Line Radiative Transfer Research Articles

Resonance line radiative transfer for hot atom coronae using Kappa distributions

Hot atomic populations are an important component of the planetary exospheres. Usually, radiative transfer models describing the scattering of light by moving atoms assume that these populations have a Maxwellian velocity distribution. However, the velocity distributions of the hot populations could actually have some more extended wings. Popular velocity distributions often used in plasma physics and recently proposed to describe neutral planetary environments are Kappa velocity function distributions. In this paper, following the work of Hummer [Non-coherent scattering: I The redistribution functions with Doppler broadening. R Astron. Soc Month Not 1962;125:21] and Cranmer [Non-Maxwellian redistribution in solar coronal Lyα emission. Astrophys J 1998;508:925–39], we calculate the frequency redistribution functions of radiation scattered by moving atoms with Kappa velocity distribution. We also present a detailed study of a radiative transfer model taking into account Kappa velocity distribution functions, for integer and semi-integer values of κ. We apply this theory to a model of Jupiter hydrogen corona containing 0.1% column density of hot hydrogen to quantify the spectroscopic and imaging differences between Kappa velocity distributions and bi-Maxwellian velocity distributions. When assuming a Kappa velocity distribution with κ=2 for the hot population, intensity increases of ∼40% occur at the bright limb and ∼15% on the disk compared with the same calculations done using a Maxwellian velocity distribution. The line profile differs slightly from a Maxwellian distribution on the disk and at the bright limb, but the difference is larger above the limb. Kappa distributions used to study the Jovian atmosphere are speculative and further studies are needed to link the formation of the hot exospheric populations to the Kappa velocity distributions.

Hot and cool water in Herbig Ae protoplanetary disks

The spatial origin and detectability of rotational H2O emission lines from Herbig Ae type protoplanetary disks beyond 70 micron is discussed. We use the recently developed disk code ProDiMo to calculate the thermo-chemical structure of a Herbig Ae type disk and apply the non-LTE line radiative transfer code Ratran to predict water line profiles and intensity maps. The model shows three spatially distinct regions in the disk where water concentrations are high, related to different chemical pathways to form the water: (1) a big water reservoir in the deep midplane behind the inner rim, (2) a belt of cold water around the distant icy midplane beyond the snow-line r>20AU, and (3) a layer of irradiated hot water at high altitudes z/r=0.1...0.3, extending from about 1AU to 30AU, where the kinetic gas temperature ranges from 200K to 1500K. Although region 3 contains only little amounts of water vapour (~3x10^-5 M_Earth), we find this warm layer to be almost entirely responsible for the rotational water emission lines, execpt for the 3 lowest excitation lines. Thus, Herschel will probe first and foremost the conditions and radial extension of region 3, where water is predominantly formed via neutral-neutral reactions and the gas is thermally decoupled from the dust T_gas>T_dust. The observations do not allow for a determination of the snow-line, because the snow-line truncates the radial extension of region 1, whereas the lines originate from region 3. Different line transfer approximations (LTE, escape probability, Monte Carlo) are discussed. A non-LTE treatment is required in most cases, and the results obtained with the escape probability method are found to underestimate the Monte Carlo results by 2%...45%.

