SUMMARY
Numerical solution of global geomagnetic induction problems in two and three spatial dimensions can be conducted with commercially available, general‐purpose, scripted, finite‐element ...software. We show that FlexPDE is capable of solving a variety of global geomagnetic induction problems. The models treated can include arbitrary electrical conductivity of the core and mantle, arbitrary spatial structure and time behaviour of the primary magnetic field. A thin surface layer of laterally heterogeneous conductivity, representing the oceans and crust, may be represented by a boundary condition at the Earth–space interface. We describe a numerical test, or validation, of the program by comparing its output to analytic and semi‐analytic solutions for several electromagnetic induction problems: (1) concentric spherical shells representing a layered Earth in a time‐varying, uniform, external magnetic field, (2) eccentrically nested conductive spheres in the same field and (3) homogeneous spheres or cylinders, initially at rest, then rotating at a steady rate in a constant, uniform, external field. Calculations are performed in both the time and frequency domains, and in both 2‐D and 3‐D computational meshes, with adaptive mesh refinement. Root‐mean‐square accuracies of better than 1 per cent are achieved in all cases. A unique advantage of our technique is the ability to model Earth rotation in both the time and the frequency domain, which is especially useful for simulating satellite data.
We present observations of the intervening O vi absorption-line system at zabs= 0.495 096 towards the quasi-stellar object (QSO) PKS 0405−123 (zem= 0.5726) obtained with the Far Ultraviolet ...Spectroscopic Explorer and Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. In addition to strong O vi, with , and moderate H i, with , this absorber shows absorption from C iii, N iv, O iv and O v, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far fewer constraints. We find that the observed ionic column densities cannot be matched by single-temperature collisional ionization models, in or out of equilibrium. Photoionization models can match all of the observed column densities, including O vi. If one assumes photoionization by an ultraviolet (UV) background dominated by QSOs, the metallicity of the gas is O/H≈−0.15, while if one assumes a model for the UV background with contributions from ionizing photons escaping from galaxies the metallicity is O/H≈−0.62. Both give N/O∼−0.6 and C/H∼−0.2 to ∼−0.1, though a solar C/O ratio is not ruled out. The choice of ionizing spectrum is poorly constrained and leads to systematic abundance uncertainties of ≈0.5 dex, despite the wide range of available ions. Multiphase models with a contribution from both photoionized gas (at T∼ 104 K) and collisionally ionized gas at T∼ (1−3) × 105 K can also match the observations for either assumed UV background giving very similar metallicities. We do not detect Ne viii or Mg x absorption. The limit on Ne viii/O vi< 0.21 (3σ) is the lowest yet observed. Thus, this absorber shows no firm evidence of the ‘warm-hot intergalactic medium’ at T∼ (0.5–3) × 106 K thought to contain a significant fraction of the baryons at low redshift. The O vi in this system is not necessarily a reliable tracer of the warm-hot intergalactic medium given the ambiguity in its origins. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions O vii or O viii even if it resided in front of the brighter X-ray sources in the sky.
We present a survey for optically thick Lyman limit absorbers at z < 2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging ...Spectrograph. We identify 206 Lyman limit systems (LLSs), increasing the number of cataloged LLSs at z < 2.6 by a factor of ~10. We compile a statistical sample of 50 Delta *tLLS >= 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z) = dn/dz, at these redshifts is well described by a single power law, l(z)(1 + z) Delta *g, with Delta *g = 1.33 ? 0.61 at z < 2.6, or with Delta *g = 1.83 ? 0.21 over the redshift range 0.2 <= z <= 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6 Gyr from z = 4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ~8 Gyr from z = 2.6 to 0.25. We show that the column density distribution function, f(N H I ), at low redshift is not well fitted by a single power-law index (f(N H I ) N -- Delta *b H I ) over the column density range 13 <= log N H I <= 22 or log N H I >= 17.2. While low- and high-redshift f(N H I ) distributions are consistent for log N H I >19.0, there is some evidence that f(N H I ) evolves with z for log N H I 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ~9 from z ~ 5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L * galaxies.
We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z = 0.27395 Lyman limit system (LLS) seen in absorption against the QSO ...PG1630+377. We detect H I absorption with log N(H I) = 17.06 ? 0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity (Mg/H = --1.71 ? 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L * star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z = 0.27406 and Delta *Dv = --26 km s--1) with near solar metallicity (O/H = --0.20 ? 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L * galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low-metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z 1 LLSs have metallicities similar to the present system and show sub-L * galaxies with Delta *r < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations.
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