Ionization Structure Research Articles

Dead Zones and the Origin of Planetary Masses

Protoplanets accrete material from their natal protostellar disks until they are sufficiently massive to open a gap in the face of the disk's viscosity that arises from the magneto-rotational instability (MRI). By computing the ionization structure within observationally well-constrained disk models, we demonstrate that poorly ionized, low viscosity stretch out to 12 AU within typical disks. We find that planets of terrestrial mass robustly form within the dead zones while massive Jovian planets form beyond. Dead zones will also halt the rapid migration of planets into their central stars. Finally, we argue that the gravitational scattering of low mass planets formed in the dead zone, to larger radii by a rapidly accreting Jupiter beyond, can explain the distribution of planetary masses in our solar system.

Infrared mergers and infrared quasi-stellar objects with galactic winds - II. NGC 5514: two extranuclear starbursts with LINER properties and a supergiant bubble in the rupture phase

A study of the morphology, kinematics and ionization structure of the IR merger NGC5514, is here presented. This study is based mainly on INTEGRAL twodimensional (2D) spectroscopy (obtained at the 4.2 m William Herschel Telescope, WHT), plus optical and near IR images. Clear evidence of two extra-nuclear starbursts with young outflows (OFs) and LINER activity are reported. One of these OFs has generated a supergiant bubble and the other is associated with an extended complex of H ii region. In the galactic bubble it was found that: (i) the [S ii], H�, [N ii], [O i] and [O iii] emission line maps show a bubble with a distorted ellipsoidal shape, with major and minor axes of �6.5 kpc (13.

Ionization of the atmosphere caused by solar protons and its influence on ozonosphere of the Earth during 1994–2003

Satellite observations of solar proton fluxes in different energetic channels have been used to investigate proton activity of the Sun during 23rd cycle of its activity. Several maxima of solar proton intensity occurred in 1998, 2000, 2001, and in 2003, with a minima in 1999. Ionization of the atmosphere caused by several strongest solar proton events (SPEs) was calculated using energetic spectrums of solar protons. It was shown on the basis of the calculations that the revealed structure of ionization caused by different SPEs looks rather different from one SPE to another. The response of chemical composition (ozone and others) at 70°N to the strongest events was calculated using photochemical model. It was shown by photochemical calculations that ozone depletion in the stratosphere and mesosphere was strong not only after famous SPE in July 2000, but also after SPE in November 2001. Special focus was on SPEs that occurred during October–November 2003 period. The results of photochemical simulations has also shown that additional production of OH molecular by solar protons leads to water vapor increase in the middle atmosphere. Calculated increase equaled about 2% in the mesosphere after SPE in July 2000. At the same time, the last result needs additional investigations involving photochemical reactions with ions.

Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber

We show that the Fe VII-Fe XII M-shell unresolved transition array (UTA) in the Chandra HETGS observation of NGC 3783 (900 ks) clearly changes in opacity on a timescale of 31 days, responding to a factor of ~2 change in the ionizing continuum. The opacity variation is observed at a level >10 σ. There is also evidence for variability in the O VI K edge (at ~3 σ). The observed changes are consistent with the gas producing these absorption features (i.e., the low-ionization component) being close to photoionization equilibrium. The gas responsible for the Fe XVII-Fe XXII L-shell absorption (i.e., the high-ionization component) does not seem to be responding as expected in photoionization equilibrium. The observed change in opacity for the UTA implies a density >1 × 104 cm-3, thus locating the gas within 6 pc of the X-ray source. The scenario in which the gas is composed of a continuous radial range of ionization structures is ruled out, as in such scenario, no opacity variations are expected. Rather, the structure of the absorber is likely composed of heavily clumped gas.

