V. I. Shematovich

A kinetic model of the formation of the hot oxygen geocorona. 1: Quiet geomagnetic conditions

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non-thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision-dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.

Observation of the proton aurora with IMAGE FUV imager and simultaneous ion flux in situ measurements

The far ultraviolet cameras on board the IMAGE satellite images the aurora in three different spectral regions. One of the channels of the spectrographic imager SI12 observes the Doppler-shifted Lyman α emission of precipitating protons. It makes it possible to spectrally discriminate between the proton and electron FUV aurora and to globally map the energetic protons. Its response depends on the auroral Lyman α line shape which reflects the characteristics of the proton pitch angle and energy distributions. We illustrate the dependence of the SI12 count rate on the characteristic energy of the proton precipitation and the viewing geometry. Simultaneous in situ observations of the precipitated protons have been collected during a FAST satellite pass when IMAGE was observing the global north polar region. The premidnight region located at the equatorward boundary of the oval is dominated by proton precipitation with a mean energy Ē = 7 keV which is separated from the electron component. The prenoon crossing exhibits a softer proton energy spectrum with Ē = 0.9 keV. The measured proton energy distribution is used as an input to a Monte Carlo model to calculate the expected SI12 signal along the magnetic footprint of the satellite orbit. If the different spatial resolution of the two types of measurements is accounted for, a good quantitative agreement is found with the IMAGE observations. Similarly, ion flux measurements collected on board the Defense Meteorological Satellite Program Fl5 satellite during an overflight in the postmidnight sector provide good agreement with the SI12 observations at the footprint aurora. The comparisons confirm the reliability of the FUV IMAGE cameras to remotely discriminate between the electron and the proton precipitations. The vertical emission rate profiles of the N2 Lyman-Birge-Hopfield and OI(1356A) emissions are calculated in the proton-dominated premidnight region. It is shown that the protons and the electrons produce FUV emissions with contributions peaking at different altitudes. Excitation by secondary electrons dominates the production of both emissions.

Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach

Abstract The formation of singly and doubly deuterated isotopomers of formaldehyde and of singly, doubly, and multiply deuterated isotopomers of methanol on interstellar grain surfaces has been studied with a semi-empirical modified rate approach and a Monte Carlo method in the temperature range 10– 20 K . Agreement between the results of the two methods is satisfactory for all major and many minor species throughout this range. If gas-phase fractionation can produce a high abundance of atomic deuterium, which then accretes onto grain surfaces, diffusive surface chemistry can produce large abundances of deuterated species, especially at low temperatures and high gas densities. Warming temperatures will then permit these surface species to evaporate into the gas, where they will remain abundant for a considerable period. We calculate that the doubly deuterated molecules CHD2OH and CH2DOD are particularly abundant and should be searched for in the gas phase of protostellar sources. For example, at 10 K and high density, these species can achieve up to 10–20% of the abundance of methanol.

A model of the Lyman‐α line profile in the proton aurora

The Lyman-α auroral emission is characterized by a broad line profile whose shape depends on the energy and pitch angle distributions of the initial proton beam, whereas its total brightness reflects the proton energy flux precipitated into the auroral upper atmosphere. Global remote sensing of the proton aurora through its ultraviolet signature makes it is increasingly important to relate the characteristics of the Lyman-α emission to the physical properties of the precipitated proton flux. We present a numerical model of proton and hydrogen flux transport and kinetics based on the direct simulation Monte Carlo method. In this approach, all elastic and inelastic processes are stochastically simulated as well as is the production of Lyman-α photons with the associated Doppler velocity component. The model also includes collisional, geomagnetic, and geometric spreading of the proton-hydrogen beam. We show that consideration of the stochastic character of the H atom velocity redistribution after collisions produces line profiles different from those obtained in the strictly forward or mean scattering angle approximations previously used in proton transport codes. In particular, the predicted fraction of photons due to backscattered particles is considerably larger when stochastic collision scattering is considered than in the strictly forward or mean scattering angle approximations. In contrast to the median wavelength, the position of the peak in the line profile shows a weak inverse dependence on the proton energy. The efficiency of the Lyman-α photon production per unit incident energy flux significantly drops as the mean proton energy increases. The line profile and the amount of blue-shifted (for downward viewing) emission depends in a complex way on the initial energy and pitch angle distribution of the protons. The line profiles expected for the noon cusp and midnight proton aurora are shown to be significantly different.

