Model calculations of the dayside ionosphere of Venus

Model calculations of the dayside ionosphere of Venus are presented. The coupled continuity and momentum equations were solved for O₂⁺, O⁺, CO₂⁺, C⁺, N⁺, He⁺, and H⁺ density distributions, which are compared with measurements from the Pioneer Venus ion mass spectrometer. The agreement between the model results and the measurements is good for some species, such as O⁺, and rather poor for others, such as N⁺, indicating that our understanding of the dayside ion composition of Venus is incomplete. The coupled heat conduction equations for ions and electrons were solved and the calculated temperatures compared with Pioneer Venus measurements. It is shown that fluctuations in the magnetic field have a significant effect on the energy balance of the ionosphere.     

Temporal and spatial variations observed in the ionospheric composition of Venus: Implications for empirical modelling

In situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about ?4° to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental results. The first results from the sorting of several prominent ions including O⁺, O₂⁺, and H⁺ and several minor ions including CO₂⁺, C⁺, and H₂⁺ reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orders of magnitude, depending upon ion mass and altitude. The concentrations of O₂⁺, O⁺, CO₂⁺ and C⁺ peak throughout the dayside decreasing sharply at the terminators to nightside levels, lower by one to two orders of magnitude relative to the dayside. The diurnal variations of the light ions H⁺ and H₂⁺ peak during the night, exhibiting asymmetric nightside bulges favoring the pre-dawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F_10.7 index, indicating a strong influence of solar variability on the ion production and distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging.     

Global comparison of the model results of GSM TIP with IRI for summer conditions

This paper presents the results of the numerical calculations thermosphere/ionosphere parameters which were executed with using of the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP)and comparison of these results with empirically-based model IRI-2001. Model GSM TIP was developed in West Department of IZMIRAN and solves self-consistently the time-dependent, 3-D coupled equations of the momentum, energy and continuity for neutral particles (O₂, N₂, O), ions (O⁺, H⁺), molecular ions (M⁺) and electrons and largescale eletric field of the dynamo and magnetospheric origin in the range of height from 80 km to 15 Earth’s radii. The empirically derived IRI model describes the E and F regions of the ionosphere in terms of location, time, solar activity and season. Its output provides a global specification not only of N_e but also on the ion and electron temperatures and the ion composition. These two models represent a unique set of capabilities that reflect major differences in along with a substantial approaches of the first-principles model and global database model for the mapping ionosphere parameters. We focus on global distribution of the N_e, T_i, T_e and TEC for the one moment UT and fixed altitudes: 110 km, h_mF2, 300 km and 1000 km. The calculations were executed with using of GSM TIP and IRI models for August 1999, moderate solar activity and quiet geomagnetic conditions. Results present as the global differences between the IRI and GSM TIP models predictions. The discrepancies between model results are discussed.     

Mesospheric H2O and H2O2 densities inferred from in situ positive ion composition measurement

The measurements of positive ion composition in the high latitude D-region have revealed an excess of 34⁺ under distrubed conditions which has been interpreted as H₂O₂⁺. At the same altitude range near the transition height oxonium ions were measured as well. This paper presents a new model for the production and loss of oxonium ions with their production from H₂O₂⁺ + H₂O → H₃O⁺ + HO₂ and their loss by attachment of N₂ and/or CO₂. A reaction constant of 8.5×10^?28 (300/T)⁴ cm⁶s^?1 has been obtained for the three body attachment H₃O⁺ + CO₂ + M → H₃O⁺.CO₂ + M from the measured density profile of 63⁺ in flight 18.1020. Mesospheric H₂O and H₂O₂ densities are inferred from measurements of four high latitude ion compositions based on the oxonium model. The mixing ratios of hydrogen peroxide are up to two orders of magnitude higher compared to previous model calculations. In order to explain the missing production of odd hydrogen, we consider larger O(¹D) densities, surface reactions of O(³P) on particles, and cathalytic photodissociation of water vapor on aerosol particles.     

The role of atomic oxygen and molecular nitrogen in the relationship between electron temperature and electron density in the ionosphere

Atomic oxygen and molecular nitrogen play several roles in controlling electron temperature in the F-region. Both O and N₂ are ionised by EUV radiation and produce photoelectrons. Ion-exchange with N₂ controls the recombination of O⁺ and hence the equilibrium electron density. In the transfer of photoelectron energy to thermal electrons competition arises from ionisation and excitation of oxygen and nitrogen. Finally, the three most important processes by which thermal electrons lose energy are Coulomb collision with 0⁺, excitation of the fine-structure states of 0 and vibrational cooling by N₂. A simple model expresses all these processes and accurately describes the observed relationship between electron temperature, electron density and solar flux.     

