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Numerical modelling of ENAs from stellar wind interactions
... Another important aspect that underlines the importance of numerical modeling is that space experiments are of an observational nature, and we cannot control the conditions of the system under study. Reproducing a system's behavior, and measurements thereof, in numerical tests is therefore critical to identify laws governing the system [2]. The existence of the solar wind was not conjectured until the latter half of the past century. ...
The physical properties of the interaction region between the solar wind and comets were studded by solution of the continuities equations using first-upwind explicit method in three dimensions. The cometary ion tail formed and shaped due to the existence of the interplanetary magnetic field (IMF) which depending on the comet type has also been made. The physical properties and its change for comets studied in this research are: mass density, velocity and magnetic field for the comet's ion tail. In this research, simulations of the interaction between solar wind and cometary ions are performed using the MHD laws. The simulations included in the present research cover two general cases (without source term where the solar wind properties are studied when there is no comet) and (with source term, in the presence of the comet). Also the effect of interplanetary magnetic field on the solar wind and cometary tail interaction has been studied in both cases. In this research, the continuity equations were solved in one and three dimensions using the First-Upwind explicit method, the results of the present research showed that the interaction near the cometary nucleus is mainly affected by the new ions added to the plasma of the solar wind, which increases the average molecular weight and result in many unique characteristics of the cometary tail. These characteristics were explained in the presence of the IMF.
representing the solar wind and the two major ionospheric ion species, respectively. The calculations indicate that the presence of a hot oxygen corona does not, within the resolution and accuracy of the model, lead to any significant effect on the dayside bow shock and ionopause positions. Next the trans-terminator fluxes and escape fluxes down the tail were calculated neglecting the effects of the crustal magnetic field. The calculated flux values are consistent with the measured escape fluxes and the calculated limiting fluxes from the dayside ionosphere. Finally, a 60-order harmonic expansion model of the measured magnetic field was incorporated into the model. The crustal magnetic field did not cause major distortions in the bow shock but certainly had an important effect within the magnetosheath and on the apparent altitude of the ionopause. The model results also indicated the presence of ''minimagnetocylinders,'' consistent with the MGS observations. We also recalculated the trans-terminator and escape fluxes, for the nominal solar wind case, in the presence of the crustal magnetic field and found, as expected, that there is a decrease in the calculated escape flux; however, it is still reasonably close to the value estimated from the Phobos-2 observations. INDEX TERMS: 2780 Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 2459 Ionosphere: Planetary ionospheres (5435, 5729, 6026, 6027, 6028); 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 2728 Magnetospheric Physics: Magnetosheath; KEYWORDS: Mars, MHD, bow shock, escape flux, solar wind interaction, crustal magnetic field
We present the first computer simulated images of energetic neutral atoms (ENAs) produced by charge exchange between solar wind protons and cometary neutrals. Proton bulk flow and temperature from MHD simulations are used along with a model of cometary neutral densities. The emission of hydrogen ENAs from the comet is then calculated using cross sections for the charge exchange collisions. The ENA production rate is integrated along lines of sight to produce an image of the fluxes. We find detection of hydrogen ENAs feasible and most favorable at solar zenith angles between 80 and 130 degrees.
The Analyzer of Space Plasma and Energetic Atoms (ASPERA-4) experiment on Venus Express provides the first measurements of energetic neutral atoms (ENAs) from Venus. The results improve our knowledge on the interaction of the solar wind with a nonmagnetized planet and they present an observational constraint to existing plasma models. We characterize the tailward flow of hydrogen ENAs observed on the nightside by providing global images of the ENA intensity. The images show a highly concentrated tailward flow of hydrogen ENAs tangential to the Venus limb around the Sun's direction. No oxygen ENAs above the instrument threshold are detected. The observed ENA intensities are reproduced with a simple ENA model within a factor of 2, indicating that the observed hydrogen ENAs originate from shocked solar wind protons that charge exchange with the neutral hydrogen exosphere.