Complex-plasma manipulation by radiofrequency biasing

This paper presents an experimental study on the nature, the dimensions and the timescale of the perturbation introduced by radiofrequency (rf) biasing of areas adjacent to the plasma. The analysis of the rf sheath, and of the charging of particles in it, has disclosed a levitation force on particles, which is substantially different from the dc one often used in complex plasmas. Experimentally, the rf heavily loaded sheath presents characteristics completely different from the normal case Vrf ⩽ Vdc. Regions of extra ionization and complex electrostatic structures arise. These have been visualized by nanoparticles grown in the plasma. A variety of equilibrium positions for a controlled number of microparticles (injected) can be achieved by fine balancing of dc and rf on a pixel with the neighbouring sheath kept under control. In certain situations gravity is completely compensated, allowing the study of three-dimensional clusters. The motion of clusters from 4 to about 100 particles is simultaneously monitored by a three-dimensional visualization based on two laser lights modulated in intensity. This method enables the study of time-varying effects, such as transitions and vibrations, as well as the study of static structures and lattice defects. At pressures below 40 Pa in large clusters a poloidal motion appears.

Constraints on the ionization sources of the high-redshift intergalactic medium

Constraints on the ionization structure of the intergalactic medium (IGM) are derived as directly imposed by observations in conjunction with the results of numerical simulations for structure formation. Under the assumption that the population of sources dominating the ultraviolet (UV) ionizing background at z < 6 is the same population which reionized the IGM, it is shown that consistency with measurements of the mean Lyα transmitted flux at high redshifts requires the epoch of hydrogen reionization of the IGM to have occurred at a redshift zri < 11, independent of the space density of the sources. The upper limit on the reionization redshift depends only on the shape of the UV spectra of the sources. Consistency with constraints on the reionization epoch from the Wilkinson Microwave Anisotropy Probe requires the sources of photoionization to have had hard spectra, such as quasi-stellar objects (QSOs) or Population III stars. The only ways to escape these conclusions are either (i) the sources which dominate the photoionization background at z < 6 are the remnants of a population that was a much more prodigious source of ionizing photons at earlier times, or (ii) the sources responsible for the photoionization of the IGM are an unknown population that contributes negligibly to the ionization of the IGM at z < 6. The evolution of the QSO luminosity function over the range 3 < z < 6 is estimated from recent QSO counts, and the fraction of ionizing photons arising in QSOs is evaluated under the assumptions of either pure luminosity evolution or pure density evolution. It is shown that QSOs dominate the UV ionizing background for z < 3.5, but that it is unlikely that more than one-half to one-third of the ionizing background radiation originates in QSOs in the redshift range 4.5 < z < 6. QSOs acting alone could not have reionized the hydrogen in the IGM prior to z≈ 4. If the QSOs had hard spectra, however, they may have reionized the helium in the IGM as early as z≈ 5. The possibility that the IGM was reionized by low-luminosity active galactic nuclei is discussed.

CiiRadiative Cooling of the Diffuse Gas in the Milky Way

The heating and cooling of the interstellar medium (ISM) allow the gas in the ISM to coexist at very different temperatures in thermal pressure equilibrium. The rate at which the gas cools or heats is therefore a fundamental ingredient for any theory of the ISM. The heating cannot be directly determined, but the cooling can be inferred from observations of ·C *, which is an important coolant in different environments. The amount of cooling can be measured through either the intensity of the 157.7 μm [C II] emission line or the C * absorption lines at 1037.018 and 1335.708 A, observable with the Far Ultraviolet Spectroscopic Explorer and the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, respectively. We present the results of a survey of these far-UV absorption lines in 43 objects situated at b 30°. Measured column densities of C *, S II, P II, and Fe II are combined with H I 21 cm emission measurements to derive the cooling rates (per H atom using H I and per nucleon using S II) and to analyze the ionization structure, depletion, and metallicity content of the low-, intermediate-, and high-velocity clouds (LVCs, IVCs, and HVCs) along the different sight lines. Based on the depletion and the ionization structure, the LVCs, IVCs, and HVCs consist mostly of warm neutral and ionized clouds. For the LVCs, the mean cooling rate in ergs s-1 per H atom is -25.70 dex (1 σ dispersion). With a smaller sample and a bias toward high H I column density, the cooling rate per nucleon is similar. The corresponding total Galactic C II luminosity in the 157.7 μm emission line is L ~ 2.6 × 107 L☉. Combining N(C *) with the intensity of Hα emission, we derive that ~50% of the C * radiative cooling comes from the warm ionized medium (WIM). The large dispersion in the cooling rates is certainly due to a combination of differences in the ionization fraction, in the dust-to-gas fraction, and physical conditions between sight lines. For the IVC Intermediate-Velocity (IV) Arch at z ~ 1 kpc we find that on average the cooling is a factor of 2 lower than in the LVCs that probe gas at lower z. For an HVC (complex C, at z > 6 kpc) we find the much lower rate of -26.99 dex, similar to the rates observed in a sample of damped Lyα absorber systems (DLAs). The fact that in the Milky Way a substantial fraction of the C II cooling comes from the WIM implies that this is probably also true in the DLAs. We also derive the electron density, assuming a typical temperature of the warm gas of 6000 K: for the LVCs, ne = 0.08 ± 0.04 cm-3, and for the IV Arch, ne = 0.03 ± 0.01 cm-3 (1 σ dispersion). Finally, we measured the column densities N(S II) and N(P II) in many sight lines and confirm that sulphur appears undepleted in the ISM. Phosphorus becomes progressively more deficient when log N(H ) > 19.7 dex, which can mean that either P becomes more depleted into dust as more neutral gas is present or P is always depleted by about -0.3 dex, but the higher value of P II at lower H I column density indicates the need for an ionization correction.