Circulation of energetic ions of terrestrial origin in the magnetosphere

Energetic ion composition measurements have now been performed from earth orbiting satellites for more than a decade. As early as 1972 we knew that energetic (keV) ions of terrestrial origin represented a non-negligible component of the storm time ring current. We have now assembled a significant body of knowledge concerning energetic ion composition throughout much of the earth's magnetosphere. We know that terrestrial ions are a common component of the hot equatorial magnetospheric plasma in the ring current and the plasma sheet out to ? 23 R_E. During periods of enhanced geomagnetic activity this component may become dominant. There is also clear evidence that the terrestrial component (specifically O⁺) is strongly dependent on solar cycle. Terrestrial ion source, transport, and acceleration regions have been identified in the polar auroral region, over the polar caps, in the magnetospheric boundary layers, and within the magnetotail lobes and plasma sheet boundary layer. Combining our present knowledge of these various magnetospheric ion populations, it is concluded that the primary terrestrial ion circulation pattern associated with enhanced geomagnetic activity involves direct injection from the auroral ion acceleration region into the plasma sheet boundary layer and central plasma sheet. The observed terrestrial component of the magnetospheric boundary layer and magnetotail lobes are inadequate to provide the required influx. They may, however, contribute significantly to the maintenence of the plasma sheet terrestrial ion population, particularly during periods of reduced geomagnetic activity. It is further concluded, on the basis of the relative energy distributions of H⁺ and O⁺ in the plasma sheet, that O⁺ probably contributes significantly to the ring current population at energies inaccessible to present ion composition instrumentation (? 30 keV).     

Hybrid quadri-ionic model of the lower ionosphere

An ion model of the lower ionosphere is proposed. It consists of four positive ions: O₂⁺, NO⁺ and two cluster ions - a simpler CI₁ and a more complex CI₂. This model well explains the normal component of the winter anomaly (WA) in the D-region, which is recorded by absorption measurements on short radiowaves and rocket experiments at middle (40°N) and high (70°) latitudes. The higher values of the electron density during the winter appear as a result of the lower recombination because of smaller rates of cluster ion formation, i.e. the normal WA can be explained and modelled by the regular seasonal variations of composition, temperature and density.     

Solar cycle effect on temporal and spatial variation of topside ion density measured by SROSS C2 and ROCSAT 1 over the Indian longitude sector

We investigated the diurnal, seasonal and latitudinal variations of ion density Ni over the Indian low and equatorial topside ionosphere within 17.5°S to 17.5°N magnetic latitudes by combining the data from SROSS C2 and ROCSAT 1 for the 9 year period from 1995 to 2003 during solar cycle 23. The diurnal maximum density is found in the local noon or in the afternoon hours and the minimum occurs in the pre sunrise hours. The density is higher during the equinoxes as compared to that in the June and December solstice. The local time spread of the daytime maximum ion density increases with increase in solar activity. A north south asymmetry with higher ion density over northern hemisphere in the June solstice and over southern hemisphere in December solstice has been observed in moderate and high solar activity years. The crest to crest distance increases with increase in solar flux. Ion density bears a nonlinear relationship with F_10.7 cm solar flux and EUV flux in general. The density increases linearly with solar flux up to ∼150 sfu (1 sfu = 10⁻²²Wm⁻²Hz⁻¹) and EUV flux up to ∼50 units (10⁹ photons cm⁻² s⁻¹). But beyond this the density saturates. Inverse saturation and linear relationship have been observed in some season or latitude also. Inter-comparison of the three solar activity indices F_10.7 cm flux, EUV flux and F_10.7P (= (F_10.7 + F_10.7A)/2, where F_10.7A is the 81 day running average value of F_10.7) shows that the ion density correlates better with F_10.7P and F_10.7 cm fluxes. The annual average daytime total ion density from 1995 to 2003 follows a hysteresis loop as the solar cycle reverses. The ion density at 500 km over the Indian longitude sector as obtained by the international reference ionosphere is in general lower than the measured densities during moderate and high solar activity years. In low solar activity years the model densities are equal or higher than measured densities. The IRI EIA peaks are symmetric (±10°) in equinox while densities are higher at 10°N in June solstice and at 10°S in the December solstice. The model density follows F_10.7 linearly up to about F_10.7 > ∼150 sfu and then saturates.     

