We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is ...used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.
ABSTRACT We perform and analyze the results of a global magnetohydrodynamic simulation of the fast coronal mass ejection (CME) that occurred on 2011 March 7. The simulation is made using the newly ...developed Alfvén Wave Solar Model (AWSoM), which describes the background solar wind starting from the upper chromosphere and extends to 24 R . Coupling AWSoM to an inner heliosphere model with the Space Weather Modeling Framework extends the total domain beyond the orbit of Earth. Physical processes included in the model are multi-species thermodynamics, electron heat conduction (both collisional and collisionless formulations), optically thin radiative cooling, and Alfvén-wave turbulence that accelerates and heats the solar wind. The Alfvén-wave description is physically self-consistent, including non-Wentzel-Kramers-Brillouin reflection and physics-based apportioning of turbulent dissipative heating to both electrons and protons. Within this model, we initiate the CME by using the Gibson-Low analytical flux rope model and follow its evolution for days, in which time it propagates beyond STEREO A. A detailed comparison study is performed using remote as well as in situ observations. Although the flux rope structure is not compared directly due to lack of relevant ejecta observation at 1 au in this event, our results show that the new model can reproduce many of the observed features near the Sun (e.g., CME-driven extreme ultraviolet EUV waves, deflection of the flux rope from the coronal hole, "double-front" in the white light images) and in the heliosphere (e.g., shock propagation direction, shock properties at STEREO A).
ABSTRACT
The ratios of the three stable oxygen isotopes 16O, 17O, and 18O on the Earth and, as far as we know in the Solar system, show variations on the order of a few per cent at most, with a few ...outliers in meteorites. However, in the interstellar medium there are some highly fractionated oxygen isotopic ratios in some specific molecules. The goal of this work is to investigate the oxygen isotopic ratios in different volatile molecules found in the coma of comet 67P/Churyumov–Gerasimenko and compare them with findings from interstellar clouds in order to assess commonalities and differences. To accomplish this goal, we analysed data from the ROSINA instrument on Rosetta during its mission around the comet. 16O/18O ratios could be determined for O2, methanol, formaldehyde, carbonyl sulfide, and sulfur monoxide/dioxide. For O2 the 16O/17O ratio is also available. Some ratios are strongly enriched in the heavy isotopes, especially for sulfur-bearing molecules and formaldehyde, whereas for methanol the ratios are compatible with the ones in the Solar system. O2 falls in-between, but its oxygen isotopic ratios clearly differ from water, which likely rules out an origin of O2 from water, be it by radiolysis, dismutation during sublimation, or the Eley–Rideal process from water ions hitting the nucleus as postulated in the literature.
The provenance of water and organic compounds on Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between ...different scenarios is by determining the deuterium-to-hydrogen (D/H) ratios in the reservoirs for comets and Earth’s oceans. Here, we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the European Space Agency’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10
−4
—that is, approximately three times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean–like water.
We present a three-dimensional compressible magnetohydrodynamics (MHD) model of the interaction of two coronal mass ejections (CMEs). Two identical CMEs are launched in the exact same direction into ...a preexisting solar wind, the second one 10 hr after the first one. Our global steady state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near-solar minimum conditions. Within this model system, we drive the CMEs to erupt by the introduction of two three-dimensional magnetic flux ropes embedded in the helmet streamer. After an initial phase, when the trailing shock and the second CME propagate into the disturbed solar wind medium, they reach the edge of the first magnetic cloud, leading to complex magnetic interactions and a steep acceleration of the shock. Later, the trailing shock reaches the dense sheath of plasma associated with the leading shock, where it decelerates to a speed about 100 km s super(-1) larger than the speed of the leading shock. Eventually, the two shocks merge and a stronger, faster shock forms in association with a contact discontinuity between the "old" and "new" downstream regions. We find that the trailing shock always remains a fast-mode shock. A reverse shock is driven after the collision of the two magnetic clouds due to the difference in speed within the reconnection region. At Earth, the two magnetic clouds can still be distinguished, with a compressed and heated first cloud and a second overexpanded cloud. The transit time of this complex ejecta is reduced by about 6 hr compared to the case of the first CME without interaction. Our simulation is able to reproduce and explain some of the general features observed in satellite data for multiple magnetic clouds.