The paper presents results of a one-dimensional hybrid simulation of the boundary region that exists between the dense release plasma and its associated diamagnetic cavity, and the oncoming solar wind flow and field. A quasi-steady field and plasma structure is found to evolve which allows momentum transfer to occur on scales which are smaller than the gyroscales of both ion species. From simple considerations an analytical estimate of the snowplough speed has been obtained which is in excellent agreement with the simulation results. Ultimately, this should allow scaling of the results presented to the parameters of the actual AMPTE releases.
Observed atomic hydrogen absorption in the stellar Lyman-alpha line by
the planet HD 209458b shows hydrogen with high velocity at great
distances from the planet. This has been interpreted as hydrogen atoms
undergoing hydrodynamic escape. We estimate the production of energetic
neutral atoms from the interaction of the stellar wind with the
extrasolar planet HD 209458b, and the implications for observations of
atomic hydrogen. The estimated production of hydrogen-energetic neutral
atoms is compared with observations.
We present a coupling between an analytical three-dimensional model covering the plasma flow behaviour through the magnetopause transition layer near a reconnection site, with results from a global MHD simulation describing the plasma flow in the magnetosheath. The structure of the plasma flow near a reconnection site at the dayside terrestrial magnetopause is investigated, together with the development of the magnetopause transition region.
We report on the completion of the first version of a new-generation simulation code, FLASH. The FLASH code solves the fully compressible, reactive hydrodynamic equations and allows for the use of adaptive mesh refinement. It also contains state-of-the-art modules for the equations of state and thermonuclear reaction networks. The FLASH code was developed to study the problems of nuclear flashes on the surfaces of neutron stars and white dwarfs, as well as in the interior of white dwarfs. We expect, however, that the FLASH code will be useful for solving a wide variety of other problems. This first version of the code has been subjected to a large variety of test cases and is currently being used for production simulations of X-ray bursts, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and thermonuclear flame fronts. The FLASH code is portable and already runs on a wide variety of massively parallel machines, including some of the largest machines now extant.
We report a spectroscopic orbit with period P = 3.52433 ± 0.00027 days for the planetary companion that transits the solar-type star HD 209458. For the metallicity, mass, and radius of the star, we derive [Fe/H] = 0.00 ± 0.02, M* = 1.1 ± 0.1 M☉, and R* = 1.2 ± 0.1 R☉. This is based on a new analysis of the iron lines in our HIRES template spectrum and also on the absolute magnitude, effective temperature, and color of the star, and it uses isochrones from four different sets of stellar evolution models. Using these values for the stellar parameters, we reanalyze the transit data and derive an orbital inclination of i = 861 ± 16. For the planet, we derive a mass of Mp = 0.69 ± 0.05 MJup, a radius of Rp = 1.40 ± 0.17 RJup, and a density of ρ = 0.31 ± 0.07 g cm-3.
The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction
with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard
the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary
disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic
field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This
paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation
results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12
and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE
observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows
inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns
observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the IMF changes
in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns
observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree
with the antiparallel merging model, though locally component merging might broaden the merging region, especially in the
region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion
of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere
are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations
emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause, as well as the intricate
connection between the bow shock and the auroral region.
When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.
A three-parameter method for modeling the position and shape of planetary bow waves was chosen to model the near portion of the Venus, earth and Mars bow shocks, and its results were compared with those of models using one to six free variables. It was found that the relative effective shapes of the near Martian, Cytherean, and terrestrial bow shocks are ellipsoidal, paraboloidal, and hyperboloidal, respectively, in response to the increasing bluntness of the obstacles that the planets present to the solar wind. No significant deviations from axial symmetry were found when the near bow waves of the earth and Venus were mapped into the aberrated terminator plane, in agreement with gas dynamic theory predictions neglecting the effects of the IMF because of their minuteness.
Absorption in the stellar Lyman-alpha (Lyalpha) line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet. This has been interpreted as hydrogen atoms escaping from the planet's exosphere, possibly undergoing hydrodynamic blow-off, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Lyalpha absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations.