Electron transfer (oxidation) of complexes between bifunctional phenols and DMSO in non-polar surroundings

Pulse-radiolysis reactions were performed to study the effect of hydrogen bonding to dimethyl sulfoxide (DMSO) on the oxidation of dihydroxy benzene and biphenyl diols to phenoxyl radicals. It was observed that with DMSO as the hydrogen-bond acceptor, the oxidation process proceeds via proton-coupled electron transfer in the case of hydroquinone. For resorcinol, DMSO acts in a similar way as in the case of hydroquinone. For other biphenols, viz., 2,2′- and 4,4′-biphenyl diols, it was found that DMSO had no effect on the electron transfer. The results are explained based on the ionization potential and structure of the phenol derivatives which probably depends on the rotation of the OH bond causing different electron distribution in the transient conformation.

Outflows near an Accreting Black Hole: Ionization and Temperature Structures

We calculate the temperature and ionization balance in an outflow from an accreting black hole under illumination by hard radiation from the central object. Electron scattering of the Fe Kα photons within the highly ionized expanding flow leads to a decrease of their energy (redshift), which is of first order in v/c, when v is the outflow velocity and v is much less than the speed of light c. This photon redshift is an intrinsic property of any outflow for which divergence is positive. We also find that the equivalent widths of red-skewed Fe Kα that originated in the wind is on the order of keV. We conclude that redshifted lines are intrinsic properties of the powerful outflows that are observed in many compact objects. Downscattering of the primary line photons generated in the outflow (a more natural and probable mechanism than the general relativistic effects in the innermost part of the accretion flow) leads to the formation of red-skewed lines.

NGC 604, the Scaled OB Association (SOBA) Prototype. I. Spatial Distribution of the Different Gas Phases and Attenuation by Dust

We have analyzed HST and ground-based data to characterize the different gas phases and their interaction with the MYC in NGC 604, a GHR in M33. The warm ionized gas is made out of two components: a high-excitation, high-surface brightness H II surface located at the faces of the molecular clouds directly exposed to the ionizing radiation of the central SOBA; and a low-excitation, low-surface brightness halo that extends to much larger distances from the ionizing stars. The cavities created by the winds and SN explosions are filled with X-ray-emitting coronal gas. The nebular lines emitted by the warm gas experience a variable attenuation as a consequence of the dust distribution, which is patchy in the plane of the sky and with clouds interspersed among emission-line sources in the same line of sight. The optical depth at H alpha as measured from the ratio of the thermal radio continuum to H alpha shows a very good correlation with the total CO (1-0) column, indicating that most of the dust resides in the cold molecular phase. The optical depth at H alpha as measured from the ratio of H alpha to H beta also correlates with the CO emission but not as strongly as in the previous case. We analyze the difference between those two measurements and we find that <=11% of the H II gas is hidden behind large-optical-depth molecular clouds. We detect two candidate compact H II regions embedded inside the molecular cloud; both are within short distance of WR/Of stars and one of them is located within 16 pc of a RSG. We estimate the age of the main stellar generation in NGC 604 to be approx. 3 Myr from the ionization structure of the H II region. The size of the main cavity is smaller than the one predicted by extrapolating from single-star wind-blown bubbles.

