Giant pulsations in the plasmasphere

It has been generally accepted up to now that giant pulsations (Pg) are auroral zone phenomena but here we present observations of a sequence of three Pg events on successive days at three stations well within the plasmasphere. Field line resonance behaviour is exhibited with one of the events clearly resonating at L ? 2.8. From the resonant frequency (10.4 mHz) equatorial mass densities are calculated and from these, and the measured azimuthai polarization at resonance, the inference is drawn that Pgs are oscillations in the fundamental guided poloidal mode. We suggest that the drift wave instability of the compressional Alfvén wave may be the source mechanism for Pgs and speculate how conditions for the instability may have arisen.     

Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high density/high velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from >60° near the surface to 20° at 3 km depth and ∼38° at 6 km depth.     

The 825-1110 angstroms EUV spectrum of Venus

On 15 August 1994 we launched the EUVS sounding rocket payload to observe the 825-1110 angstrom region of Venus's far ultraviolet airglow spectrum. The EUVS telescope/spectrograph obtained good data at five times higher spectral resolution than was previously available in the far ultraviolet. We present these data and compare our results to those obtained by the Galileo UVS and Venera 11/12 UV spectrophotometers. We identify several new spectral emission features, including both singly ionized nitrogen and molecular nitrogen in Venus's spectrum. We also see evidence for electron-impact-induced emission from CO. Finally, the EUVS data indicate that the "Ar" emissions detected in Venus's far ultraviolet spectrum by Venera 11/12 spectrophotometers are in fact not due to argon, thus eliminating the discrepancy between in situ and remote sensing measurements.     

A close look at dihedral angles and melt geometry in olivine-basalt aggregates: a TEM study

Olivine-basalt aggregates sintered at high P/T have been used as a simplest approximation of partially molten upper mantle peridotite. In the past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most other crystalline materials) is distinctively anisotropic, the simple surface energy balance defining the dihedral angles cos(Θ/2)=_gb/2_sl is not valid and melt geometry is more complicated than can be expressed by a single dihedral angle value. We examine in detail melt geometry in aggregates held at high temperature and pressure for very long times (240–612 h). We show the simple dihedral angle concept to be invalid via transmission electron microscope images. Olivine-basalt interfaces are frequently planar crystal faces (F-faces) which are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. They may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not present (possibly at large grain size and very low melt fraction). Our measurements, excluding F-faces, give a range of dihedral angle values from 0 to 10° which is significantly lower than reported previously (20–50°). The nature of 0° angles (films and layers up to 1 μm in thickness) is unclear but their frequency compared to dry grain boundaries depends on grain size and melt fraction (e.g. 70% for grain size 43 μm and melt fraction 2%). Received: 13 April 1997 / Accepted: 2 October 1997     

The 7C(G) survey of radio sources at 151 MHz — the Galactic plane at 80° < l < 104° and 126° < l < 180°, for |b|≤ 55°.

Results from a survey of the northern Galactic plane (at declination ≳ 30°) at 151 MHz made with the Cambridge Low Frequency Synthesis Telescope are presented. This survey is designated the 7C(G) — i.e. the Galactic portion of the ongoing 7C surveys. This covers the regions 80° <  l  104° and 126° <  l  < 180°, for | b | ≤ 55^°_., and has some coverage to | b | ≈ 9°, with a resolution of ≈ 70 × 70 cossec (δ) arcsec² (RA × Dec.). The observations, data reduction and calibration of this survey are described, and a catalogue of 6262 compact sources, with a completeness limit of ≈ 0.25 Jy over most of the survey region, is presented. The catalogue has an rms positional accuracy of better than 10 arcsec, and the flux densities are tied to the scale of Roger, Bridle & Costain with an accuracy of better than 10 per cent.     

Electron excitation of a Jovian aurora

The planet Jupiter, like the Earth, possesses a magnetic field, and, therefore, auroral activity is very likely. In this work, the auroral emissions due to electron precipitation are estimated for a model atmosphere with and without helium. The incident primary electrons, which are characterized by representative spectra, are degraded in energy by applying the continuous slow down approximation. All secondaries, tertiaries, and higher generation electrons are assumed to be absorbed locally. A compilation of excitation, dissociation, and ionization cross section data for H, H₂, and He are used to model all aspects of the energy deposition process. Volume emission rates are calculated from the total direct excitation rates, and appropriate corrections for cascading are applied. Integrated column intensities of several kiloRayleighs are obtained for the various vibrational levels of the Lyman and Werner bands of H₂, as well as the triplet continuum a³Σ_g⁺ → b³Σ_u⁺. Helium emissions are relatively small because the majority of electrons are absorbed above the region of maximum He concentration. Atomic hydrogen emissions are due mainly to dissociative excitation of molecular hydrogen rather than direct excitation.     

Martian topography and surface properties as seen by radar: The 1971 opposition

Taking advantage of the favorable opposition of 1971, the Goldstone radar system, operating at 2388MHz, was used to scan the Martian surface. Measurements of altitude and reflected power were taken approximately every 3 days. Each measurement represents an area 8km E-W × 80km N-S, the highest resolution attained to date. Altitude measurements obtained on different observing days were combined to produce altitude profiles for three complete rotations, each at different latitudes. Large-scale variations in altitudes cover a range of 14km. Altitude changes of 5 in 30km of longitude were observed. The altitude profiles show the heavy cratering of the surface, and several large craters (50–100km) 1–2 km deep are easily seen. Reflected power for different angles of incidence was measured, yielding the scattering properties of the surface as a function of longitude. Correlation was found between the peak intensity of the reflected signal and the width of the scattering function. The average relationship between the intensity and the width suggests a power reflection coefficient of at least 0.064 ± 0.012. Departures from the average are interpreted as deviations in the reflection coefficient from the mean value. These variations are presented as a function of longitude for each of the three rotations.     

Mariner 9—Image processing and products

The purpose of this paper is to describe the system for the display, processing, and production of image-data products created to support the Mariner 9 Television Experiment. Of necessity, the system was large in order to respond to the needs of a large team of scientists with a broad scope of experimental objectives. The desire to generate processed data products as rapidly as possible to take advantage of adaptive planning during the mission, coupled with the complexities introduced by the nature of the vidicon camera, greatly increased the scale of the ground-image processing effort.This paper describes the systems that carried out the processes and delivered the products necessary for real-time and near-real-time analyses. References are made to the computer algorithms used for the, different levels of decalibration and analysis.     

Jovian proton Aurora

The planet Jupiter possesses a magnetic field and is surrounded by a magnetosphere. The occurrence of auroral and polar cap phenomena similar to those found on earth is very likely. In this work auroral and polar cap emissions in a model Jovian atmosphere are determined for proton precipitation. The incident protons, which are characterized by representative spectra, are degraded in energy by applying the continuous slowing down approximation. All secondary and higher generation electrons are assumed to be absorbed locally and their contributions to the total emissions are included. Volume emission rates are calculated from the total direct excitation rates with corrections for cascading applied. Results show that most molecular hydrogen and helium emissions for polar cap precipitation are below the ambient dayglow values. Charge capture by precipitating protons is an important source of Lyman α and Balmer α emissions and offers a key to the detection of large fluxes of low energy protons.