Introduction to Space Physics - edited by Margaret G. Kivelson April 1995
It has often been said that 99% of the matter in the universe is in the plasma state; that is, in the form of an electrified gas with the atoms dissociated into positive ions and negative electrons. This estimate may not be very accurate, but it is certainly a reasonable one in view of the fact that stellar interiors and atmospheres, gaseous nebulae, and much of the interstellar hydrogen are plasmas. In our own neighborhood, as soon as one leaves the earth’s atmosphere, one encounters the plasma comprising the Van Allen radiation belts and the solar wind. On the other hand, in our everyday lives encounters with plasmas are limited to a few examples: the flash of a lightning bolt, the soft glow of the Aurora Borealis, the conducting gas inside a fluorescent tube or neon sign, and the slight amount of ionization in a rocket exhaust. It would seem that we live in the 1% of the universe in which plasmas do not occur naturally.
This book provides a comprehensive introduction to the physical
phenomena that result from the interaction of the sun and the planets -
often termed space weather. Physics of the Space Environment explores
the basic processes in the Sun, in the interplanetary medium, in the
near-Earth space, and down into the atmosphere. The first part of the
book summarizes fundamental elements of transport theory relevant for
the atmosphere, ionosphere and the magnetosphere. This theory is then
applied to physical phenomena in the space environment. The fundamental
physical processes are emphasized throughout, and basic concepts and
methods are derived from first principles. This book is unique in its
balanced treatment of space plasma and aeronomical phenomena. Students
and researchers with a basic mathematics and physics background will
find this book invaluable in the study of phenomena in the space
environment.
Most of the solar systems are in the plasma state and its subtle non-linear interaction with the magnetic field is described for many purposes adequately by the equations of magnetohydrodynamics (MHD). These equations are also applicable to the electrically conducting liquid cores of planets, such as our Earth, where magnetic fields are being generated by dynamo action. Indeed, this realisation of the existence of a strong coupling between a magnetic field and a plasma has revolutionised our understanding of the solar system. Furthermore, the complementary remote sensing observations at high resolution of the Sun and in-situ spacecraft measurements of solar system plasmas have revealed a spectacular world of intriguing dynamic activity over a wide range of length- and time-scales.
The physics of ionized gases is a relatively new science. Not until the development of the electrical industry were controlled experiments on ionized gases possible, and so plasma physics is only about 100 years old. The early part of this century saw some pioneering studies of gas discharges and radio propagation in the ionosphere. However, the real impetus came with the initiation of the controlled thermonuclear reaction programs in the 1950s and with the discoveries of the Van Allen belts and the solar wind in the 1960s. Studies in these areas showed that plasma behavior is much more complex than had been anticipated. Computer models such as the ‘numerical tokamak’ will advance all areas of plasma science, including basic plasma physics, fusion physics, space plasmas and industrial plasma processing.
Charge exchange between solar wind protons and neutral atmospheric atoms is expected to affect the solar wind interaction with Mars, but its influences and significance have only been touched upon in previous work. Here several features associated with the charge exchange process between the solar wind protons and Martian neutral upper atmospheres are described. The analysis is based on an empirical proton model derived from Phobos 2 observations interacting with the Martian atomic (H) and molecular (H2) hydrogen, and oxygen (O) upper atmospheres representing solar minimum and solar maximum conditions. The region where the largest fraction of solar wind protons is lost by the charge exchange process is found to be a thin layer above the surface of Mars on the dayside resulting from charge exchange with the thermal oxygen. In general, the magnetosheath and ``magnetosphere'' (where the observed plasma takes on a different character in the Phobos 2 data) produce two distinguishable regions where the loss rate of solar wind protons is highest. Increasing solar activity increases the loss rate in the magnetosheath but decreases it in the magnetosphere. No significant increase of the absorption of the solar wind was found near the ``magnetopause'' suggesting that the decrease of the solar wind protons observed by Phobos 2 are not due to the charge exchange process. In addition to a reduction in the solar wind density, the charge exchange reaction results in energetic neutral atom (ENA) production. This paper considers some of the detailed properties expected for the ENA population at Mars. The ENA differential fluxes were found to be typically 106-107cm-2s-1keV-1 in the energy range 0.01-1 keV. During solar minimum, the ENA production rate and ENA integral fluxes were found to be highest in the magnetosheath. At solar maximum the ENA production rate is highest in the magnetosphere, and ENA integral fluxes in the dayside magnetosphere appear to become comparable to the fluxes in the magnetosheath if the proton temperature in the magnetosphere is low. It is found that 1-3% of the original solar wind proton flux converts into ENAs before the bow shock. The ENAs produced upstream are undeflected and so may precipitate into the Martian upper atmosphere, depositing an energy flux of up to 3×109eVcm-2s-1 derived from the solar wind. These results both suggest the possible benefits of observing ENA fluxes around Mars and suggest the necessary parameters for detector design.
