Transport of ionizing radiation in terrestrial-like exoplanet atmospheres

The propagation of ionizing radiation through model atmospheres of terrestrial-like exoplanets is studied for a large range of column densities and incident photon energies using a Monte Carlo code we have developed to treat Compton scattering and photoabsorption. Incident spectra from parent star flares, supernovae, and gamma-ray bursts are modeled and compared to energetic particles in importance. Large irradiation events with fluences of 10⁶-10⁹ erg cm⁻² at the conventional habitable zone can occur at a rate from many per day (flares from young low-mass parent stars) to ∼100 per Gyr (supernovae and gamma-ray bursts). We find that terrestrial-like exoplanets with atmospheres thinner than about 100 g cm⁻² block nearly all X-rays, but transmit and reprocess a significant fraction of incident γ-rays, producing a characteristic, flat surficial spectrum. Thick atmospheres (?100 g cm⁻²) efficiently block even γ-rays, but nearly all the incident energy is redistributed into diffuse UV and visible aurora-like emission, increasing the effective atmospheric transmission by many orders of magnitude. Depending on the presence of molecular UV absorbers and atmospheric thickness, up to 10% of the incident energy can reach the surface as UV reemission. For the Earth, between 2×10⁻³ and 4×10⁻² of the incident flux reaches the ground in the biologically effective 200-320 nm range, depending on O₂/O₃ shielding. For atmospheres thicker than ∼50 g cm⁻² in the case of pure Rayleigh scattering and ∼100 g cm⁻² in the case of O₂/O₃ absorption, the UV reemission exceeds the surficial transmitted ionizing radiation. We also discuss the effects of angle of incidence and derive a modified two-stream approximation solution for the UV transfer. Finally, we suggest that transient atmospheric ionization layers can be frequently created at altitudes lower than the equilibrium layers that result from steady irradiation and winds from the parent star. We suggest that these events can produce frequent fluctuations in atmospheric ionization levels and surficial UV fluxes on terrestrial-like planets.     

NIGHTGLOW: an instrument to measure the Earth’s nighttime ultraviolet glow—results from the first engineering flight

We have designed and built an instrument to measure and monitor the “nightglow” of the Earth’s atmosphere in the near ultraviolet (NUV). In this paper we describe the design of this instrument, called NIGHTGLOW. NIGHTGLOW is designed to be flown from a high altitude research balloon, and circumnavigate the globe. NIGHTGLOW is a NASA, University of Utah, and New Mexico State University project. A test flight took place from Palestine, Texas on July 5, 2000, lasting about 8 h. The instrument performed well and landed safely in Stiles, Texas with little damage. The resulting measurements of the NUV nightglow are compared with previous measurements from sounding rockets and balloons.     

The chronology of a large ice-dammed lake and the Barents–Kara Ice Sheet advances, Northern Russia

Beach and shoreface sediments deposited in the more than 800-km long ice-dammed Lake Komi in northern European Russia have been investigated and dated. The lake flooded the lowland areas between the Barents–Kara Ice Sheet in the north and the continental drainage divide in the south. Shoreline facies have been dated by 18 optical stimulated luminescence (OSL) dates, most of which are closely grouped in the range 80–100 ka, with a mean of 88±3 ka. This implies that that the Barents–Kara Ice Sheet had its Late Pleistocene maximum extension during the Early Weichselian, probably in the cold interval (Rederstall) between the Brørup and Odderade interstadials of western Europe, correlated with marine isotope stage 5b. This is in strong contrast to the Scandinavian and North American ice sheets, which had their maxima in isotope stage 2, about 20 ka. Field and air photo interpretations suggest that Lake Komi was dammed by the ice advance, which formed the Harbei–Harmon–Sopkay Moraines. These has earlier been correlated with the Markhida moraine across the Pechora River Valley and its western extension. However, OSL dates on fluvial sediments below the Markhida moraine have yielded ages as young as 60 ka. This suggests that the Russian mainland was inundated by two major ice sheet advances from the Barents–Kara seas after the last interglacial: one during the Early Weichselian (about 90 ka) that dammed Lake Komi and one during the Middle Weichselian (about 60 ka). Normal fluvial drainage prevailed during the Late Weichselian, when the ice front was located offshore.     

General relativistic MHD simulations of black hole accretion disks and jets

Accretion disks orbiting black holes power high-energy systems such as X-ray binaries and Active Galactic Nuclei. Observations are providing increasingly detailed quantitative information about such systems. This data has been interpreted using standard toy-models that rely on simplifying assumptions such as regular flow geometry and a parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions and, in principle, lead to a genuinely predictive theory. In recent years we have developed a fully three-dimensional general relativistic magnetohydrodynamic simulation code that evolves time-dependent inflows into Kerr black holes. Although the resulting global simulations of black hole accretion are still somewhat simplified, they have brought to light a number of interesting results. These include the formation of electro-magnetically dominated jets powered by the black hole’s rotation, and the presence of strong stresses in the plunging region of the accretion flow. The observational consequences of these features are gradually being examined. Increasing computer power and increasingly sophisticated algorithms promise a bright future for the computational approach to black hole accretion.     

Graded-Density Reservoirs for Accessing High Stress Low Temperature Material States

In recently developed laser-driven shockless compression experiments an ablatively driven shock in a primary target is transformed into a ramp compression wave in a secondary target via unloading followed by stagnation across an intermediate vacuum gap. Current limitations on the achievable peak longitudinal stresses are limited by the ability of shaping the temporal profile of the ramp compression pulse. We report on new techniques using graded density reservoirs for shaping the loading profile and extending these techniques to high peak pressures.     

A new tool for the study of periodic orbits

We present the generalization of the method of rational Fourier-series approximations for nonlinear ordinary differential equations in galactic dynamics. In previous papers we presented the formulae which describe simple families of periodic orbits with very good accuracy. In this Letter we generalize our formulae to describe periodic orbits of any resonancem:n.