APEX-CHAMP+high-JCO observations of low-mass young stellar objects

AIMS Our aim is to characterize the size, mass, density and temperature profiles of the protostellar envelope of HH~46 IRS 1 and its surrounding cloud material as well as the effect the outflow has on its environment.METHODS The CHAMP+ and LABOCA arrays on the APEX telescope, combined with lower frequency line receivers, are used to obtain a large continuum map and smaller heterodyne maps in various isotopologues of CO and HCO+. The high-J lines of CO (6--5 and 7--6) and its isotopologues together with [C I] 2--1, observed with CHAMP+, are used to probe the warm molecular gas in the inner few hundred AU and in the outflowing gas. The data are interpreted with continuum and line radiative transfer models. RESULTS Broad outflow wings are seen in CO low- and high-J lines at several positions, constraining the gas temperatures to a constant value of ~100 K along the red outflow axis and to ~60 K for the blue outflow. The derived outflow mass is of order 0.4--0.8 M_sol, significantly higher than previously found. The bulk of the strong high-J CO line emission has a surprisingly narrow width, however, even at outflow positions. These lines cannot be fit by a passively heated model of the HH 46 IRS envelope. We propose that it originates from photon heating of the outflow cavity walls by ultraviolet photons originating in outflow shocks and the accretion disk boundary layers. At the position of the bow shock itself, the UV photons are energetic enough to dissociate CO. The envelope mass of ~5 M_sol is strongly concentrated towards HH 46 IRS with a density power law of -1.8.

Crashα: coupling continuum and line radiative transfer

The detection of lya lines from local and distant objects has always been of great importance in astrophysics. It has been extensively used as indicator of redshift, as a measurement of the star formation activity of galaxies and as a probe of their internal structure. In the last few years an increasing interest has been devoted to the search of lya emitters (LAEs) at high redshift, which are expected to be characterized by a strong lya emission, but significantly attenuated by dust absorption. Emission of lya photons from high redshift sources has also an impact on the detectability of 21~cm line from neutral hydrogen in the Intergalactic Medium (IGM). This work is focused on the study, analysis and implementation of a new radiative transfer scheme (CRLya) which, for the first time in the literature, follows simultaneously the propagation of lya and ionizing radiation self-consistently. This allows to investigate the effects of evolving ionization configurations on the propagation of lya radiation and on the shaping of the line emerging from single objects. The implementation introduces the time evolution for lya photons (a feature commonly neglected in line radiative transfer codes) and, to reduce the computational time needed to follow each scattering, adopts a statistical approach to the lya treatment by making extensive use of pre-compiled tables. I find that the emerging spectra keep memory of the ionization history which generates a given ionization configuration of the gas and, to properly account for this effect, a self-consistent joint evolution of line and ionizing continuum radiation as implemented in CRLya is necessary. As next step, I implemented new features in CRLya: a new emission system, diffuse lya photons from recombination and treatment of dust. With a new test performed on a simulated galaxy at redshift z~10, I show the ability of the code to determine the variations of the lya radiation field while the gas in which it propagates is affected by the ionizing radiation emitted by the stars within the galaxy. In this test, the lya radiation is generated by both stars and recombining ionized gas. Finally I tackle the impact of lya photon scattering on the IGM temperature. This is an important issue since lya photons could be able to heat the IGM temperature above the CMB temperature and render the 21 cm line visible in emission. In the last Chapter, I discuss the implementation of the lya heating in \CRLya and I analyze how the temperature is modified by changing the most relevant physical parameters in the simulation.

Measurement of K–L radiative vacancy transfer probabilities in selected rare earth elements bombarded with 3–4 MeV protons

K-shell X-ray intensity ratios for selected rare earth elements were measured following irradiation with proton beams having energies between 3 and 4 MeV. Using the X-ray intensity ratios, the radiative vacancy transfer probabilities from the shell K to the L sub-shells were determined. The experimental data were compared to theoretical predictions for X-ray line intensities and radiative vacancy transfer probabilities. The results showed a good agreement between theory and experiment.

Characterizing the velocity field in hydrodynamical simulations of low-mass star formation using spectral line profiles

