Kilometric Type III Radio Bursts,Electron Beams,and Interplanetary Density Structures

We briefly review kilometric type III radio bursts and what their physical characteristics reveal about the propagation and characteristics of suprathermal electron beams and about density structures in interplanetary medium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Jupiter's radiation belts

The recent close encounters of Pioneer-10 (December 1973) and Pioneer-11 (December 1974) with the planet Jupiter provided the first in situ observations of zenomagnetically trapped particle radiation. Such observations represented a major advance in planetary research. Prior estimates of radiation intensities (particle fluxes) at Jupiter had necessarily relied (in the case of electrons) upon inferences from Jovian decimetric radio emission observed at the Earth and (in the case of protons) upon postulates for the numerical scaling from terrestrial proton intensities. The Pioneer-10 and Pioneer-11 observations have stimulated continuing theoretical efforts to understand the reported findings and to extrapolate from them to other planets and other epochs. While the analysis of trapped-radiation data from the Pioneer spacecraft is far from being completed, a consensus has developed with respect to the physical mechanisms that must be considered. The observed radiation belts seem to be populated by radial diffusion from an external source. The diffusion coefficient seems to be that derived from fluctuations in the polarization electric field produced by neutral winds in the Jovian ionosphere, which is coupled to the magnetosphere by equipotential B-field lines. Radiation-belt electrons lose energy and change their equatorial pitch angles by virtue of synchrotron emission. Radiation-belt ions and electrons both may be subject to pitch-angle diffusion caused by waves that the respective particle anisotropies have created through plasma instabilities. Finally, radiation-belt ions and electrons seem to experience absorption by the inner Jovian satellites (moons) in a manner that may depend upon the species and energy of the incident radiation-belt particle. It is not yet known whether satellite-associated clouds of sodium and sulfur contribute substantially to the inferred particle absorption. Also still open is the question of whether the satellites provide a substantial source of radiation-belt particles. Moreover, there remains doubt concerning the configuration of the outer Jovian magnetosphere and the influence of this configuration on the zenomagnetic trapping of energetic charged particles.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Black hole accretion and X-ray spectra of QSOS

Changes in Eddington accretion ratios are thought to result in X-ray spectral index changes in Galactic binary black hole systems. Objects with higher Eddington ratios have softer X-ray spectra. Can we apply this result to much more massive black hole systems such as QSOs? If so, X-ray observations will give us valuable insight into the physics of QSOs. Among QSOs, X-ray spectral index is part of a large set of correlated optical and UV observational properties, especially optical Fe II and [O III] strengths in the Hβ region. To investigate whether this set of correlations is related to Eddington ratio, we use as probes, BALQSOs that have been suggested to be youthful super-accretors. We conducted infrared spectroscopy of the Hβ rest wavelength region for a sample of BALQSOs and compared line measurements with those for high and low redshift non-BAL QSOs. Hβ line widths and bolometric luminosity are used to calculate QSO black hole masses and Eddington ratios. Our results support the hypothesis that optical Fe II and [O III] line strengths are Eddington ratio indicators in QSO central engines. A possible explanation is that strong Fe II and weak [O III] indicate abundant cold gas that could fuel near Eddington accretion.     

Travel, Sex, and Food: What's Speciation Got to Do with It?

Abstract We discuss the potential interactions among travel (dispersal and gene flow), bacterial "sex" (mainly as horizontal gene transfer), and food (metabolic plasticity and responses to nutrient availability) in shaping microbial communities. With regard to our work at a unique desert oasis, the Cuatro Ciénegas Basin in Coahuila, Mexico, we propose that diversification and low phosphorus availability, in combination with mechanisms for nutrient recycling and community cohesion, result in enhanced speciation through reproductive as well as geographic isolation. We also discuss these mechanisms in the broader sense of ecology and evolution. Of special relevance to astrobiology and central to evolutionary biology, we ask why there are so many species on Earth and provide a working hypothesis and a conceptual framework within which to consider the question. Key Words: Microbial ecology-Microbial mats-Evolution-Horizontal gene transfer-Metabolism. Astrobiology 12, 634-640.     

Bayesian linking of geosynchronous orbital debris tracks as seen by the Large Synoptic Survey Telescope

We describe a Bayesian sampling model for linking and constraining orbit models from angular observations of “streaks” in optical telescope images. Our algorithm is particularly suited to situations where the observation times are small fractions of the orbital periods of the observed objects or when there is significant confusion of objects in the observation field. We use Markov Chain Monte Carlo to sample from the joint posterior distribution of the parameters of multiple orbit models (up to the number of observed tracks) and parameters describing which tracks are linked with which orbit models. Using this algorithm, we forecast the constraints on geosynchronous (GEO) debris orbits achievable with the planned Large Synoptic Survey Telescope (LSST). Because of the short 15 s exposure times, preliminary orbit determinations of GEO objects from LSST will have large and degenerate errors on the orbital elements. Combined with the expected crowded fields of GEO debris it will be challenging to reliably link orbital tracks in LSST observations given the currently planned observing cadence.     

A numerical model characterizing internal gravity wave propagation into the upper atmosphere

A new two-dimensional, time-dependent and fully nonlinear model is developed to numerically simulate plane wave motions for internal gravity waves in a non-isothermal and windy atmosphere that accounts for the dissipation due to eddy and molecular processes. The atmosphere has been treated as a well mixed total gas with a constant mean molecular weight. The effects of Rayleigh friction and Newtonian cooling are applied near the upper boundary of the model to simulate the radiation conditions as well as to act as a sponge layer; lateral boundaries are periodic over a horizontal wavelength to simulate a horizontally infinite domain. The thermal excitation to initiate upward propagating waves is spatially localized in the troposphere and is a Gaussian function of time. A time-splitting technique is applied to the finite difference equations that are derived from the Navier–Stokes equations. The time integration for these highly coupled equations is performed using an explicit second order Lax–Wendroff scheme and an implicit Newton–Raphson scheme. The wave solutions derived from the model are found to be broadly agreeable with those derived from a Wentzel–Kramers–Brillouin theory.     

Transverse Oscillations of Coronal Loops

On 14 July 1998 TRACE observed transverse oscillations of a coronal loop generated by an external disturbance most probably caused by a solar flare. These oscillations were interpreted as standing fast kink waves in a magnetic flux tube. Firstly, in this review we embark on the discussion of the theory of waves and oscillations in a homogeneous straight magnetic cylinder with the particular emphasis on fast kink waves. Next, we consider the effects of stratification, loop expansion, loop curvature, non-circular cross-section, loop shape and magnetic twist. An important property of observed transverse coronal loop oscillations is their fast damping. We briefly review the different mechanisms suggested for explaining the rapid damping phenomenon. After that we concentrate on damping due to resonant absorption. We describe the latest analytical results obtained with the use of thin transition layer approximation, and then compare these results with numerical findings obtained for arbitrary density variation inside the flux tube. Very often collective oscillations of an array of coronal magnetic loops are observed. It is natural to start studying this phenomenon from the system of two coronal loops. We describe very recent analytical and numerical results of studying collective oscillations of two parallel homogeneous coronal loops. The implication of the theoretical results for coronal seismology is briefly discussed. We describe the estimates of magnetic field magnitude obtained from the observed fundamental frequency of oscillations, and the estimates of the coronal scale height obtained using the simultaneous observations of the fundamental frequency and the frequency of the first overtone of kink oscillations. In the last part of the review we summarise the most outstanding and acute problems in the theory of the coronal loop transverse oscillations.