D₂O and HDS in the coma of 67P/Churyumov—Gerasimenko Altwegg, K.; Balsiger, H.; Berthelier, J. J. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
07/2017, Letnik:
375, Številka:
2097
Journal Article
Recenzirano
Odprti dostop
The European Rosetta mission has been following comet 67P/Churyumov-Gerasimenko for 2 years, studying the nucleus and coma in great detail. For most of these 2 years the Rosetta Orbiter Sensor for ...Ion and Neutral Analysis (ROSINA) has analysed the volatile part of the coma. With its high mass resolution and sensitivity it was able to not only detect deuterated water HDO, but also doubly deuterated water, D₂O and deuterated hydrogen sulfide HDS. The ratios for HDO/H₂O, D₂O/HDO and HDS/H₂S derived from our measurements are (1.05 ± 0.14) × 10⁻³, (1.80 ± 0.9) × 10⁻² and (1.2 ± 0.3) × 10⁻³, respectively. These results yield a very high ratio of 17 for D₂O/HDO relative to HDO/H₂O. Statistically one would expect just 1/4. Such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. The high HDS/H₂S ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary H₂S with presolar grain surface chemistry. This article is part of the themed issue 'Cometary science after Rosetta'.
ABSTRACT The classic accepted view of the heliosphere is a quiescent, comet-like shape aligned in the direction of the Sun's travel through the interstellar medium (ISM) extending for thousands of ...astronomical units (AUs). Here, we show, based on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the twisted magnetic field of the Sun confines the solar wind plasma beyond the termination shock and drives jets to the north and south very much like astrophysical jets. These jets are deflected into the tail region by the motion of the Sun through the ISM similar to bent galactic jets moving through the intergalactic medium. The interstellar wind blows the two jets into the tail but is not strong enough to force the lobes into a single comet-like tail, as happens to some astrophysical jets. Instead, the interstellar wind flows around the heliosphere and into the equatorial region between the two jets. As in some astrophysical jets that are kink unstable, we show here that the heliospheric jets are turbulent (due to large-scale MHD instabilities and reconnection) and strongly mix the solar wind with the ISM beyond 400 AU. The resulting turbulence has important implications for particle acceleration in the heliosphere. The two-lobe structure is consistent with the energetic neutral atom (ENA) images of the heliotail from IBEX where two lobes are visible in the north and south and the suggestion from the Cassini ENAs that the heliosphere is "tailless."
By analogy with the Solar system, it is believed that stellar winds will form bow shocks around exoplanets. For hot Jupiters the bow shock will not form directly between the planet and the star, ...causing an asymmetric distribution of mass around the exoplanet and hence an asymmetric transit. As the planet orbits through varying wind conditions, the strength and geometry of its bow shock will change, thus producing transits of varying shape. We model this process using magnetic maps of HD 189733 taken one year apart, coupled with a 3D stellar wind model, to determine the local stellar wind conditions throughout the orbital path of the planet. We predict the time-varying geometry and density of the bow shock that forms around the magnetosphere of the planet and simulate transit light curves. Depending on the nature of the stellar magnetic field, and hence its wind, we find that both the transit duration and ingress time can vary when compared to optical light curves. We conclude that consecutive near-UV transit light curves may vary significantly and can therefore provide an insight into the structure and evolution of the stellar wind.
In order to better describe the space plasmas where pressure anisotropy has prominent effects, we extend the BATS‐R‐US magnetohydrodynamics (MHD) model to include anisotropic pressure. We implement ...the anisotropic MHD equations under the double adiabatic approximation with an additional pressure relaxation term into BATS‐R‐US and perform global magnetospheric simulations. The results from idealized magnetospheric simulations confirm previous studies: pressure anisotropy widens the magnetosheath, increases the density depletion in the vicinity of the magnetopause, enhances the nightside plasma pressure, and introduces an eastward ring current. In addition, we find that the flow speed in the magnetotail is significantly reduced by including pressure anisotropy in MHD simulations. Our model is validated through comparing the simulations to the THEMIS data on both the dayside and nightside of the magnetosphere during quiet times. The comparison to the results from isotropic MHD simulations implies that although anisotropic MHD is comparable to isotropic MHD in matching the measurement, it improves the simulated plasma velocity in some cases.
Key Points
We extend the BATS‐R‐US MHD model to include anisotropic pressure
The anisotropic MHD model is validated by global magnetospheric simulations
We compare the anisotropic MHD model to the standard isotropic MHD model
Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the ...main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta spacecraft. A N2/CO ratio of (5.70 ± 0.66) × 10–3 (2σ standard deviation of the sampled mean) corresponds to depletion by a factor of ∼25.4 ± 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ∼30 kelvin.
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