octave A₀ was about 10¹⁵ electrons/m², or 0.6 per cent of the mean electron content. The amplitude increased during the day, particularly in winter when periodic components predominated. The cut-off at about 17 min was sharply defined, giving a mean scale height for the neutral atmosphere (at 300 km) of about 43 km in summer, 47 km on winter days and 42 km on winter nights.

From 12 hr to 1 month A₀ was about 12 per cent of the mean electron content in both summer and winter at 34°S, and 10 per cent at 42°S. The 24 hr and 27 day peaks were largest just before sunspot maximum, and almost disappeared near sunspot minimum. Variations between 1 and 27 days reflect the random occurrence of ionospheric storms and show no consistent peaks. Day to day and night to night variations were both about 10 per cent of the background content for periods from 2 days to 2 yr, apart from a slight decrease between 1 and 6 months.