In order to explore the physical nature of the cometopause observed at
Comet Halley by the Vega spacecraft and by the Giotto probe, the
chemical compositional changes and variations of the thermal-energy
distributions of the water-group ions are examined, adopting a
two-dimensional cometary-plasma flowfield model based on
three-dimensional MHD simulations of Fedder et al. (1986). The
charge-exchange loss of hot cometary ions and the solar-wind protons
could be used to explain the observed number-density profiles
quantitatively. The resulting exponential depletion of the hot-ion
populations with a scale length of about 10,000 km occurs near
60,000-80,000 km along the trajectory of Giotto, as indicated by both
theoretical computations and the ion-mass-spectrometer measurements. The
formation of the cometopause located at about 140,000 km is therefore
not necessarily as closely related to the charge-exchange process.
BROWN dwarfs are substellar objects with too little mass to ignite hydrogen in their cores. Despite considerable effort to detect brown dwarfs astrometrically1–4, photometrically4–9, and
spectroscopi-cally10–12, only a few good candidates have been discovered. Here we present spectroscopic evidence for a probable brown-dwarf companion to the solar-type star HD114762. This star undergoes periodic variations in radial velocity which we
attribute to orbital motion resulting from the presence of an unseen companion. The rather short period of 84 days places the companion in an orbit similar to that of Mercury around the Sun, whereas the rather low velocity amplitude of about 0.6 km s-1
implies that the mass of the companion may be as low as 0.011 solar masses, or 11 Jupiter masses. This leads to the suggestion that the companion is probably a brown dwarf, and may even be a giant planet. However, because the inclination of the orbit to the line of sight
is unknown, the mass of the companion may be considerably larger than this lower limit.
Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD 209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. However, difficulties in reconciling evaporation models with observations call for a reassessment of the problem. Here we use HST archive data to report a new absorption rate of ~8.9% ± 2.1% by atomic hydrogen during the HD 209458b transit and show that no sign of evaporation could be detected for the exoplanet. We also report evidence of time variability in the HD 209458 Lyα flux, a variability that was not accounted for in previous studies, which corrupted their diagnostics. Mass-loss rates thus far proposed in the literature in the range 5 × (1010–1011) g s-1 must induce a spectral signature in the Lyα line profile of HD 209458 that cannot be found in the present analysis. Either an unknown compensation effect is hiding the expected spectral feature or else the mass-loss rate of neutrals from HD 209458 is modest.