Initial Ionization of Compressible Turbulence

We study the effects of the initial conditions of turbulent molecular clouds on the ionization structure in newly formed H_{ii} regions, using three-dimensional, photon-conserving radiative transfer in a pre-computed density field from three-dimensional compressible turbulence. Our results show that the initial density structure of the gas cloud can play an important role in the resulting structure of the H_{ii} region. The propagation of the ionization fronts, the shape of the resulting H_{ii} region, and the total mass ionized depend on the properties of the turbulent density field. Cuts through the ionized regions generally show ``butterfly'' shapes rather than spherical ones, while emission measure maps are more spherical if the turbulence is driven on scales small compared to the size of the H_{ii} region. The ionization structure can be described by an effective clumping factor $\zeta= \cdot / ^2$, where $n$ is number density of the gas. The larger the value of $\zeta$, the less mass is ionized, and the more irregular the H_{ii} region shapes. Because we do not follow dynamics, our results apply only to the early stage of ionization when the speed of the ionization fronts remains much larger than the sound speed of the ionized gas, or Alfv\'en speed in magnetized clouds if it is larger, so that the dynamical effects can be negligible.

Chemical abundances in QSO host galaxies and environments from narrow absorption line systems

We determined C, N and a-element relative abundances in the gas surrounding six quasi-stellar objects (QSOs) at an average redshift of z 2.4, by studying six narrow associated absorption systems in Ultraviolet Visual Echelle Spectrograph (UVES) high-resolution spectra. We found five systems with a metallicity (measured by C/H) consistent or above the solar value. The ionization structure observed in the associated systems is clearly different from that of the intervening ones, indicating that the associated systems are influenced by the strong UV flux from the QSO. There is a possible correlation (anticorrelation) between [N/C] ([Si/C]) and [C/H] of the studied associated systems, and [N/C]= 0 when [C/H]= 0. We have compared these observational results with the predictions of a model simulating the joint evolution of QSOs and their spheroidal hosts. The agreement turns out to be very good; in particular, the case envisaging massive haloes and high star formation rates recovers both the correlation between [N/C] and [C/H] and the anticorrelation for [Si/C] versus [C/H]. Narrow associated absorption systems prove to be powerful tracers of the chemical abundances in gas belonging to high-redshift spheroidal galaxies. The outflow of this same gas, triggered by the QSO feedback, is probably going to contribute to the early enrichment of the surrounding intergalactic medium. A larger sample, possibly increasing the number of ionization stages, chemical elements and the redshift range, would allow us to put firm constraints on detailed chemical evolution models of galaxies at high redshifts.

Dusty, Radiation Pressure–Dominated Photoionization. I. Model Description, Structure, and Grids

We present the implementation of dusty, radiation pressure-dominated photoionization models applicable to the narrow-line regions (NLRs) of active galactic nuclei, using the MAPPINGS III code. We give a grid of the predicted intensities of the most commonly used diagnostic spectral lines in the UV, optical, and IR, covering a wide range of density, metallicity, the power-law index characterizing the photoionizing source, and photoionization parameter, for use in the diagnosis of NLRs. We examine the temperature, density, and ionization structure of these models, investigating the effect of variation of these parameters in order to gain a better understanding of NLR clouds themselves.