When low-mass stars form, the collapsing cloud of gas and dust goes through several stages which are usually characterized by the shape of their spectral energy distributions. Such classification is based on the cloud morphology only and does not address the dynamical state of the object. In this paper we investigate the initial cloud collapse and subsequent disk formation through the dynamical behavior as reflected in the sub-millimeter spectral emission line profiles. If a young stellar object is to be characterized by its dynamical structure it is important to know how accurately information about the velocity field can be extracted and which observables provide the best description of the kinematics. Of particular interest is the transition from infalling envelope to rotating disk, because this provides the initial conditions for the protoplanetary disk, such as mass and size. We use a hydrodynamical model, describing the collapse of a core and formation of a disk, to produce synthetic observables which we compare to calculated line profiles of a simple parameterized model. Because we know the velocity field from the hydrodynamical simulation we can determine in a quantitative way how well our best-fit parameterized velocity field reproduces the original. We use a molecular line excitation and radiation transfer code to produce spectra of both our hydro dynamical simulation as well as our parameterized model. We find that information about the velocity field can reasonably well be derived by fitting a simple model to either single-dish lines or interferometric data, but preferentially by using a combination of the two. Our result shows that it is possible to establish relative ages of a sample of young stellar objects using this method, independently of the details of the hydrodynamical model.

Primordial hydrogen recombination dynamics with recoil upon scattering in the Ly-α line

We show that including the recoil upon scattering in the Ly-α line can lead to a noticeable acceleration of the primordial hydrogen recombination. Thus, for example, for the ΛCDM model, the decrease in the degree of ionization exceeds 1% at redshifts z = 800–1050, reaching ≈1.3% at z = 900. The corresponding corrections to the calculated cosmic microwave background power spectra reach 1.1% and 1.7% for TT and EE spectra, respectively. The radiative transfer in these calculations was treated in the quasi-stationary approximation.We have also obtained numerical solutions (in the diffusion approximation) to the nonstationary problem of radiative transfer in the Ly-α line for a partial frequency redistribution with recoil. We trace the evolution of the local line profile and the relative number of uncompensated transitions from the 2p state to the 1s state. We show that including the nonstationarity of the Ly-α line radiative transfer can lead to an additional acceleration of the primordial hydrogen recombination.

The Role of Galactic Winds on Molecular Gas Emission from Galaxy Mergers

Galactic winds from starbursts and active galactic nuclei (AGNs) are thought to play an important role in driving galaxies along the starburst-AGN sequence. Here, we assess the impact of these winds on the CO emission from galaxy mergers and, in particular, search for signatures of starburst and AGN-feedback-driven winds in the simulated CO morphologies and emission-line profiles. We do so by combining a three-dimensional non-LTE molecular line radiative transfer code with smoothed particle hydrodynamic (SPH) simulations of galaxy mergers that include prescriptions for star formation, black hole growth, a multiphase interstellar medium (ISM), and the winds associated with star formation and black hole growth. Our main results are (1) Galactic winds can drive outflows of masses ~108-109 M☉ which may be imaged via CO emission-line mapping. (2) AGN-feedback-driven winds are able to drive detectable CO outflows for longer periods of time than starburst-driven winds owing to the greater amount of energy imparted to the ISM by AGN feedback compared to star formation. (3) Galactic winds can control the spatial extent of the CO emission in postmerger galaxies, and may serve as a physical motivation for the subkiloparsec scale CO emission radii observed in local advanced mergers. (4) Secondary emission peaks at velocities greater than the circular velocity are seen in the CO emission lines in all models, regardless of the associated wind model. In models with winds, however, these high-velocity peaks are seen to preferentially correspond to outflowing gas entrained in winds, which is not the case in the model without winds. The high-velocity peaks seen in models without winds are typically confined to velocity offsets (from the systemic) 1.7 times the circular velocity, whereas the models with AGN-feedback-driven winds can drive high-velocity peaks to ~2.5 times the circular velocity.

Chemical and Thermal Structure of Protoplanetary Disks as Observed with ALMA

We predict how protoplanetary disks around low-mass young stars would appear in molecular lines observed with the ALMA interferometer. Our goal is to identify those molecules and transitions that can be used to probe and distinguish between chemical and physical disk structure and to define necessary requirements for ALMA observations. Disk models with and without vertical temperature gradient as well as with uniform abundances and those from a chemical network are considered. As an example, we show the channel maps of HCO$^+$(4-3) synthesized with a non-LTE line radiative transfer code and used as an input to the GILDAS ALMA simulator to produce noise-added realistic images. The channel maps reveal complex asymmetric patterns even for the model with uniform abundances and no vertical thermal gradient. We find that a spatial resolution of $0.2-0.5\arcsec$ and 0.5--10 hours of integration time will be needed to disentangle large-scale temperature gradients and the chemical stratification in disks in lines of abundant molecules.