We present a uniform catalog of stellar properties for 1040 nearby F, G, and K stars that have been observed by the Keck, Lick, and AAT planet search programs. Fitting observed echelle spectra with synthetic spectra yielded effective temperature, surface gravity, metallicity, projected rotational velocity, and abundances of the elements Na, Si, Ti, Fe, and Ni, for every star in the catalog. Combining V-band photometry and Hipparcos parallaxes with a bolometric correction based on the spectroscopic results yielded stellar luminosity, radius, and mass. Interpolating Yonsei-Yale isochrones to the luminosity, effective temperature, metallicity, and α-element enhancement of each star yielded a theoretical mass, radius, gravity, and age range for most stars in the catalog. Automated tools provide uniform results and make analysis of such a large sample practical. Our analysis method differs from traditional abundance analyses in that we fit the observed spectrum directly, rather than trying to match equivalent widths, and we determine effective temperature and surface gravity from the spectrum itself, rather than adopting values based on measured photometry or parallax. As part of our analysis, we determined a new relationship between macroturbulence and effective temperature on the main sequence. Detailed error analysis revealed small systematic offsets with respect to the Sun and spurious abundance trends as a function of effective temperature that would be inobvious in smaller samples. We attempted to remove these errors by applying empirical corrections, achieving a precision per spectrum of 44 K in effective temperature, 0.03 dex in metallicity, 0.06 dex in the logarithm of gravity, and 0.5 km s-1 in projected rotational velocity. Comparisons with previous studies show only small discrepancies. Our spectroscopically determined masses have a median fractional precision of 15%, but they are systematically 10% higher than masses obtained by interpolating isochrones. Our spectroscopic radii have a median fractional precision of 3%. Our ages from isochrones have a precision that varies dramatically with location in the Hertzsprung-Russell diagram. We plan to extend the catalog by applying our automated analysis technique to other large stellar samples.
We have observed four transits of the planet of HD 209458 using the STIS spectrograph on the Hubble Space Telescope (HST). Summing the recorded counts over wavelength between 582 and 638 nm yields a photometric time series with 80 s time sampling and relative precision of about 1.1 × 10-4 per sample. The folded light curve can be fitted within observational errors using a model consisting of an opaque circular planet transiting a limb-darkened stellar disk. In this way we estimate the planetary radius Rp = 1.347 ± 0.060 RJup, the orbital inclination i = 866 ± 014, the stellar radius R* = 1.146 ± 0.050 R☉, and one parameter describing the stellar limb darkening. Our estimated radius is smaller than those from earlier studies but is consistent within measurement errors and also with theoretical estimates of the radii of irradiated Jupiter-like planets. Satellites or rings orbiting the planet would, if large enough, be apparent from distortions of the light curve or from irregularities in the transit timings. We find no evidence for either satellites or rings, with upper limits on satellite radius and mass of 1.2 R⊕ and 3 M⊕, respectively. Opaque rings, if present, must be smaller than 1.8 planetary radii in radial extent. The high level of photometric precision attained in this experiment confirms the feasibility of photometric detection of Earth-sized planets circling Sun-like stars.
1] Energetic neutral atom (ENA) images are computed from an empirical solar wind proton model and recent models of the Martian exosphere. The proton model describes the proton bulk velocity in the interaction region and the neutral model the profiles of H, H 2 and O (hot) and O (thermal). We assume a Maxwellian proton velocity distribution. The ENA production model is analytical; thus an ENA image can be constructed for any vantage point by line of sight integration, and we present examples of such images. In this work we study the influence of different parameters in the input models on the generated images and examine the relative importance of the different parameters on the global ENA production rate as well as on image morphology for particular vantage points. All together our parameter set includes 14 variables for solar wind conditions, flow geometry, and the exosphere model. Changing the parameters also makes it possible to mimic various features of the solar wind-Mars interaction, such as some of the effects caused by the recently observed magnetic anomalies. It is found that the exobase temperature of atomic hydrogen is the parameter that affects the ENA production and images most. We also investigate the backscattering of ENAs that precipitate on the Martian atmosphere. It is found to be an important effect for views of Mars in the antisunward direction. The outflux of ENAs from the near Mars region is studied, and two maxima are found. One at an angle of approximately 115° to the Mars-Sun line corresponding to ENAs produced downstream of the bow shock and one at approximately 150° corresponding to ENAs produced upstream of the bow shock in the solar wind.
We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future.
