Photochemical Equilibrium Condition Research Articles

The dayside ionosphere of Mars: Comparing a one-dimensional photochemical model with MAVEN Deep Dip campaign observations

A one dimensional photochemical model for the dayside ionosphere of Mars has been developed for calculating the density profiles of ions and electrons under steady state photochemical equilibrium condition. The study focuses on the Deep Dip campaigns of the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) and used the in-situ measurements of neutral density profiles, solar flux and electron temperatures from instruments onboard MAVEN as input to the model. An energy deposition model is employed for calculating the attenuated photon flux and photoelectron flux at different altitudes in the ionosphere. The Analytical Yield Spectrum approach is used for calculating the photoelectron fluxes. Volume production rates of major primary ions, CO2+, CO+, O+, C+, N2+, and N+, due to photon and photoelectron impact are calculated and used as input to the model in which ion-neutral chemistry in the dayside ionosphere is simulated. The modeled ion profiles are compared with the ion mode observations of Neutral Gas Ion Mass Spectrometer (NGIMS) and electron density estimates from Langmuir Probe and Waves (LPW). The model could reproduce the observed structure of the major ion profiles O2+, CO2+, and electron density reasonably well. However, when the magnitudes are compared, the modeled values are roughly a factor of 2 larger than observation. By reducing the neutral CO2 density, the model outputs and the observed ion profiles and electron density profile can become closer. The model also calculated the densities of 11 other ions, viz. N+, C+, O+, NO+, N2H+, HCO+, N2+, CO+, OCOH+, HNO+, and OH+. Profiles of each of these ions are compared with the NGIMS observations during the respective orbits and are discussed in detail. Such a comparison for the deep dip periods is reported for the first time, which showcases the level of the current understanding of the ion chemistry in the Martian ionosphere.

A Model for Negative Ion Chemistry in Titan’s Ionosphere

Abstract We developed a one-dimensional photochemical model for the dayside ionosphere of Titan for calculating the density profiles of negative ions under steady-state photochemical equilibrium condition. We concentrated on the T40 flyby of the Cassini orbiter and used the in situ measurements from instruments on board Cassini as input to the model. Using the latest available reaction rate coefficients and dissociative electron attachment cross sections, the densities of 10 anions are calculated. Our study shows CN− as the dominant anion, followed by C3N−, which agrees with the results of previous calculations. We suggest that H− could be an important anion in Titan’s ionosphere and is the second most abundant anion at altitudes greater than 1200 km. The main production channel of the major ion CN− is the reaction of H− with HCN. The H− also play a major role in the production of anions C2H−, C6H−, and OH−. We present a comparison of the calculated ion density profiles with the relative density profiles derived using recently reported Cassini CAPS/ELS observations.

Dayside ionosphere of Titan: Impact on calculated plasma densities due to variations in the model parameters

A one dimensional photochemical model for the dayside ionosphere of Titan has been developed for calculating the density profiles of ions and electrons under steady state photochemical equilibrium condition. We concentrated on the T40 flyby of Cassini orbiter and used the in-situ measurements from instruments onboard Cassini as input to the model. An energy deposition model is employed for calculating the attenuated photon flux and photoelectron flux at different altitudes in Titan’s ionosphere. We used the Analytical Yield Spectrum approach for calculating the photoelectron fluxes. Volume production rates of major primary ions, like, N2+, N+, CH4+, CH3+, etc due to photon and photoelectron impact are calculated and used as input to the model. The modeled profiles are compared with the Cassini Ion Neutral Mass Spectrometer (INMS) and Langmuir Probe (LP) measurements. The calculated electron density is higher than the observation by a factor of 2 to 3 around the peak. We studied the impact of different model parameters, viz. photoelectron flux, ion production rates, electron temperature, dissociative recombination rate coefficients, neutral densities of minor species, and solar flux on the calculated electron density to understand the possible reasons for this discrepancy. Recent studies have shown that there is an overestimation in the modeled photoelectron flux and N2+ ion production rates which may contribute towards this disagreement. But decreasing the photoelectron flux (by a factor of 3) and N2+ ion production rate (by a factor of 2) decreases the electron density only by 10 to 20%. Reduction in the measured electron temperature by a factor of 5 provides a good agreement between the modeled and observed electron density. The change in HCN and NH3 densities affects the calculated densities of the major ions (HCNH+, C2H5+, and CH5+); however the overall impact on electron density is not appreciable ( < 20%). Even though increasing the dissociative recombination rate coefficients of the ions C2H5+ and CH5+ by a factor of 10 reduces the difference between modeled and observed densities of the major ions, the modeled electron density is still higher than the observation by ∼ 60% at the peak. We suggest that there might be some unidentified chemical reactions that may account for the additional loss of plasma in Titan’s ionosphere.

