Electron content modelling: The significance of protonospheric contents

The principal advance of the ATS-6 satellite beacon experiment was the ability to deduce continuously the electron content along the entire slant path from ground-based measurements of the signal group delay. This feature has been exploited in conjunction with the more usual Faraday rotation technique to separate the total electron content into ionospheric and protonospheric components. The physical validity of the deduced quantities is investigated using a mathematical model of the plasmasphere in which integration of the time-dependent continuity and momentum equations for oxygen and hydrogen ions along selected L shells yields the ion concentrations and field-aligned fluxes. The ion concentrations are then integrated along the propagation path to various ground stations from ATS-6 to give computed values for comparison with observations. The mathematical model is used with different sets of atmospheric parameters to investigate the significance of ionospheric and protonospheric contents as derived from beacon data.The calculated electron concentrations are able to reproduce mid-latitude equinoctial electron content observations. The shape parameters τ and F can also be simulated by day, but night-time values do not match the observations well, a greater protonospheric content being required. The calculations show that the quantity N_p, which is readily derived from ATS-6 observations, may be interpreted as the slant H⁺ content above some fixed height in the case of some stations (but not others) if the plasmasphere is reasonably full. The total slant content of H⁺ is approx. twice the value of N_p, though it appears that for the Lancaster raypath a closer relationship exists between N_p and the H⁺ tube content at L = 1.8. In general,N_p is most closely related to the tube content for an L value slightly greater than the minimum L intersected along the raypath.     

Particle dynamics in reconnection field configurations

Trajectories of test particles are studied numerically in two types of reconnection magnetic field configurations, a single X-line magnetic field configuration and a tearing magnetic field configuration. Both adiabatic and nonadiabatic motions are examined, with special emphasis on net energy gain and time spent in the neutral line regions. They spend typically one characteristic gyroperiod in the X-line region and are ejected predominantly along field lines in the vicinity of the separatrix. Both adiabatic and nonadiabatic test particles in the tearing-type field configuration are channelled into and accelerated along the O-line region. It may be inferred from these test particle results that particle energizations are significant along the O-line region, but not along the X-line region. These results are in qualitative agreement with those obtained by a self-consistent particle simulation.     

PKS2250-41 and the Role of Jet Cloud Interactions in Powerful Radio Galaxies

We have studied the effects of the interaction between the radio jet and the ambient gas in the powerful radio galaxy PKS2250-41 (z = 0.31). Our results show that the gas has been accelerated, compressed, heated and collisionally ionized by the shock. This study helps us to understand the processes which determine the observed properties of many high redshift radio galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Dynamics of Dust Grains in a Hierarchical Environment

Most main sequence stars are binaries or higher multiplicity Systems and it appears that at birth most stars have circumstellar disks. It is commonly accepted that planetary systems arise from the material of these disks; consequently, binary and multiple systems may have a main role in planet formation. In this paper, we study the stage of planetary formation during which the particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of the particles, in a protoplanetary disk with radius R_D = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600 AU) orbit. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations using a direct integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components of the binary. The evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative revolution of the stellar companion, and on the temperature and mass of the circumstellar disk. Our results show that for binary companions near the limit of tidal truncation of the disk, the perturbation leads to an enhanced accretion rate onto the primary, decreasing the lifetime of the particles in the protoplanetary disk with respect to the case of a single star. As a consequence of an enhanced accretion rate the mass of the disk decreases faster, which leads to a longer resultant lifetime for particles in the disk. On the other hand, binary companions may induce tidal arms in the dust phase of protoplanetary disks. Spiral perturbations with m = 1 may increase in a factor 10 or more the dust surface density in the neighbourhood of the arm, facilitating the growth of the particles. Moreover, in a massive disk (0.01M⊙) the survival time of particles is significantly shorter than in a less massive nebula (0.001M⊙) and the temperature of the disk severely influences the spiral-in time of particles. The rapid evolution of the dust component found in post T Tauri stars can be explained as a result of their binary nature. Binarity may also influence the evolution of circumpulsar disks. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Method to Determine the Solar Synodic Rotation Rate and the Height of Tracers

