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Possible reasons for the non-detection of absorption in the metastable HeI(2^3S) line at transit observations of warm Neptune GJ436b, in spite of the well pronounced strong absorption features measured earlier in Ly{\alpha} for this planet, are investigated. We perform numeric simulations of the escaping upper atmosphere of this planet and its HeI(2^3S) triplet absorption with a global 3D multi-fluid self-consistent hydrodynamic model. By fitting the model parameters to the lowest detection level of absorption measurements, we constrain an upper limit the He/H abundance three times smaller than the solar value. We demonstrate that neither the significant changes of the stellar wind related with possible stellar coronal mass ejections (CMEs), or possible variations in the stellar ionization radiation, nor the presence of heavy trace elements have crucial effect on the absorption at the 10830{\AA} line of HeI(2^3S) triplet. The main reason of weak signature is that the region populated by the absorbing metastable helium is rather small (<3R_p), as well as the small size of the planet itself, in comparison to the host star. We show that the radiation pressure force acting on the HeI(2^3S) atoms spreads them along the line of sight and around the planet, thus further reducing peak absorption., Comment: 17 pages, 7 figures

The paper presents results of laboratory experiment modeling the interaction between Lunar magnetic anomalies and Solar wind. To model the LMA we use quadrupole magnetic field. The main dimentionless parameter of the problem, the ion inertia length relative to the mini-magnetosphere size, well corresponds between experiment and LMA conditions. The main result is measurement of the magnetically reflected proton fluxes, which show qualitative agreement to available satellite data.

A 3D fully self-consistent multi-fluid hydrodynamic aeronomy model is applied to simulate the hydrogen-helium expanding upper atmosphere of the hot Jupiter HD189733b, and related absorption in the Lya line and the 10830 A line of metastable helium. We studied the influence of a high-energy stellar flux, stellar wind, and Lya cooling to reproduce the available observations. We found that to fit the width of the absorption profile in 10830 A line the escaping upper atmosphere of planet should be close to the energy limited escape achieved with a significantly reduced Lya cooling at the altitudes with HI density higher than 3*10^6 cm^-3. Based on the preformed simulations, we constrain the helium abundance in the upper atmosphere of HD189733b by a rather low value of He/H~0.005. We show that under conditions of a moderate stellar wind similar to that of the Sun the absorption of Lya line takes place mostly within the Roche lobe due to thermal broadening at a level of about 7%. At an order of magnitude stronger wind, a significant absorption of about 15% at high blue shifted velocities of up to 100 km/s is generated in the bowshock region, due to Doppler broadening. These blue shifted velocities are still lower than those (~200 km/s) detected in one of the observations. We explain the differences between performed observations, though not in all the details, by the stellar activity and the related fluctuations of the ionizing radiation (in case of 10830 A line), and stellar wind (in case of Lya line).

The hot Jupiter WASP-80b has been identified as a possible excellent target for detecting and measuring HeI absorption in the upper atmosphere. We observed 4 primary transits of WASP-80b in the optical and near-IR using the HARPS-N and GIANO-B high-resolution spectrographs, focusing on the HeI triplet. We further employed a three-dimensional hydrodynamic aeronomy model to understand the observational results. We did not find any signature of planetary absorption at the position of the HeI triplet with an upper limit of 0.7% (i.e. 1.11 planetary radii; 95% confidence level). We re-estimated the stellar high-energy emission that we combined with a stellar photospheric model to generate the input for the hydrodynamic modelling. We obtained that, assuming a solar He to H abundance ratio, HeI absorption should have been detected. Considering a stellar wind 25 times weaker than solar, we could reproduce the non-detection only assuming a He to H abundance ratio about 16 times smaller than solar. Instead, considering a stellar wind 10 times stronger than solar, we could reproduce the non-detection only with a He to H abundance ratio about 10 times smaller than solar. We attempted to understand this result by collecting all past HeI measurements looking for correlations with stellar high-energy emission and planetary gravity, but without finding any. WASP-80b is not the only planet with a sub-solar estimated He to H abundance ratio, suggesting the presence of efficient physical mechanisms (e.g. phase separation, magnetic fields) capable of significantly modifying the He to H content in the upper atmosphere of hot Jupiters. The planetary macroscopic properties and the shape of the stellar spectral energy distribution are not sufficient for predicting the presence or absence of detectable metastable He in a planetary atmosphere, as also the He abundance appears to play a major role., Comment: Accepted for publication on A&A