Longitudinal distribution of thermospheric densities and ionization rates in the upper atmosphere of Mars at mid-latitude using MGS ACC data

The Accelerometer Experiment (ACC) onboard Mars Global Surveyor (MGS) measured 1600 density profiles in the upper atmosphere of Mars during aerobraking. These measurements reveal large-scale and small-scale structure in the thermosphere of Mars. Here, the measurements of mass density for 115 orbits (#P0670–P0789) from November 1 to 30, 1998, under spring equinox and medium solar activity conditions (average F 10.7 ∼ 137) during phase 2 of the aerobraking in the thermosphere of Mars at different altitudes and longitudes are presented for northern mid-latitude (17–42°N) in the dayside atmosphere using ACC onboard MGS. From these mass densities, the neutral densities of different gases are derived from their mixing ratios. Using these neutral densities, the longitudinal distribution of photoionization rates and photoelectron impact ionization rates are calculated at wavelength range 1–102.57 nm due to EUV and soft X-ray radiation under photochemical controlled region using Analytical Yield Spectrum approach (AYS). These conditions are appropriate for MGS Phase 2 aerobraking period from which the accelerometer data is used. Under the photochemical equilibrium condition, the electron density near the peak varies as the square root of the total peak ionization rate. Using this fact, an attempt is being made to estimate the mean primary and secondary peak electron density by averaging the longitudinal variations of total peak ionization rates in the northern mid-latitude (17–42°N) ionosphere of Mars, as there is no radio science measurement at this latitude region by MGS.

Estimation of peak electron density in upper ionosphere of Mars at high latitude (50°–70°N) using MGS ACC data

Under photochemical equilibrium condition, electron density near peak varies as the square root of total peak ionization rate. Thus by averaging the longitudinal variation of latter, the mean primary and secondary peak electron densities in upper ionosphere of Mars using measured mass densities at LST 16 hrs are estimated to be ∼8.56 × 104 cm−3 and ∼1.81 × 104 cm−3. On comparison with mean electron densities at LST 3–4 hrs for observational period Dec. 24–31, 1998, at same solar zenith angle, it is found that the estimated primary peak electron density at LST 16 hrs is more by a factor of ∼1.06. With this analogy, the estimated secondary peak electron density should be ∼3.76 × 104 cm−3 instead of ∼1.81 × 104 cm−3. Thus, in order to obtain the estimated secondary peak electron density of ∼3.76 × 104 cm−3, soft X‐ray flux should be increased by a factor of ∼4.31.

Radial distribution of production rates, loss rates and densities corresponding to ion masses ⩽40 amu in the inner coma of Comet Halley: Composition and chemistry

In this paper we have studied the chemistry of C, H, N, O, and S compounds corresponding to ions of masses ⩽40 amu in the inner coma of the Comet 1P/Halley. The production rates, loss rates, and ion mass densities are calculated using the Analytical Yield Spectrum approach and solving coupled continuity equation controlled by the steady state photochemical equilibrium condition. The primary ionization sources in the model are solar EUV photons, photoelectrons, and auroral electrons of the solar wind origin. The chemical model couples ion–neutral, electron–neutral, photon–neutral and electron–ion reactions among ions, neutrals, electrons, and photons through over 600 chemical reactions. Of the 46 ions considered in the model the chemistry of 24 important ions (viz., CH 3OH + 2, H 3CO +, NH + 4, H 3S +, H 2CN +, H 2O +, NH + 3, CO +, C 3H + 3, OH +, H 3O +, CH 3OH +, C 3H + 4, C 2H + 2, C 2H +, HCO +, S +, CH + 3, H 2S +, O +, C +, CH + 4, C + 2, and O + 2) are discussed in this paper. At radial distances <1000 km, the electron density is mainly controlled by 6 ions, viz., NH + 4, H 3O +, CH 3OH + 2, H 3S +, H 2CN +, and H 2O +, in the decreasing order of their relative contribution. However, at distances >1000 km, the 6 major ions are H 3O +, CH 3OH + 2, H 2O +, H 3CO +, C 2H + 2, and NH + 4; along with ions CO +, OH +, and HCO +, whose importance increases with further increase in the radial distance. It is found that at radial distances greater than ∼1000 km (±500 km) the major chemical processes that govern the production and loss of several of the important ions in the inner coma are different from those that dominate at distances below this value. The importance of photoelectron impact ionization, and the relative contributions of solar EUV, and auroral and photoelectron ionization sources in the inner coma are clearly revealed by the present study. The calculated ion mass densities are compared with the Giotto Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km. There is a reasonable agreement between the model calculation and the Giotto measurements. The nine major peaks in the IMS spectra between masses 10 and 40 amu are reproduced fairly well by the model within a factor of two inside the ionopause. We have presented simple formulae for calculating densities of the nine major ions, which contribute to the nine major peaks in the IMS spectra, throughout the inner coma that will be useful in estimating their densities without running the complex chemical models.