The dependence of the measured apparent synodic solar rotation rate on the height of the chosen tracer is studied. A significant error occurs if the rotation rate is determined by tracing the apparent position of an object above the photospheric level projected on the solar disc. The centre-to-limb variation of this error can be used to determine simultaneously the height of the object and the true synodic rotation rate. The apparent (projected) heliographic coordinates are presented as a function of the height of the traced object and the coordinates of its footpoint. The relations obtained provide an explicit expression for the apparent rotation rate as a function of the observed heliographic coordinates of the tracer, enabling an analytic least-squares fit expression to determine simultaneously the real synodic rotation rate and the height of the tracer.     

THE SAN JUAN CAPISTRANO METEORITE

The San Juan Capistrano chondrite fell on 15 March 1973; the total recovered mass was 56 g. Electron microprobe, chemical and petrographic studies show it to be a member of the H group and of petrologic type 6. Rare gas studies show that only minor radiogenic gas loss has occurred and yield a K-Ar age of 4.6 × 10⁹ years and a Kr-Kr exposure age of 29 × 10⁶ years.     

COMPOSITIONAL STUDY OF THE LONE TREE,IOWA, CHONDRITE

A meteorite, named for the location of its discovery near Lone Tree, Iowa, was found by Loren Westfall in May 1971. Electron microprobe and petrographic studies reveal its mineral composition to be olivine, low-calcium clinopyroxene, high-calcium clinopyroxene, troilite, kamacite, taenite and iron oxides. On the basis of texture, olivine composition (19% Fa), low-calcium clinopyroxene composition (17% Fs, 2% Wo) and metal (determined by modal analysis), this meteorite is classified as an H group bronzite chondrite. While it has characteristics of classes 3 and 4 (Van Schmus and Wood, 1967, Table 2) it fits class 4 better since low-calcium pyroxene has a MD of 5.6%, olivine has a MD of 3.2%, turbid glass is present in chondrules, feldspar is absent, and the matrix is opaque. The opacity of the matrix may be due to iron oxides in microfractures in a microcrystalline matrix.     

A trigger mechanism for the emerging flux model of solar flares

The energetics of a current sheet that forms between newly emerging flux and an ambient field are considered. As more and more flux emerges, so the sheet rises in the solar atmosphere. The various contributions to the thermal energy balance in the sheet are approximated and the resulting equation solved for the internal temperature of the sheet. It is found that, for certain choices of the ambient magnetic field strength and velocity, the internal temperature increases until, when the sheet reaches some critical height, no neighbouring equilibrium state exists. The temperature then increases rapidly, seeking a hotter branch of the solution curve. During this dynamic heating, the threshold temperature for the onset of plasma microinstabilities may be attained. It is suggested that this may be a suitable trigger mechanism for the recently proposed emerging flux model of a solar flare.This work was done while the author was participating in the CECAM workshop on Plasma Physics applied to Active Solar Phenomena, August–September 1976 at Orsay, France, and the Skylab Solar Workshop on Solar Flares (sponsored by NASA and NSF and managed by the High Altitude Observatory).     

On making radio contact with extraterrestrial civilizations

The problem of, attempting to make radio contact, with other technological civilizations is examined. It is argued that there are few (perhaps indeed no) omnidirectional radio beacons in the Galaxy primarily because any such beacon, to be effective, would have to be designed for a call-response time of at least some 10⁴ years, which is much too long to be likely to be acceptable to governments. A consequence, is that any search program by us would have little chance of detecting incoming signals.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978.     

26Al and the partial ionization of the solar nebula

Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of²⁶Al will result in strong coupling of the gas to magnetic fields. In the absence of²⁶Al,⁴⁰K will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.also Department of Astronomy.