The signs of an expanding atmosphere of HD209458b have been observed with far-ultraviolet transmission spectroscopy and in the measurements of transit absorption by metastable HeI. These observations are interpreted using the hydrodynamic and Monte-Carlo numerical simulations of various degree of complexity and consistency. At the same time, no attempt has been made to model atmospheric escape of a magnetized HD209458b, to see how the planetary magnetic field might affect the measured transit absorption lines. This paper presents the global 3D MHD self-consistent simulations of the expanding upper atmosphere of HD209458b interacting with the stellar wind, and models the observed HI (Lya), OI (1306 A), CII (1337 A), and HeI (10830 A) transit absorption features. We find that the planetary dipole magnetic field with the equatorial surface value of Bp = 1 G profoundly changes the character of atmospheric material outflow and the related absorption. We also investigate the formation of planetary magnetosphere in the stellar wind and show that its size is more determined by the escaping atmosphere flow rather than by the strength of magnetic field. Fitting of the simulation results to observations enables constraining the stellar XUV flux and He abundance at ~10 erg cm2/s (at 1 a.u.) and He/H=0.02, respectively, as well as setting an upper limit for the dipole magnetic field of Bp<0.1 G on the planetary surface at the equator. This implies that the magnetic dipole moment of HD209458b should be less than 0.06 of the Jovian value.

Transmission spectroscopy of WASP-107b revealed 7-8% absorption at the position of metastable HeI triplet at 10830 {\AA} in Doppler velocity range of [-20; 10] km/s, which is stronger than that measured in other exoplanets. With a dedicated 3D self-consistent hydrodynamic multi-fluid model we calculated the expanding upper atmosphere of WASP-107b and reproduced within the observations accuracy the measured HeI absorption profiles, constraining the stellar XUV flux to (6-10) erg cm-2 s-1 at 1 a.u., and the upper atmosphere helium abundance He/H to 0.075-0.15. The radiation pressure acting on the metastable HeI atoms was shown to be an important factor affecting the shape of the absorption profiles. Its effect is counterbalanced by the processes of collisional depopulation of the HeI metastable state. Altogether, the observed HeI absorption in WASP-107b can be interpreted with the expected reasonable parameters of the stellar-planetary system and appropriate account of the electron and atom impact processes., Comment: to be published

Warm Neptune GJ3470b has been recently observed in 23S-23P transition of metastable helium, yielding absorption of about 1% in Doppler velocity range of [-40; 10] km/s. Along with previous detection of absorption in Ly{\alpha} with depth of 20-40% in the blue and red wings of the line, it offers a complex target for simulation and testing of the current models. Obtained results suggest that absorption in both these lines comes from interaction of expanding upper planetary atmosphere with stellar plasma wind, allowing to constrain the stellar plasma parameters and the helium abundance in planet atmosphere.

The absorption of stellar radiation observed by the HD209458b in resonant lines of OI and CII has not yet been satisfactorily modeled. In our previous 2D simulations we have shown that the hydrogen-dominated upper atmosphere of HD209458b, heated by XUV radiation, expands supersonically beyond the Roche lobe and drags the heavier species along with it. Assuming solar abundances, OI and CII particles accelerated by tidal forces to velocities up to 50 km/s should produce the absorption due to Doppler resonance mechanism at the level of 6-10%, consistent with the observations. Since the 2D geometry does not take into account the Coriolis force in the planet reference frame, the question remained to which extent the spiraling of the escaping planetary material and its actually achieved velocity may influence the conclusions made on the basis of 2D modeling. In the present paper we apply for the first time in the study of HD209458b a global 3D hydrodynamic multi-fluid model that self-consistently describes the formation and expansion of the escaping planetary wind, affected by the tidal and Coriolis forces, as well as by the surrounding stellar wind. The modeling results confirm our previous findings that the velocity and density of the planetary flow are sufficiently high to produce the absorption in HI, OI, and CII resonant lines at the level close to the in-transit observed values. The novel finding is that the matching of the absorption measured in MgII and SiIII lines requires at least 10 times lower abundances of these elements than the Solar system values., Comment: Monthly Notices of the Royal Astronomical Society (2019)