Molecular Line Radiative Transfer in Protoplanetary Disks: Monte Carlo Simulations versus Approximate Methods

We analyze the line radiative transfer in protoplanetary disks using several approximate methods and a well-tested Accelerated Monte Carlo code. A low-mass flaring disk model with uniform as well as stratified molecular abundances is adopted. Radiative transfer in low and high rotational lines of CO, C18O, HCO+, DCO+, HCN, CS, and H2CO is simulated. The corresponding excitation temperatures, synthetic spectra, and channel maps are derived and compared to the results of the Monte Carlo calculations. A simple scheme that describes the conditions of the line excitation for a chosen molecular transition is elaborated. We find that the simple LTE approach can safely be applied for the low molecular transitions only, while it significantly overestimates the intensities of the upper lines. In contrast, the Full Escape Probability (FEP) approximation can safely be used for the upper transitions ($J_{\rm up} \ga 3$) but it is not appropriate for the lowest transitions because of the maser effect. In general, the molecular lines in protoplanetary disks are partly subthermally excited and require more sophisticated approximate line radiative transfer methods. We analyze a number of approximate methods, namely, LVG, VEP (Vertical Escape Probability) and VOR (Vertical One Ray) and discuss their algorithms in detail. In addition, two modifications to the canonical Monte Carlo algorithm that allow a significant speed up of the line radiative transfer modeling in rotating configurations by a factor of 10--50 are described.

Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

Abstract. In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1–2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5–1 K) and moistening (3%–12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.

On the Transfer of Resonant‐Line Radiation in Mesh Simulations

The last decade has seen applications of Adaptive Mesh Refinement (AMR) methods for a wide range of problems from space physics to cosmology. With the advent of these methods, in which space is discretized into a mesh of many individual cubic elements, the contemporary analog of the extensively studied line radiative transfer (RT) in a semi-infinite slab is that of RT in a cube. In this study we provide an approximate solution of the RT equation, as well as analytic expressions for the probability distribution functions (pdfs) of the properties of photons emerging from a cube, and compare them with the corresponding slab problem. These pdfs can be used to perform fast resonant-line RT in optically thick AMR cells where, otherwise, it could take unrealistically long times to transfer even a handful of photons.

The effect of a strong external radiation field on protostellar envelopes in Orion

We discuss the effects of an enhanced interstellar radiation field (ISRF) on the observables of protostellar cores in the Orion cloud region. Dust radiative transfer is used to constrain the envelope physical structure by reproducing SCUBA 850 µm emission. Previously reported 13 CO, C 17 O and H2CO line observations are reproduced through detailed Monte Carlo line radiative transfer models. It is found that the 13 CO line emission is marginally optically thick and sensitive to the physical conditions in the outer envelope. An increased temperature in this region is needed in order to reproduce the 13 CO line strengths and it is suggested to be caused by a strong heating from the exterior, corresponding to an ISRF in Orion 10 3 times stronger than the “standard” ISRF. The typical temperatures in the outer envelope are higher than the desorption temperature for CO. The C 17 O emission is less sensitive to this increased temperature but rather traces the bulk envelope material. The data are only fit by a model where CO is depleted, except in the inner and outermost regions where the temperature increases above 30–40 K. The fact that the temperatures do not drop below ≈25 K in any of the envelopes whereas a significant fraction of CO is frozen-out suggest that the interstellar radiation field has changed through the evolution of the cores. The H2CO lines are successfully reproduced in the model of an increased ISRF with constant abundances of 3–5 × 10 −10 .