Carina's defiant Finger:HSTobservations of a photoevaporating globule in NGC 3372★

We present Hubble Space Telescope Wide Field Planetary Camera 2 images of a prominent externally-ionized molecular globule in the Carina Nebula (NGC 3372), supplemented with ground-based infrared images and visual-wavelength spectra. This molecular globule has a shape resembling a human hand, with an extended finger that points toward its likely source of ionizing radiation. Following an analysis of the spatially-resolved ionization structure and spectrum of the photoevaporative flow from the Finger, we conclude that the dominant ionizing source is either the WNL star WR25 (HD 93162), the adjacent O4 If-type star Tr16-244, or perhaps both. We estimate a mass-loss rate of ∼2×10 −5 M⊙ yr −1 from the main evaporating surface of the globule, suggesting a remaining lifetime of 10 5.3 to 10 6 years. We find a total mass for the entire globule of more than 6 M⊙, in agreement with previous estimates. The hydrogen column density through the globule derived from extinction measurements is a few times 10 22 cm −2 , so the photodissociation region behind the ionization front should be limited to a thin layer compared to the size of the globule, in agreement with the morphology seen in H2 images. Although a few reddened stars are seen within the boundary of the globule in near-infrared continuum images, these may be background stars. We do not detect a reddened star at the apex of the finger, for example, down to a limiting magnitude of mK ≃17. However, considering the physical properties of the globule and the advancing ionization front, it appears that future star formation is likely in the Finger globule, induced by radiation-driven implosion.

A Study of the Reionization History of Intergalactic Helium withFUSEand the Very Large Telescope

We obtained high-resolution Far Ultraviolet Spectroscopic Explorer (FUSE; R ~ 20,000) and Very Large Telescope (VLT; R ~ 45,000) spectra of the quasar HE 2347-4342 in order to study the properties of the intergalactic medium between redshifts z = 2.0 and 2.9. The high-quality optical spectrum allows us to identify approximately 850 H I absorption lines with column densities between N ~ 5 × 1011 and 1018 cm-2. The reprocessed FUSE spectrum extends the wavelength coverage of the He II absorption down to an observed wavelength of 920 A. Source flux is detected to rest-frame wavelengths as short as ~237 A. Approximately 1400 He II absorption lines are identified, including 917 He II Lyα systems and some of their He II Lyβ, Lyγ, and Lyδ counterparts. The ionization structure of He II is complex, with approximately 90 absorption lines that are not detected in the hydrogen spectrum. These features may represent the effect of soft ionizing sources. The ratio η = N(He )/N(H ) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indexes of quasars. This provides evidence that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured active galactic nuclei, which do not ionize helium. We find an evolution in η toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z > 2.7, the large but finite increase in the He II opacity, τ = 5 ± 1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He+ ions. For line widths b = ξbH, we find ξ = 0.95 ± 0.12, indicating a velocity field in the intergalactic medium dominated by turbulence. At hydrogen column densities N < 3 × 1012 cm-2, the number of forest lines shows a significant deficit relative to a power law and becomes negligible below N = 1011 cm-2.

Jump conditions in hypersonic shocks

We study the quantitative effects of excitation, ionization, radiation energy and pressure, on the jump conditions in hypersonic shocks in a real gas. The ionization structure and excitation energies are calculated from the local temperature and density, using the Screened Hydrogenic Model. We assume an optically thick medium and no radiation flux through the shock front. We investigate the jump conditions in different gases and propose a phenomenological description of compression for different shock velocities. We find that the excitation energy term is the dominant term in ionized gases at low velocities. Consequently, higher shock velocities than the values predicted by standard calculations in a perfect gas must be reached in order to observe the effects of radiation in the compression ratio. Our results provide constraints for the design of future radiative shock experiments on the next generation of powerful nanosecond lasers or on Z-pinches.

Infrared mergers and infrared quasi-stellar objects with galactic winds - I. NGC 2623: nuclear outflow in a proto-elliptical candidate