Using a global 3D, fully self-consistent, multi-fluid hydrodynamic model, we simulate the escaping upper atmosphere of the warm Neptune GJ436b, driven by the stellar XUV radiation impact and gravitational forces and interacting with the stellar wind. Under the typical parameters of XUV flux and stellar wind plasma expected for GJ436, we calculate in-transit absorption in Ly{\alpha} and find that it is produced mostly by Energetic Neutral Atoms outside of the planetary Roche lobe, due to the resonant thermal line broadening. At the same time, the influence of radiation pressure has been shown to be insignificant. The modelled absorption is in good agreement with the observations and reveals such features as strong asymmetry between blue and red wings of the absorbed Ly{\alpha} line profile, deep transit depth in the high velocity blue part of the line reaching more than 70%, and the timing of early ingress. On the other hand, the model produces significantly deeper and longer egress than in observations, indicating that there might be other processes and factors, still not accounted, that affect the interaction between the planetary escaping material and the stellar wind. At the same time, it is possible that the observational data, collected in different measurement campaigns, are affected by strong variations of the stellar wind parameters between the visits, and therefore, they cannot be reproduced altogether with the single set of model parameters., Comment: 34 pages and 18 figures

We present a 3D fully selfconsistent multi-fluid hydrodynamic aeronomy model to study the structure of a hydrogen dominated expanding upper atmosphere around the hot Jupiter HD 209458b and the warm Neptune GJ 436b. In comparison to previous studies with 1D and 2D models, the present work finds such 3D features as zonal flows in upper atmosphere reaching up to 1 km/s, the tilting of the planetary outflow by Coriolis force by up to 45 degrees and its compression around equatorial plane by tidal forces. We also investigated in details the influence of Helium (He) on the structure of the thermosphere. It is found that by decrease of the barometric scale-height, the He presence in the atmosphere strongly affects the H2 dissociation front and the temperature maximum.

To shed more light on the nature of the observed Ly{\alpha} absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6-8 percent can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10-15 percent absorption at the blue wing of the Lya line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption., Comment: open access

The interaction of escaping upper atmosphere of a hydrogen rich non-magnetized analog of HD209458b with a stellar wind of its host G-type star at different orbital distances is simulated with a 2D axisymmetric multi-fluid hydrodynamic model. A realistic sun-like spectrum of XUV radiation which ionizes and heats the planetary atmosphere, hydrogen photo-chemistry, as well as stellar-planetary tidal interaction are taken into account to generate self-consistently an atmospheric hydrodynamic outflow. Two different regimes of the planetary and stellar winds interaction have been modelled. These are: 1) the "captured by the star" regime, when the tidal force and pressure gradient drive the planetary material beyond the Roche lobe towards the star, and 2) the "blown by the wind" regime, when sufficiently strong stellar wind confines the escaping planetary atmosphere and channels it into the tail. The model simulates in details the hydrodynamic interaction between the planetary atoms, protons and the stellar wind, as well as the production of energetic neutral atoms (ENAs) around the planet due to charge-exchange between planetary atoms and stellar protons. The revealed location and shape of the ENA cloud either as a paraboloid shell between ionopause and bowshock (for the "blown by the wind" regime), or a turbulent layer at the contact boundary between the planetary stream and stellar wind (for the "captured by the star" regime) are of importance for the interpretation of Ly{\alpha} absorption features in exoplanetary transit spectra and characterization of the plasma environments., Comment: Published on ApJ