We present the results of narrowband photometry of 85 comets observed over a period of 17 years. The data have been reduced homogeneously to molecular production rates and a proxy for the dust production rate. We confirm previous investigations, both our own and those of others, showing that there is no differentiation with depth in the cometary nucleus, that most comets are very similar to each other in chemical composition, and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is little variation of relative abundances with heliocentric distance, implying that for the species we observe the role of density-dependent processes in the coma is small. There is also little variation from one apparition to the next for most short-period comets.
Measurements of energetic neutral atoms (ENA) generated in the magnetosheath at Mars are reported. These ENAs are the result of charge exchange collisions between solar wind protons and neutral oxygen and hydrogen in the exosphere of Mars. The peak of the observed ENA flux is . For the case studied here, i.e., the passage of Mars Express through the martian magnetosheath around 20:15 UT on 3 May 2004, the measurements agree with an analytical model of the ENA production at the planet. It is possible to find parameter values in the model such that the observed peak in the ENA count rate during the spacecraft passage through the magnetosheath is reproduced.
We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.
We report high-precision, high-cadence photometric measurements of the star HD 209458, which is known from radial velocity measurements to have a planetary-mass companion in a close orbit. We detect two separate transit events at times that are consistent with the radial velocity measurements. In both cases, the detailed shape of the transit curve due to both the limb darkening of the star and the finite size of the planet is clearly evident. Assuming stellar parameters of 1.1 R middle dot in circle and 1.1 M middle dot in circle, we find that the data are best interpreted as a gas giant with a radius of 1.27+/-0.02 RJup in an orbit with an inclination of 87&fdg;1+/-0&fdg;2. We present values for the planetary surface gravity, escape velocity, and average density and discuss the numerous observations that are warranted now that a planet is known to transit the disk of its parent star.
The ASPERA-4 instrument on board the Venus Express spacecraft offers for the first time the possibility to directly measure the emission of energetic neutral atoms (ENAs) in the vicinity of Venus. When the spacecraft is inside the Venus shadow a distinct signal of hydrogen ENAs usually is detected. It is observed as a narrow tailward stream, coming from the dayside exosphere around the Sun direction. The intensity of the signal reaches several 105cm-2sr-1s-1, which is consistent with present theories of the plasma and neutral particle distributions around Venus.
The Neutral Particle Detector (NPD), an Energetic Neutral Atom (ENA) sensor of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) on board Mars Express, detected intense fluxes of ENAs emitted from the subsolar region of Mars. The typical ENA fluxes are (4–7)×105 cm−2 sr−1 s−1 in the energy range 0.3–3 keV. These ENAs are likely to be generated in the subsolar region of the martian exosphere. As the satellite moved away from Mars, the ENA flux decreased while the field of view of the NPD pointed toward the subsolar region. These decreases occurred very quickly with a time scale of a few tens of seconds in two thirds of the orbits. Such a behavior can be explained by the spacecraft crossing a spatially constrained ENA jet, i.e., a highly directional ENA emission from a compact region of the subsolar exosphere. This ENA jet is highly possible to be emitted conically from the subsolar region. Such directional ENAs can result from the anisotropic solar wind flow around the subsolar region, but this can not be explained in the frame of MHD models.
In this paper, we describe a community toolkit which is designed to provide parallel support with adaptive mesh capability for a large and important class of computational models, those using structured, logically cartesian meshes. The package of Fortran 90 subroutines, called PARAMESH, is designed to provide an application developer with an easy route to extend an existing serial code which uses a logically cartesian structured mesh into a parallel code with adaptive mesh refinement. Alternatively, in its simplest use, and with minimal effort, it can operate as a domain decomposition tool for users who want to parallelize their serial codes, but who do not wish to use adaptivity. The package can provide them with an incremental evolutionary path for their code, converting it first to uniformly refined parallel code, and then later if they so desire, adding adaptivity.
