Context. The heating efficiency ηhν is defined as the ratio of the net local gas-heating rate to the rate of stellar radiative energy absorption. It plays an important role in thermal-escape ...processes from the upper atmospheres of planets that are exposed to stellar soft X-rays and extreme ultraviolet radiation (XUV). Aims. We model the thermal-escape-related heating efficiency ηhν of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the extrasolar gas giant HD 209458b. The model result is then compared with previous thermal-hydrogen-escape studies, which assumed ηhν values between 10–100%. Methods. The photolytic and electron impact processes in the thermosphere were studied by solving the kinetic Boltzmann equation and applying a Direct Simulation Monte Carlo model. We calculated the energy deposition rates of the stellar XUV flux and that of the accompanying primary photoelectrons that are caused by electron impact processes in the H2→ H transition region in the upper atmosphere. Results. The heating by XUV radiation of hydrogen-dominated upper atmospheres does not reach higher values than 20% above the main thermosphere altitude, if the participation of photoelectron impact processes is included. Conclusions. Hydrogen-escape studies from exoplanets that assume ηhν values that are ≥20% probably overestimate the thermal escape or mass-loss rates, while those who assumed values that are <20% produce more realistic atmospheric-escape rates.
Far ultraviolet spectral observations have been made with the Hubble Space Telescope in the time‐tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in ...such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long‐wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere‐ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three‐dimensional modeling of the distribution of particle heat sources into the high‐latitude Jovian upper atmosphere.
Key Points
We present the first maps of the absorption of auroral FUV emission by methaneThe penetration depth of auroral electrons layer is highly spatially variableThe electron energy reaches as much as 500 keV in the polar region
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We present the kinetic Monte-Carlo model, which was developed to describe the influence of a proton flux from the undisturbed solar wind on the daytime atmosphere of Mars. For the first time, the ...degradation of the solar-wind proton spectrum in the cascade charge-exchange process in the extended hydrogen corona of Mars has been self-consistently modeled; and the energy fluxes and energy spectra of hydrogen atoms penetrating into the daytime upper atmosphere through the boundary of the induced magnetosphere have been determined. The obtained characteristics make it possible to model numerically the proton auroral phenomena observed in the upper atmosphere of Mars with the Imaging Ultraviolet Spectrometer onboard the Mars Atmosphere and Volatile Evolution spacecraft. In our future studies, we plan to compare the results of these calculations to those of observations, which will provide a unique opportunity to determine more accurately the properties of the atmosphere and the magnetic field of Mars, as well as will improve the techniques for determining the solar wind parameters.
Because of their activity, late-type stars are known to host powerful flares producing intense high-energy radiation on short timescales that may significantly affect the atmosphere of nearby ...planets. We employ a one-dimensional aeronomic model to study the reaction of the upper atmosphere of the hot Jupiter HD 209458b to the additional high-energy irradiation caused by a stellar flare. Atmospheric absorption of the additional energy produced during a flare leads to local atmospheric heating, accompanied by the formation of two propagating shock waves. We present estimates of the additional atmospheric loss occurring in response to the flare. We find the mass-loss rate at the exobase level to significantly increase (3.8 × 1010, 8 × 1010, and 3.5 × 1011 g s−1 for 10, 100, and 1000 times the high-energy flux of the quiet star, respectively) in comparison to that found considering the inactive star (2 × 1010 g s−1).
The paper discusses the formation and dynamics of the rarefied gas envelope near the icy surface of Jupiter’s moon Ganymede. Being the most massive icy moon, Ganymede can form a rarefied exosphere ...with a relatively dense near-surface layer. The main parent component of the gas shell is water vapor, which enters the atmosphere due to thermal degassing, nonthermal radiolysis, and other active processes and phenomena on the moon’s icy surface. A numerical kinetic simulation is performed to investigate, at the molecular level, the formation, chemical evolution, and dynamics of the mainly H
2
O- and O
2
-dominant rarefied gas envelopes. The ionization processes in these rarefied gas envelopes are due to exposure to ultraviolet radiation from the Sun and the magnetospheric plasma. The chemical diversity of the icy moon’s gas envelope is attributed to the primary action of ultraviolet solar photons and plasma electrons on the rarefied gas in the H
2
O- or O
2
-dominant atmosphere. The model is used to calculate the formation and development of the chemical diversity in the relatively dense near-surface envelope of Ganymede, where an important contribution comes from collisions between parent molecules and the products of their photolysis and radiolysis.