Conditions in the Martian ionosphere/atmosphere from a comparison of a thermospheric model with radio occultation data

In this paper a possibility to find out some effect of dipole-like intrinsic magnetic field on the plasma scale height at altitudes 180–240 km in the Martian ionosphere, that is above the topside boundary of the chemically controlled region predicted by existing models of the magnetic field-free ionosphere, is examined. It is assumed that the effect could be detected if the relationship between plasma and neutral atmosphere scale heights which is expected under the photochemical equilibrium conditions in the multicomponent atmosphere remains valid at altitude region 180–240 km. The plasma scale heights assessed from the data collected during the Mariner 9 and Viking 1 sessions are statistically analyzed and its trends with the solar zenith angle and F 10.7 flux are evaluated. A half of statistically averaged scale height presented as a function of the solar zenith angle (SZA) and F 10.7 flux is compared with the neutral atmosphere scale heights, calculated from the model of Izakov and Roste (1996) for the corresponding SZA and F 10.7 fluxes. Since the conditions for the radio occultation experiments during the Mariner 9 and Viking 1 sessions are different a ratio of assessed plasma scale height to the appropriate neutral atmosphere scale height suits better for comparison than the average scale heights itself does. Employing such ratios the effect of dust content within the lower atmosphere on the thermal balance within the topside atmosphere at altitude 180–200 km is estimated.

Recent measurements of trace constituents from rocket and balloon probes

During the past few years several field campaigns each employing a number of different experiments and of different experimental techniques have given new insights into the behaviour of trace constituent distributions in the middle atmosphere. Results from the MAP/WINE and from the Energy Budget Campaings are presented and compared with results previously obtained. The measured trace gas distributions are often very structured in time and/or space. These variations cannot be understood if photochemical equilibrium conditions are assumed. It is therefore concluded that dynamical processes are of major importance to the trace constituent distributions even at altitudes where gases are believed to be in photochemical equilibrium.

Diagnostic analysis of two-dimensional monthly average ozone balance with Chapman chemistry

Chapman chemistry has been used in a two-dimensional model to simulate ozone balance phenomenology. The similarity between regions of ozone production and loss calculated using Chapman chemistry and those computed using LIMS and SAMS data with a photochemical equilibrium model indicate that such simplified chemistry is useful in studying gross features in stratospheric ozone balance. Net ozone production or loss rates are brought about by departures from the photochemical equilibrium (PCE) condition. If transport drives ozone above its PCE condition, then photochemical loss dominates production. If transport drives ozone below its PCE condition, then photochemical production dominates loss. Gross features of ozone loss/production ( L/P) inferred for the real atmosphere from data are also simulated using only eddy diffusion. This indicates that one must be careful in assigning a transport scheme for a two-dimensional model that mimics only behavior of the observed ozone L/P.

Doubly charged atomic oxygen ions in the thermosphere, 1. Photochemistry

Measurements of O++ concentrations made by the Atmosphere Explorer C satellite are analyzed for altitudes where photochemical equilibrium conditions prevail in order to determine the photochemical sources and sinks of the doubly charged ion. The major loss process is found to be through partial charge exchange with neutral atomic oxygen with a rate coefficient of 1 × 10−11 cm³ s−1 with an uncertainty of 40%. Above 220 km the major source is photo-ionization of O+. However, X ray ionization of O(λ ≤ 23.3 Å) producing O++ directly through the Auger process provides a better fit to the observed profile at lower altitudes. At solar maximum, particularly during solar disturbances, this process could be a major source of ionospheric O++ up to 500 km.