Energetic neutral atom (ENA) images obtained by the ENA imager on- board the Astrid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst greater than or equal to 80 nT) on 8 February 1995 are simulated using a parameterized model of the equatorial ion distribution and a six-component Chamberlain exo-sphere with parameters from the MSISE-90 model. By changing the ion parameters until a matching ENA image is obtained one can extract the equatorial ion distribution. Four consecutive images from different view points several of minutes apart are simulated assuming H(+) and O(+), respectively, as parent ions. The optimal set of parameters is extracted by minimizing the chi(exp 2) difference between simulated and observed ENA image using Powell's minimization algorithm. The optimal equatorial model ion distribution consists of O(+) peaked in around dusk. The lower intensity of fluxes obtained from vantage points closer to the pole is an effect of the loss cone of the parent ion distribution being empty.
The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed. Together with radial velocity measurements, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption. Here we report the detection of atomic hydrogen absorption in the stellar Lyman alpha line during three transits of HD209458b. An absorption of 15 +/- 4% (1sigma) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.
The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's
magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA)
continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly
ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable
ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density
decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity
of the interactions between Titan's upper atmosphere and Saturn's space environment.
The Sun-like star HD209458 harbors a close-in giant planet which transits across the star's disk, and thus allows an unprecedented access to the basic parameters of the planet, given a certain knowledge of the basic parameters of the star, namely its mass and radius. We present theoretical stellar evolution model calculations for HD 209458 and discuss the uncertainties involved in deriving the stellar mass and radius. We derive the mass, M=1.06 Msun, radius, R=1.18 Rsun,and age, t=5.2 Gyr of the star with uncertainties of 10% or more. The dominant sources of uncertainty remain to be the helium abundance estimate and the treatment of convection, even after an optimistic estimate for the effective temperature of the star. However, we find that in deriving the radius of the planet, R_p, the relevant stellar model input is the M/R relation, which runs orthogonal to a degeneracy in the transit light curve solution and greatly improves the estimate of R_p. Theoretically the M/R relation has a lower uncertainty than the M and R separately. We estimate the planet radius and mass to be R_p =1.42 +0.10/-0.13 R_J and M_p =0.69 +/- 0.02 M_J. Comment: 24 pages, 5 figures, accepted by the Astrophysical Journal; added correct version of figure 5 and fixed references
We use multi-band photometry to refine estimates for the planetary radius and orbital inclination of the transiting planet system HD 209458. We gathered 1066 spectra over four distinct transits with the STIS spectrometer on the Hubble Space Telescope using two gratings with a resolution R=1500 and a combined wavelength range of 290-1030 nm. We divide the spectra into ten spectrophotometric bandpasses, five for each grating, of equal wavelength span within each grating, and fit a transit curve over all bandpasses simultaneously. In our fit we use theoretical values for the stellar limb-darkening to further constrain the planetary radius. We find that the radius of HD 209458b is 1.320 +/- 0.025 R_Jup, which is a factor of two more precise than current estimates. We also obtain improved estimates for the orbital period P and time of center of transit T_C. Although in principle the photon-limited precision of the STIS data should allow us to measure the timing of individual transits to a precision of 2-7 s, we find that systematic instrumental offsets in the measured flux from one orbit of the spacecraft to the next degrade these measurements to a typical precision of +/- 14 s. Within this level of error, we find no significant variations in the timing of the eight events examined in this work.
MHD modeling of the interaction between the solar wind and solar system objects
- A Ekenbäck
- M Holmström
Ekenbäck, A. & Holmström, M. MHD modeling of the interaction between the solar
wind and solar system objects. In Dongarra, J., Madsen, K. & Wasniewski, J. (eds.),
Proceedings of PARA'04 State-of-the-Art, volume 3732 of Lecture Notes in Computer Science, pp. 554-562. Springer-Verlag, Berlin, Germany, 2006.
The Extrasolar Planets Encyclopaedia
- J Schneider
Schneider, J.
The Extrasolar Planets Encyclopaedia, 2008.
URL
http://tinyurl.com/5o78cw.
Hybrid simulation codes with applications to shocks and upstream waves
- D Winske
Winske, D. Hybrid simulation codes with applications to shocks and upstream waves.
Space Science Reviews, 42 (1-4), 53-66, 1985.