A Rotating Disk around the Very Young Massive Star AFGL 490

We observed the embedded, young 8-10 M☉ star AFGL 490 at subarcsecond resolution with the Plateau de Bure Interferometer (PdBI) in the C17O (2-1) transition and found convincing evidence that AFGL 490 is surrounded by a rotating disk. Using two-dimensional modeling of the physical and chemical disk structure coupled to line radiative transfer, we constrain its basic parameters. We obtain a relatively high disk mass of 1 M☉ and a radius of ~1500 AU. A plausible explanation for the apparent asymmetry of the disk morphology is given.

A new exact method for line radiative transfer

We present a new method, the Coupled Escape Probability (CEP), for exact calculation of line emission from multi-level systems, solving only algebraic equations for the level populations. The CEP formulation of the classical two-level problem is a set of {\em linear equations}, and we uncover an exact analytic expression for the emission from two-level optically thick sources that holds as long as they are in the effectively thin regime. In comparative study of a number of standard problems, the CEP method outperformed the leading line transfer methods by substantial margins. The algebraic equations employed by our new method are already incorporated in numerous codes based on the escape probability approximation. All that is required for an exact solution with these existing codes is to augment the expression for the escape probability with simple zone-coupling terms. As an application, we find that standard escape probability calculations generally produce the correct cooling emission by the CII 158 mic line but not by the $^3$P lines of OI.

Millimeter Observations and Modeling of the AB Aurigae System

We present the results of millimeter observations and a suitable chemical and radiative transfer model of the AB Aurigae (HD 31293) circumstellar disk and surrounding envelope. The integral molecular content of this system is studied by observing CO, C18O, CS, HCO+, DCO+, H2CO, HCN, HNC, and SiO rotational lines with the IRAM 30 m antenna, while the disk is mapped in the HCO+ (1-0) transition with the Plateau de Bure Interferometer. Using a flared disk model with a vertical temperature gradient and an isothermal spherical envelope model with a shadowed midplane and two unshielded cones together with a gas-grain chemical network, time-dependent abundances of observationally important molecules are calculated. Then a two-dimensional non-LTE line radiative transfer code is applied to compute excitation temperatures of several rotational transitions of HCO+, CO, C18O, and CS molecules. We synthesize the HCO+ (1-0) interferometric map along with single-dish CO (2-1), C18O (2-1), HCO+ (1-0), HCO+ (3-2), CS (2-1), and CS (5-4) spectra and compare them with the observations. Our disk model successfully reproduces observed interferometric HCO+ (1-0) data, thereby constraining the following disk properties: (1) the inclination angle i=17+6-3 deg, (2) the position angle φ=80deg+/-30deg, (3) the size Rout=400+/-200 AU, (4) the mass Mdisk=1.3×10-2 Msolar (with a factor of ~7 uncertainty), and (5) that the disk is in Keplerian rotation. Furthermore, indirect evidence for a local inhomogeneity of the envelope at >~600 AU is found. The single-dish spectra are synthesized for three different cases, namely, for the disk model, for the envelope model, and for their combination. An overall reasonable agreement between all modeled and acquired line intensities, widths, and profiles is achieved for the latter model, with the exception of the CS (5-4) data that require the presence of high-density clumpy structures in the model. This allows us to constrain the physical structure of the AB Aur inner envelope: (1) its mass-average temperature is about 35+/-14 K; (2) the density goes inversely down with the radius, ρ~r-1.0+/-0.3, starting from an initial value n0~3.9×105 cm-3 at 400 AU; and (3) the mass of the shielded region within 2200 AU is about 4×10-3 Msolar (the latter two quantities are uncertain by a factor of ~7). In addition, evolutionary nature and lifetime for dispersal of the AB Aur system and Herbig Ae/Be systems in general are discussed.