The review presents the results of a wide range of studies on modeling the atmospheres and shells of exoplanets and studying the processes associated with the activity of the parent star, performed ...at the Institute of Astronomy of the Russian Academy of Sciences in recent years. The developed methods of analyzing superflares in solar-type stars are applied to stars with planetary systems and the obtained estimates are used to detail the conditions of extreme stellar activity in the study of atmospheric losses for Earth-type planets in low orbits—super-Earths and sub-Neptunes. The results of calculations of the rate of atmospheric loss for exoplanets in orbits close to the parent star (close-in exoplanets) are presented and it is shown that under conditions of a high level of stellar radiation rigidity, and even more so under conditions of a stellar flare, the contribution of exothermic photochemistry processes to the formation of a flow of supra-thermal hydrogen atoms escaping from the atmosphere becomes significant and comparable to the flow of hydrodynamic outflow. Accordingly, this source of supra-thermalhydrogen atoms should be included in modern aeronomic models of physical and chemical processes in the upper atmospheres of hot exoplanets. Taking into account the contribution of supra-thermal particles in the aeronomic model made it possible to clarify the heating rates of atmospheric gas due to the absorption of hard radiation from the parent star. This, in turn, made it possible to more accurately calculate the values of the rate of mass loss by atmospheres and, accordingly, to study the evolutionary properties of the atmospheres of hot exoplanets. It is also important to note that the kinetic and aeronomic models presented in the review, developed in recent years at the Institute of Astronomy of the Russian Academy of Sciences, will be used to analyze and interpret existing and expected observations of the atmospheres of Earth-type exoplanets. Such work will allow to impose additional restrictions on the models and thereby make them more reliable.
We present model results of the interaction of proton and hydrogen atom precipitation with the Martian atmosphere. We use a kinetic Monte Carlo model developed earlier for the analysis of the ...Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐3) Mars Express data. With the availability of Mars Atmosphere and Volatile Evolution Mission in situ measurements, not only the flux of protons incident on the atmosphere but also their degradation along the orbit may now be described. The comparison of the simulations with data collected with the Solar Wind Ion Analyzer shows that the Monte Carlo model reproduces some of the measured features. The results of comparison between simulations and measurements of the proton fluxes at low altitudes make it possible to infer the efficiency of charge exchange between solar wind and the extended hydrogen corona if the value of the magnetic field is measured simultaneously. We also find that the induced magnetic field plays a very important role in the formation of the backscattered flux and strongly controls its magnitude. At the same time, discrepancies between the modeled and the measured energy spectra of the backscattered protons are pointed out. We suggest that some of the physical processes controlling the upward flux are not fully understood or that the data processing of the measured backscattered proton flux should be improved.
Key Points
We present results from a kinetic Monte Carlo model of proton and hydrogen atom precipitation into the Martian atmosphere
Comparison of simulations with SWIA MAVEN data makes it possible to infer the efficiency of charge exchange between solar wind and H corona
Discrepancy with the measured spectra suggests that some of physical processes are not understood or that additional data should be analyzed
We present the model calculation results of the atomic oxygen loss rate from the Martian atmosphere induced by precipitation of high-energy protons and hydrogen atoms (H/H
+
) from the solar wind ...plasma. Penetration of energetic protons and hydrogen atoms from the solar wind plasma to the upper atmosphere of Mars at altitudes of 100−250 km is accompanied by the momentum and energy transfer in collisions with the main component, atomic oxygen. This process is considered as atmospheric gas sputtering during proton auroral events, which is accompanied by formation of the suprathermal hydrogen and oxygen atom fluxes escaping from the atmosphere. When calculating the formation rate of suprathermal atoms, the modified Monto Carlo kinetic model was used. This model was earlier developed to analyze the data of the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) and the Solar Wind Ion Analyzer (SWIA) onboard the Mars Express (MEX) and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, respectively. We study the processes of kinetics and transport of hot oxygen atoms in the transition zone (from the thermosphere to the exosphere) of Mars’ upper atmosphere. The kinetic energy distribution functions for suprathermal oxygen atoms were calculated. It has been shown that, during proton auroral events on Mars, the exosphere is populated with a significant number of suprathermal oxygen atoms, the kinetic energy of which reaches the escape energy, 2 eV. In addition to photochemical sources, a hot fraction is formed in the oxygen corona; and a nonthermal flux of atomic oxygen escaping from the Martian atmosphere is produced during proton aurora events. Proton aurorae are sporadic auroral events. Consequently, according to the estimates obtained from the recent MAVEN observations the magnitude of the precipitation-induced escaping flux of hot oxygen atoms may become prevailing over the photochemical sources under conditions of the extreme solar events such as solar flares and coronal mass ejections.
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 2.3–2.9×1025s−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 3.2×1024s−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.
•Collisions are treated on the basis of recent total and differential cross sections.•Dissociative recombination of O2+ and CO2+ appear equally important for O escape.•Little variation of hot oxygen exosphere density with solar activity.•Loss of C due to photodissociation of CO higher than in previous calculations.•Ratio of O and C loss due to dissociative recombination of CO2+ is 5:1.
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