Hybrid simulations of the O ion escape from Venus: Influence of the solar wind density and the IMF x component

As an initial effort to study the evolution of the Venus atmosphere, the influence of the solar wind density and the interplanetary magnetic field (IMF) x component (the x-axis points from Venus towards the Sun) on the O⁺ ion escape rate from Venus is investigated using a three-dimensional quasi-neutral hybrid (HYB-Venus) model. The HYB-Venus model is first applied to a case of the high-density (100 cm⁻³) solar wind interaction with Venus selected from the Pioneer Venus Orbiter observations to demonstrate its capability for the study. Two sets of simulations with a wide range of solar wind densities and different IMF x components are then performed. It is found that the O⁺ ion escape rate increases with increasing solar wind density. The O⁺ ion escape rate saturates when the solar wind density becomes high (above 100 cm⁻³). The results also suggest that the IMF x component enhances the O⁺ ion escape rate, given a fixed IMF component perpendicular to the x-axis. Finally, the results imply a higher ion loss rate for early-Venus, when solar conditions were dramatically different.     

Models of Venus neutral upper atmosphere: Structure and composition

Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO₂, O, CO, He, N, N₂, and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides.Estimates are made of other species, including O₂ and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.     

In situ results on the variation of neutral atmospheric hydrogen at venus

The concentrations of neutral hydrogen within the atmosphere of Venus are investigated for the period 1979–1980. During this period, the planet made nearly three orbits about the Sun, so that nearly three complete diurnal cycles were observed from the Pioneer Venus Orbiter (PVO). Values of n(H) are derived from in-situ ion and neutral composition measurements from the Orbiter Ion Mass Spectrometer (OIMS) and the Orbiter Neutral Mass Spectrometer (ONMS) using a charge exchange relationship involving O⁺, H⁺, O and CO₂. The dawn bulge in the diurnal distribution of n(H), reported from the first diurnal cycle by Brinton et al., is found to persist with n(H) peaking at levels near 2 - 5 × 10⁷/cm³ at altitudes below 165 km. At peak levels, the bulge exhibits a concentration ratio up to 400/1 relative to dayside values. Large day to day variations of up to a factor of five in n(H) are frequently encountered, and are attributed to perturbations induced by the solar wind interaction. These short term variations, plus a suggestion of some local time variation in the bulk location, make precise assessment of interannual variations in the n(H) difficult. Between the first diurnal cycle in early 1979 and the third in mid 1980, the decline in solar euv flux was of the order of 10% or less. Allowing for uncertainties due to short term variations, no clear evidence is found for an interannual variation in the hydrogen concentrations.     

Ion precipitation into the ionosphere during geomagnetic storms

This review presents numerous recent examples of interesting variations in the composition and intensity of the hot ion flux (10 eV - 15 keV/e) provided by the AUREOL-3 satellite as a function of latitude and local time during periods of magnetic activity. In particular, these results reveal that although H⁺ is the most abundant ion during magnetically quiet periods, the ion composition of hot plasma at ionospheric altitudes is quite variable, and depends strongly on magnetic activity; results obtained during main and recovery phases of several magnetic storms demonstrate clearly (below 15 keV/Q) the great importance of the low altitude ionospheric source (H⁺, O⁺, and to a lesser degree He⁺) particularly at low latitudes (L ～ 3 - 4) where the flux of O⁺ ions becomes very large and even dominates. The results of the AUREOL-3 ion spectrometers establish the fact that upflowing suprathermal ionospheric ions (E_i < 100 eV/e) appear over large regions of the auroral ionosphere, the polar caps, and the polar cusp, as well as in or at the boundary of the plasmasphere during magnetospheric substorms or magnetic storms, and may consequently contribute significantly to the plasma sheet and to the inner storm time ring current. Most of the properties of the storm time ring current found by the GEOS, SCATHA, and ISEE satellites apply to lower altitudes, although the role of the ionospheric and/or plasmaspheric source appears accentuated.     