An atomic and molecular database for analysis of submillimetre line observations

Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, including energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies. These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/~moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calculations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

Evolution of Chemistry and Molecular Line Profiles during Protostellar Collapse

Understanding the chemical evolution in star-forming cores is a necessary precondition to correctly assessing physical conditions when using molecular emission. We follow the evolution of chemistry and molecular line profiles through the entire star formation process, including a self-consistent treatment of dynamics, dust continuum radiative transfer, gas energetics, chemistry, molecular excitation, and line radiative transfer. In particular, the chemical code follows a gas parcel as it falls toward the center, passing through regimes of density, dust temperature, and gas temperature that are changing because of both the motion of the parcel and the evolving luminosity of the central source. We combine a sequence of Bonnor-Ebert spheres and the inside-out collapse model to describe dynamics from the pre-protostellar stage to later stages. The overall structures of abundance profiles show complex behavior that can be understood as interactions between freezeout and evaporation of molecules. We find that the presence or absence of gas-phase CO has a tremendous effect on the less abundant species. In addition, the ambient radiation field and the grain properties have important effects on the chemical evolution, and the variations in abundance have strong effects on the predicted emission-line profiles. Multitransition and multiposition observations are necessary to constrain the parameters and interpret observations correctly in terms of physical conditions. Good spatial and spectral resolution is also important in distinguishing evolutionary stages.

The sensitivity of infall molecular line profiles to the ambient radiation field

In cold molecular clouds, submillimetre emission lines are excited by the ambient radiation field. The pumping is dominated by the cosmic microwave background (CMB). It is usual in molecular line radiative transfer modelling to assume simply that this is the only incident radiation field. In this paper, a molecular line transport code and a dust radiative transfer code are used to explore the effects of the inclusion of a full interstellar radiation field (ISRF) on a simple test molecular cloud. It is found that in many galactic situations, the shape and strength of the line profiles that result are robust to variations in the ISRF and thus that in most cases, it is safe to adopt the CMB radiation field for the molecular line transport calculations. However, we show that in two examples, the inclusion of a plausible radiation field can have a significant effect on the line profiles. First, in the vicinity of an embedded massive star, there will be an enhanced far-infrared component to the radiation field. Secondly, for molecular clouds at large redshift, the CMB temperature increases and this of course also alters the radiation field. In both these cases, the line profiles are weakened significantly compared with a cloud exposed to a standard radiation field. Therefore this effect should be accounted for when investigating pre-stellar cores in massive star-forming regions and when searching for molecular clouds at high redshift.

Constraints on Jupiter's hydrogen corona from Galileo UVS observations

During Galileo orbits C10 and E11, the Ultraviolet Spectrometer (UVS) obtained H Lyα scans at low latitudes across the disk of Jupiter from a phase angle of 90–109° and a distance of 18–21 jovian radii. Jupiter's nightside Lyα brightness was about 0.5kR, approximately half as bright as interplanetary space. From the nightside to the dayside, the Lyα brightness rose to a peak of ∼15kR near the sub-solar limb and then dropped sharply to interplanetary brightness levels. On orbit C10 three disk scans were obtained (near 30°N latitude, 20°S latitude, and along the equator), while a single equatorial scan was made on orbit E11. The steep falloff in Lyα brightness at the sub-solar point provides a strong constraint on the distribution of atomic hydrogen in the upper atmosphere. All disk scans are reasonably well fit by a resonance line radiative transfer model which assumes a total H column density of ∼1017cm−2 and a temperature profile consistent with the Galileo probe results with an exospheric temperature of 940K. However, we find that the inclusion of a small amount of “hot” hydrogen, with an abundance described by a Chapman profile, can substantially improve the fit to the UVS limb scan data. Exactly such a hot hydrogen component is expected in the upper atmosphere of Jupiter, and has been previously invoked to explain observations of the jovian Lyα line profile made from Earth orbit. The best fits to the UVS data occur with a hot H column density that is 0.1% of the ambient H column, with an effective temperature of 25,000K and a topside scale height of 1000km (in our empirical representation, the hot H scale height is unrelated to the hot H temperature). Other combinations of column abundance and the effective temperature can also provide good fits to the data. As long as the hot H is located above the homopause, there is no strong dependence on the height of the Chapman profile.