Nitrogen loss from Titan

and carbon-containing pickup ions is described using a Monte Carlo model. The interaction of these ions with the atmospheric neutrals leads to the production of fast neutrals that collide with other atmospheric neutrals producing heating and ejection of atoms and molecules. Results from Brecht et al. [2000] are used to estimate the net flux and energy spectra of the magnetospheric and pickup ions onto the exobase. Sputtering is primarily responsible for any ejected molecular nitrogen, and, for the ion fluxes used, we show that the total sputtering contribution is comparable to or larger than the dissociation contribution giving a total loss rate of � 3.6 � 10 25 nitrogen neutrals per

On the master equation approach to diffusive grain-surface chemistry: The H, O, CO system

We have used the master equation approach to study a moderately complex network of diffusive reactions occurring on the surfaces of interstellar dust particles. This network is meant to apply to dense clouds in which a large portion of the gas-phase carbon has already been converted to carbon monoxide. Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust particles and their chemistry is followed. The stable molecules produced are oxygen, hydrogen, water, carbon dioxide (CO 2 ), formaldehyde (H 2 CO), and methanol (CH 3 OH). The surface abundances calculated via the master equation approach are in good agreement with those obtained via a Monte Carlo method but can differ considerably from those obtained with standard rate equations.

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

[1] A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO2 doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166 during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15–20%.

Stochastic models of hot planetary and satellite coronas: A photochemical source of hot oxygen in the upper atmosphere of Mars

The processes of the kinetics and transport of hot oxygen atoms in the upper atmosphere of Mars are studied. A reaction of dissociative recombination of the main ionospheric ion O2+ with thermal electrons is considered as a photochemical source of suprathermal oxygen atoms. Oxygen atoms are formed in the dissociative recombination reaction with an excess of kinetic energy of about 0.4–4 eV and lose that energy in elastic and inelastic collisions with the ambient thermal atmospheric gas. The altitude distributions of the concentrations of neutral and ionized components, as well as their temperatures, were taken from Krasnopolsky (2002). Unlike the models published earlier, detailed calculations of the formation, collisional kinetics, and transport of suprathermal oxygen atoms in the thermosphere-exosphere transition region of the upper atmosphere of Mars have been made for the first time. For this, we used a stochastic model of the formation of a hot planetary corona (Shematovich, 2004). It has been shown that the considered photochemical source of suprathermal oxygen leads to the formation of the hot corona and to higher nonthermal losses of oxygen from the upper atmosphere of Mars due to escape fluxes. The detailed energy spectra of the fluxes of suprathermal atomic oxygen were calculated for the thermosphere-exosphere transition region of the Martian atmosphere.

Hot oxygen and carbon escape from the martian atmosphere

Abstract The escape of hot O and C atoms from the present martian atmosphere during low and high solar activity conditions has been studied with a Monte-Carlo model. The model includes the initial energy distribution of hot atoms, elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient cooler neutral atmosphere, and applies energy dependent total and differential cross sections for the determination of the collision probability and the scattering angles. The results yield a total loss rate of hot oxygen of 2.3 – 2.9 × 10 25 s − 1 during low and high solar activity conditions and is mainly due to dissociative recombination of O2+ and CO2+. The total loss rates of carbon are found to be 0.8 and 3.2 × 10 24 s − 1 for low and high solar activity, respectively, with photodissociation of CO being the main source. Depending on solar activity, the obtained carbon loss rates are up to ~40 times higher than the CO2+ ion loss rate inferred from Mars Express ASPERA-3 observations. Finally, collisional effects above the exobase reduce the escape rates by about 20–30% with respect to a collionless exophere.

Thermalization of O(1D) atoms in the thermosphere

Measurements of the Doppler width of the 6300 A airglow emission line have been extensively used to determine the thermospheric temperature. This technique is based on the assumption that the bulk of the emitting O(1D) atoms are thermalized in the region of the airglow source (200–300 km). A Monte Carlo stochastic model is used to calculate the energy distribution function of O(1D) atoms in the daytime and nighttime thermosphere. Hot O(1D) atoms are produced by exothermic processes and their thermalization is controlled by the competition between radiation, collisional quenching, and relaxation. It is found that the O(1D) temperature departs from the background gas temperature not only in the upper thermosphere but also in the region of the bulk 6300 A emission. At 300 km for low solar activity conditions, the model predicts an excess O(1D) temperature of ∼ 180 K during daytime and ∼ 950 K at night. The temperature departure persists at lower altitudes as a result of the major contribution of the O+2 dissociative recombination source of hot 1D atoms. Experimental evidence based on the Fabry-Perot interferometer measurements on board the Dynamics Explorer satellite confirms the existence of an O(1D) temperature excess over the mass spectrometer/incoherent scatter (MSIS) value. It is concluded that temperatures deduced from the 6300 A airglow line width may significantly exceed the ambient gas temperature in a way depending on solar activity, local time, and observation geometry.