We present the first results of a study of the morphology, kinematics and ionization structure of infrared (IR) mergers/quasi-stellar objects (QSOs) with galactic winds. This study is based mainly on integral two-dimensional (2D) fibre spectroscopy [obtained on the 4.2-m William Herschel Telescope (WHT), La Palma] combined with high-resolution Hubble Space Telescope (HST) observations. Clear evidence of outflow (OF) from the nucleus of the luminous infrared merger NGC 2623 is reported. Specifically: (i) the integral 2D Hα, [N ii] and [S ii] emission line maps depict a cone-shaped extended nebula that emerges from the nucleus, with an aperture angle θ= 100°± 5° and reaching a distance of ∼3.2 kpc from the nucleus; (ii) inside the nebula and in the central region, all the emission-line WHT spectra show low velocity blue/OF components, with 〈VOF〉= (−405 ± 35) km s−1; and (iii) in the OF nebula, the emission line ratios are consistent with ionization by a dusty nuclear starburst plus shock heating. These results are consistent with a galactic wind process powered mainly by a nuclear starburst. The integral 2D Hα and [N ii]λ6583 velocity field (VF) maps for the main body of NGC 2623 (16.4 × 12.3 arcsec2; ∼5.9 × 4.4 kpc2) show outflow motion in the nuclear and the Hα+[N ii] nebular regions superposed on a general circular motion. This circular motion prevails inside r∼ 1.5 kpc, and for larger radii we detected non-circular motions. In the central region, the average observed rotation curve was fitted with a model corresponding to a single-component Plummer spherical potential. After the subtraction of the Plummer and an axisymmetric polynomial model, the residues of the VF in both cases indicate ejection as the origin of the cone nebula. The fitted Plummer model implies a total mass of MT= 1.5 × 1010 M⊙ and a spherical distribution of matter in the central region. The high-resolution HST WFPC2 F555W (∼V) and F814W (∼I) broad-band images display a strongly obscured nucleus in the apex of a small nuclear cone, an asymmetrical clumpy spiral arm located to the east of the nucleus, a ring plus an arc to the west and several large-scale filaments of dust. A good r1/4-law fit to the HST WFPC2 I band luminosity profile was found. In 85 per cent of the integral 2D field we measure very high values (>1) of the [N ii]λ6583/Hα and [S ii]λ6717 + 31/Hα ratios, suggesting that shocks are important on large scales (in almost all the main body). Furthermore, the 2D full width at half maximum FWHM–[N ii] and VF residual maps show a good spatial correlation, suggesting that the OF shocks ionize the gas and broaden the emission lines. However, close to the nucleus, the OF nebula shows low values of the [N ii]λ6583/Hα ratio (in the range 0.1–0.4), indicating that photoionization by a dusty nuclear starburst also plays a significant role in the excitation of the nebula. The characteristics of the nucleus of NGC 2623 could be associated with a starburst-related LINER. The properties found in IR mergers/QSOs with galactic winds mainly underline the importance of studying the possible link between IR mergers with starburst + galactic wind IR QSOs with composite nature + galactic wind, and elliptical galaxies.

High‐Resolution X‐Ray Spectroscopy of the Fe K Complex in IC 4329A

We report the detection of complex Fe K line emission from a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of the Seyfert 1 galaxy IC 4329A. The line emission is double-peaked, one peak centered at ~6.3 keV and the other at ~6.9 keV in the source rest frame. When modeled by Gaussians, the lower energy peak is resolved by the high-energy grating at greater than 99% confidence, while the higher energy peak is resolved at only less than 90% confidence. The best-fitting widths are ~21,000 and ~4000 km s-1 FWHM for the ~6.3 and ~6.9 keV peaks, respectively. If the peaks correspond to two distinct emission lines, then the peak energies are redshifted with respect to the expected line energies of Fe I Kα and Fe XXVI Lyα by at least 650 and 950 km s-1, respectively. Alternatively, the Fe K line profile may be due to a single line from a relativistic accretion disk. In that case, the inclination angle of the disk is required to be 24 degrees, the outer radius is constrained to several tens of gravitational radii, and the radial line emissivity is flatter than r-0.7. Another possibility is that both peaks are due to distinct lines but that each one is relativistically broadened by a disk. In that case, the lower energy peak could correspond to emission from Fe in a low ionization state and the high-energy peak to Fe XXVI Lyα emission. Then the inclination angle is even less, restricted to a few degrees. However, the radial emissivity law is allowed to be steeper (~r-2.5), and the outer radius does not have to be fine-tuned. Yet another scenario is that the lower energy peak originates in a disk but the higher energy peak originates in more distant matter. The disk inclination angle is then intermediate between the last two cases, but the emissivity is again required to be flat. We cannot rule out Fe XXV He-like absorption modifying the observed line profile. However, the data and the inferred emission-line parameters are insensitive to the presence of a Compton reflection continuum. Including Compton reflection does, however, allow a steeper radial emissivity law for the relativistic line. Future missions, such as Astro-E2, will be able to break a lot of the degeneracy in the physically distinct models that can all account for the Chandra data. Since IC 4329A is one of the brightest Seyfert 1 galaxies, it should be a good astrophysical laboratory for studying the ionization structure of accretion disks around supermassive black holes.