Stratospheric ozone: An introduction to its study

An analysis is made of the various reactions in which ozone and atomic oxygen are involved in the stratosphere. At the present time, hydrogen, nitrogen, and chlorine compounds in the ranges parts per million, parts per billion, and parts per trillion may have significant chemical effects. In the upper stratosphere, above the ozone peak, where there is no strong departure from photochemical equilibrium conditions, the action of hydroxyl and hydroperoxyl radicals of nitrogen dioxide and chlorine monoxide on atomic oxygen and of atomic chlorine on ozone can be introduced. A precise determination of their exact effects requires knowledge of (1) the vertical distribution of the H2O, CH4, and H2dissociation by reaction of these molecules with electronically excited oxygen atom O(¹D); (2) the ratio of the OH and HO2concentrations and their absolute values, which depend on insufficiently known rate coefficients; (3) the various origins of nitric oxide production, with their vertical distributions related to latitude and season; and (4) the various sources giving different chlorine compounds that may be dissociated in the stratosphere. In the lower stratosphere, below the ozone peak, there is no important photochemical production of O3, but there exist various possibilities of transport. The predictability of the action of chemical reactions depends strongly on important interactions between OH and HO2radicals with CO and NO, respectively, which affect the ration(OH)/n(HO2) at the tropopause level; between OH and NO2, which lead to the formation of nitric acid with its downward transport toward the troposphere; between NO and HO2, which lead to NO2and its subsequent photodissociation; between ClO and NO, which also lead to NO2and become more important than the reaction of ClO with O; and between Cl and various molecules, such as CH4and H2, which lead to HCl with its downward transport toward the troposphere. All these chemical processes are subject to many changes since they occur in the lower stratosphere where seasonal, latitudinal, and even day‐to‐day variations of the ozone concentrations are observed in association with advective and dynamic transports, which depend on meteorological conditions as indicated by variations of tropopause heights.

The effects of water vapour and oxides of nitrogen on ozone and temperature structure of the stratosphere

A steady-state mean meridional model of the stratosphere is used to investigate the effects of water vapour and nitrogen oxides on ozone and temperature distributions in the stratosphere. The Chapman classical photochemical scheme for ozone is extended to include the dominant reactions involving hydrogen compounds and nitrogen oxides. The ozone and temperature changes are studied under radiative-photochemical equilibrium conditions and in a model incorporating both transport and radiative-photochemical processes. It is found that both hydrogen and nitrogen reactions contribute to substantial decreases in ozone and temperature under photochemical equilibrium conditions, but the computed distribution do not resemble those observed. The effect of transport processes is to reduce the deviations in the ozone mixing ratio and temperature with the computed distributions having many features in common with the observations. It is found that the ozone and temperature respond more readily to the arbitrary increase of nitrogen oxides than to that of water vapour.

The Effects of Water Vapor and Oxides of Nitrogen on the Ozone and Temperature Structure of the Stratosphere

A steady-state mean meridional model of the stratosphere is used to investigate the effects of water vapor and nitrogen oxides on ozone and temperature distributions in the stratosphere. Chapman's classical photochemical scheme for ozone is extended to include the dominant reactions involving hydrogen compounds and nitrogen oxides. The ozone and temperature changes are studied under radiative-photochemical equilibrium conditions and in a model incorporating both transport and radiative-photochemical processes. It is found that both hydrogen and nitrogen reactions contribute to substantial decreases in ozone and temperature under photochemical equilibrium conditions, but the computed distributions do not resemble those observed. The effect of transport processes is to reduce the deviations in ozone mixing ratio and temperature, with the computed distributions having many features in common with the observations. It is found that ozone and temperature respond more readily to an arbitrary increase of nitrogen oxides than to that of water vapor.

Photochimie de l'ozone dans la stratosphère sous l'action des oxydes d'azote et des composés de l'hydrogène

The nature of the minor constituents which may have a controlling effect on the stratospheric ozone is discussed and the results of calculation are given. It is shown that there is practically no direct action on the ozone molecules as it is always assumed. All reactions involve atomic oxygen in photochemical equilibrium with ozone. A catalytic destruction of ozone by water vapor occurs through reactions of hydroxyl and hydroperoxyl radicals produced in the oxidation of H2O by excited oxygen atoms O(1D). With mixing ratios greater than 3 ppmv, water vapor represents a significant ozone-destroying agent in the upper stratosphere. The catalytic destruction by nitrogen oxides requires a concentration of NO2 molecules greater than 109 cm-3 above 25 km. Injection of water vapor and of nitric oxide at stratospheric levels will have a controlling effect on the ozone concentration in the upper stratosphere if it leads to mixing ratios greater than 3 ppmv and 3 ppbv for H2O and NO2, respectively. In the lower stratosphere (&lt; 25 km) dynamic considerations must be introduced since there is a complete departure from photochemical equilibrium conditions which require several years.