Modulation of dayside ion and neutral distributions at Venus: Evidence of direct and indirect solar energy inputs

In-situ measurements of ion and neutral composition and temperature across the dayside of Venus during 1979–1980 exhibit long and short-term changes attributed to solar variations. Following solar maximum, dayside concentrations of CO⁺ and the neutral gas temperature are relatively smoothly modulated with a 28-day cycle reasonably matching that of the solar F_10.7 and EUV fluxes. Measurements some 6–8 months earlier show less pronounced and more irregular modulation, and short-term day-to-day fluctuations in the ions and neutrals are relatively more conspicuous than in the later period. During the earlier period, the solar wind at Venu exhibits relatively large velocity enhancements, which appear to be consistent with differences in solar coronal behavior during the two periods. It is suggested that through the solar wind variations and associated changes in the draping of the interplanetary magnetic field about the dayside, fluctuating patterns of joule heating may occur, producing the observed short term ion and neutral variations. This indirect energy effect, if verified, presents a complication for quantitatively analyzing the modulation in neutral temperature and ion concentration produced by changes in direct EUV radiation.     

Study of maximum electron density NmF2 at Karachi and Islamabad during solar minimum (1996) and solar maximum (2000) and its comparison with IRI

The monthly hourly medians of maximum electron density, N_mF₂, at two Pakistani ionospheric stations, Karachi and Islamabad, have been determined for solar minimum (1996) and solar maximum (2000) and compared with IRI predictions using the URSI coefficients. At night and pre-noon period the N_mF₂ values at both stations are almost equal during the 2 years. However, at post-noon the values at Karachi are considerably larger than those at Islamabad due to the equatorial or geomagnetic anomaly. Karachi (geomag. coord. 16.44°N, 139.08°E) lies near the region of the equatorial anomaly (+20 and −20 geomagnetic latitude), so most of the N_mF₂ values at Karachi are larger than those at Islamabad (geomag. coord. 24.46°N, 145.67°E). The maximum monthly values of N_mF₂ show a semi-annual variation at Karachi and Islamabad both during 1996 and 2000 as predicted by IRI.     

Results of measurements of the response of a delta-doped CCD to neutral and charged particle beams

We report the measurements of the response of a delta-doped Charge Coupled Device (CCD) in imaging mode to beams of charged and neutral particles. That is, the detector imaged the incident beam over its 1024 × 1024 pixels, integrating the number of particles counted in each pixel during the exposure period. In order to count individual particles the exposure time would have had to be reduced considerably compared to the typical ?5 s used in these studies. Our CCD thus operated in a different manner than do conventional particle detectors such as the CEM and MCP that normally are used in a particle counting mode. The measurements were carried out over an energy range from 0.8 to 30 keV. The species investigated include H, H⁺, He⁺, N⁺, N₂⁺, and Ar⁺. The energy and ion mass covered wider ranges than previous measurements for the CCD. The results of these measurements show, as in the case of the previous measurement, for a given ion the CCD response increases with energy and for a given particle energy the response decreases with increasing mass of the particle. These results are in agreement with predictions of the theory of the range of ions in solids. The results also show the possibility for the application of the delta doped CCD as a detector for low energy particle measurements for space plasma physics applications.     

Occurrence features of simultaneous H+- and He+-band EMIC emissions in the outer radiation belt

As an important loss mechanism of radiation belt electrons, electromagnetic ion cyclotron (EMIC) waves show up as three distinct frequency bands below the hydrogen (H⁺), helium (He⁺), and oxygen (O⁺) ion gyrofrequencies. Compared to O⁺-band EMIC waves, H⁺- and He⁺-band emissions generally occur more frequently and result in more efficient scattering removal of <～5?MeV relativistic electrons. Therefore, knowledge about the occurrence of these two bands is important for understanding the evolution of the relativistic electron population. To evaluate the occurrence pattern and wave properties of H⁺- and He⁺-band EMIC waves when they occur concurrently, we investigate 64 events of multi-band EMIC emissions identified from high quality Van Allen Probes wave data. Our quantitative results demonstrate a strong occurrence dependence of the multi-band EMIC emissions on magnetic local time (MLT) and L-shell to mainly concentrate on the dayside region of L?=?～4–6. We also find that the average magnetic field amplitude of H⁺-band waves is larger than that of He⁺-band waves only when L?<?4.5 and AE¹?<?300?nT, and He⁺-band emissions are more intense under all other conditions. In contrast to 5 events that have average H⁺-band amplitude over 2 nT, 19 events exhibit >2 nT He⁺-band amplitude, indicating that the He⁺-band waves can be more easily amplified than the H⁺-band waves under the same circumstances. For simultaneous occurrences of the two EMIC wave bands, their frequencies vary with L-shell and geomagnetic activity: the peak wave frequency of H⁺-band emissions varies between 0.25 and 0.8 f_cp with the average between 0.25 and 0.6 f_cp, while that of He⁺-band emissions varies between 0.03 and 0.23 f_cp with the average between 0.05 and 0.15 f_cp. These newly observed occurrence features of simultaneous H⁺- and He⁺-band EMIC emissions provide improved information to quantify the overall contribution of multi-band EMIC waves to the loss processes of radiation belt electrons.     