A power law power spectral density model of total electron content structure in the polar region

Measurements by the early warning radar at Thule, Greenland, together with GPS measurements from the Air Force Research Laboratory (AFRL) Ionospheric Measurement System (IMS) receiver, have been analyzed to develop a model for the structure of the total electron content (TEC) of the polar ionosphere. For the model the TEC measurements are related to the Wide Band Model (WBMOD), a climatological model of small‐scale ionization structure. The TEC data agree very well with the predictions of a simple extension of WBMOD to larger scales, where a slightly steeper spectral slope (p ≈ 3) is used for the TEC structure (compared to p = 2.7 for the small‐scale structure of WBMOD's polar region model). The benefit of this approach is that the TEC model subsumes the climatology of WBMOD, which is built upon two solar cycles of ionospheric measurements.

Inference of hot star density stream properties from data on rotationally recurrent DACs

The information content of data on rotationally periodic recurrent discrete absorption components (DACs) in hot star wind emission lines is discussed. The data comprise optical depths τ(w, φ) as a function of dimensionless Doppler velocity w = (∆λ/λ0)(c/v∞) and of time expressed in terms of stellar rotation angle φ. This is used to study the spatial distributions of density, radial and rotational velocities, and ionisation structures of the corotating wind streams to which recurrent DACs are conventionally attributed. The simplifying assumptions made to reduce the degrees of freedom in such structure distribution functions to match those in the DAC data are discussed and the problem then posed in terms of a bivariate relationship between τ(w, φ) and the radial velocity vr(r), transverse rotation rate Ω(r) and density ρ(r ,φ ) structures of the streams. The discussion applies to cases where: the streams are equatorial; the system is seen edge on; the ionisation structure is approximated as uniform; the radial and transverse velocities are taken to be functions only of radial distance but the stream density is allowed to vary with azimuth. The last kinematic assumption essentially ignores the dynamical feedback of density on velocity and the relationship of this to fully dynamical models is discussed. The case of narrow streams is first considered, noting the result of Hamann et al. (2001) that the apparent acceleration of a narrow stream DAC is higher than the acceleration of the matter itself, so that the apparent slow acceleration of DACs cannot be attributed to the slowness of stellar rotation. Thus DACs either involve matter which accelerates slower than the general wind flow, or they are formed by structures which are not advected with the matter flow but propagate upstream (such as Abbott waves). It is then shown how, in the kinematic model approximation, the radial speed of the absorbing matter can be found by inversion of the apparent acceleration of the narrow DAC, for a given rotation law. The case of broad streams is more complex but also more informative. The observed τ(w, φ) is governed not only by vr(r) and Ω(r) of the absorbing stream matter but also by the density profile across the stream, determined by the azimuthal (φ0) distribution function F0(φ0) of mass loss rate around the stellar equator. When F0(φ0 )i s fairly wide inφ0, the acceleration of the DAC peak τ(w, φ )i nw is generally slow compared with that of a narrow stream DAC and the information on vr(r), Ω(r) and F0(φ0) is convoluted in the data τ(w, φ). We show that it is possible, in this kinematic model, to recover by inversion, complete information on all three distribution functions vr(r), Ω(r )a ndF0(φ0) from data on τ(w, φ )o f sufficiently high precision and resolution since vr(r )a ndΩ(r) occur in combination rather than independently in the equations. This is demonstrated for simulated data, including noise effects, and is discussed in relation to real data and to fully hydrodynamic models.