Positive ion composition and electron density in a combined auroral and NLC event

The positive ion composition and electron density were measured in the lower ionosphere above Kiruna in salvo A of CAMP (Cold Arctic Mesopause Project). The CAMP/P (S37/P) payload carrying a magnetic ion spectrometer, positive ion and electron probes, and propagation experiments was launched on 3 August 1982 2332 UT during extended Noctilucent Clouds (NLC) and auroral activities over Kiruna. The measured electron density was 5×10³cm^?3 at 80 km and 2.5×10⁵cm^?3 at 90 km. The increase of ion and electron densities in the D- and E-region during twilight was caused by precipitating auroral particles. The height distribution of the positive ions measured by the mass spectrometer in the mass range 19–280 amu is different from a winter flight with similar auroral conditions. Below 85.5 km proton hydrates H⁺(H₂O)₃ ? H⁺(H₂O)₈ were the dominant ions. The heaviest proton hydrates H⁺(H₂O)₇ and H⁺(H₂O)₈ were most abundant at 82–85.5 km, the altitude of visible NLC. Above 85.5 km O₂⁺ and NO⁺ became dominant. A small metal ion layer was observed between 90.5–93 km with a maximum ion density of 10% of the total positive ion density at 91 km altitude. The metal ion density disappeared within about a km below 90.5 km.     

Rotational constants of linear and/or bent Cn+1H and CnN(n = 1–6): A DFT study

The geometries, dipole moments, and rotational constants for the linear and/or bent cations, C_n+1H⁺ and C_nN⁺(n = 1–6), were studied by the B3LYP method with the modest basis sets. For C_nH⁺(n = odd; 3, 5, 7) and C_nN⁺(n = even; 2, 4, 6), the theoretical rotational constants (B_es) of closed-shell singlet C₃H⁺, C₅H⁺, C₇H⁺, CCN⁺, C₄N⁺, and C₆N⁺ were calculated to be about 11,244, 2420, 885.2, 11,970, 2439, and 880.8 MHz, respectively. By contrast, the triplets are stable than the corresponding singlets for C_nH⁺(n = odd; 2, 4, 6) and C_nN⁺(n = even; 3, 5) except CN⁺.     

Recent advances in upper atmospheric structure

A possible quantitative explanation of the semi-annual variation in thermospheric density has been obtained in terms of a semi-annual variation in the computed globally averaged vertical energy carried by propagating tides from the lower and middle atmosphere into the thermosphere. The effect is primarily due to seasonal changes in the distribution of water vapor and in the solar declination angle and Sun-Earth distance. An MSIS-83 empirical model of the thermosphere, representing a revision of the earlier MSIS models, has been prepared. The database used covers a wider range of solar activity than previous models and an improved magnetic storm representation is included. Atomic oxygen profiles in the 100 to 160 km altitude region of the auroral thermosphere have been recalculated from measured quenching of N₂(A³∑_u⁺) using the latest laboratory rates and the results are in good agreement with the mean CIRA 1972 profile. A new empirical model of thermospheric variations with geomagnetic activity has been developed incorporating variations with local magnetic time, latitude dependent terms which can vary with the magnitude of the geomagnetic disturbance, and an altitude dependent expression for the equatorial wave. A new index ML, derived from the AL index, has been developed that appears to have promise to represent the variations of thermospheric species with geomagnetic activity. Satellite measured values of solar UV flux, ground-based observations of CaK plages, sunspot numbers and 10.7 cm solar radio flux have been analyzed for temporal variations. Some differences have been identified and the significance to empirical and theoretical upper atmosphere models is discussed.