D. A. Bryant

The Giotto three-dimensional positive ion analyser

The three-dimensional velocity distribution of positive ions in the neighbourhood of Comet Halley was measured by an instrument which included two complementary sensors. The fast ion sensor measured the energy/charge distribution from 10 eV/q to 20 keV/q once per revolution of the spacecraft. It obtained the characteristics of the solar wind flow near the comet. The implanted ion sensor measured the energy/charge distribution from 90 eV/q to 90 keV/q with discrimination into five mass groups in a period of 32 spacecraft revolutions. These observations provided the angular distribution of the cometary ions formed by the ionisation of gas molecules sublimed from the cometary nucleus. The relation between the raw count rates and the plasma parameters for the fast ion sensor is derived on the basis of a complete energy angle scan of the sensor in a calibration source. The accuracy of the analysis was tested by two techniques using data collected during the mission.

Waves in the magnetic field and solar wind flow outside the bow shock at comet P/Halley

The existence of hydromagnetic waves in the mass-loaded solar wind upstream from the bow shock of a comet is well-established both for comet Giacobini-Zinner and for comet Halley. Whereas previous reports have been concerned either with the magnetic field observations or with plasma observations, here we combine observations of the magnetic field with the solar wind proton and alpha particle distributions. This allows the three possible modes of propagation for these waves to be separated. The magnetic component is predominantly transverse to the magnetic field and linearly polarised. The flow vector also has a substantial amount of power parallel to the magnetic field. A examination of the pressure variations shows that slow magnetosonic waves are more common than the fast mode.

In-situ observations of a bi-modal ion distribution in the outer coma of comet P/Halley

Observations obtained by the Johnstone Plasma Analyzer on the Giotto fly-by of comet Halley showed a fairly sudden decrease in the count rate of energetic (~ 30 keV) water-group ions inside about 5 105 km from the nucleus. This decrease was accompanied by the appearance of a new water-group ion population at slightly lower energies (< 10 keV). Close inspection reveals that this lower-energy peak was also present somewhat earlier in the post-shock flow but only became prominent near the sudden transition just described. It is shown that the observed bimodal ion distribution is well explained in terms of the velocity history of the accreting solar wind flow in the outer coma. The decline in count rate of the energetic pick-up distribution is due to a relatively sudden slowing of the bulk flow there and not to a loss of particles. Hence charge-exchange cooling of the flow is probably not important at these distances from the nucleus. Finally, the observations suggest that pitch-angle scattering is fairly efficient at least after the bow shock, but that energy diffusion is probably not very efficient.

Auroral electron acceleration

Two theories of auroral electron acceleration are discussed. Section 1 examines the currently widely held view that the acceleration is an ordered process in a quasi-static electric field. It is suggested that, although there are many factors seeming to support this theory, the major qualifications and uncertainties that have been identified combine to cast serious doubt over its validity. Section 2 is devoted to a relatively new interpretation in terms of stochastic acceleration in turbulent electric fields. This second theory, which appears to account readily for most known features of the electron distribution function, is considered to provide a more promising approach to this central question in magnetospheric plasma physics.

Alfvénic turbulence in the solar wind flow during the approach to comet P/Halley

The Johnstone Plasma Analyser on Giotto operated almost continuously for two days before the encounter with comet Halley. Throughout this period it made observations of the proton and alpha particle distributions in the solar wind with 8 seconds time resolution. As the comet was approached, fluctuations were observed in all the primary bulk parameters, i.e. density, temperature and flow velocity, of both distributions at levels above the usual solar wind turbulence. We present here a survey of the data from a distance of 5 106 km from the nucleus up to the cometary foreshock at 1.4 106 km. Early in this period the variance was higher than in the solar wind on a similar day but the activity may have been associated with a crossing of the heliospheric current sheet. From a distance of 2.7 106 km the power level gradually increased, as the mass loading of the solar wind increased, to levels well above normal solar wind turbulence. We estimate that up to 16% of the free energy associated with the implanted cometary ion distribution went into the wave energy, similar to the proportion which has been estimated by quasilinear theory. Although the solar wind convection time through the mass-loaded region is short compared with the development time for a shell distribution in the cometary ions estimated from numerical simulation, the observations indicate that the cometary ion distribution is still basically ring-like.

The AMPTE UKS Spacecraft

The decision to include a third spacecraft, the UKS, in the AMPTE mission was made in 1981. The reasons for this are presented, together with a description of the spacecraft, its subsystems, and a summary of its early orbit performance. The UKS scientific instruments, and early results from them, are described in companion papers in this issue.

At the edge of the Earth's magnetosphere: a survey by AMPTE-UKS

A survey is made, by using measurements from the Active Magneto spheric Particle Tracer Explorers - United Kingdom Satellite, of the interaction between plasmas of solar and terrestrial origin at the outer edge of the Earth’s magnetosphere. The position of the boundary and its rate of movement are related statistically to solarwind dynamic pressure and its variations. The first results are presented of a new type of analysis which aims to clarify the nature of the boundary layer that develops between the two plasmas by reordering, on the basis of a consistent relation between electron density and temperature, the normally erratic progress made by a spacecraft across the constantly moving region. Distinctive patterns found consistently for the electron and ion transitions suggest that diffusion, viscosity and loss to the atmosphere govern the boundary layer. Various possibilities are discussed for the topology of the region. Electron acceleration within the boundary layer is identified; its cause and relevance to dayside auroral precipitation are discussed. There is an indication that the transition in the magnetic field, across the magnetopause current layer, lies within, rather than immediately outside, the boundary layer.

Solar wind flow through the comet P/Halley bow shock

The bow shock of a comet is formed by the interaction of three different particle populations; solar wind ions, cometary ions and electrons. We follow the behaviour of the solar wind protons through the Giotto inbound shock crossing at comet P/Halley. A foreshock boundary is seen at ~ 1.4 106 km where the level of solar wind fluctuations increases substantially. The shock itself is seen some 2.5 105 km closer. It is a complex structure with high-amplitude waves in the cometary ion foot of the shock preceding a permanent drop in the solar wind speed. The width of the shock structure is ~ 40,000 km. There is evidence for a further excursion back into the cometary ion foot after the initial shock crossing. The cometary ion density is inferred from the solar wind speed changes and is compared with measurements.

Data Handling for the UKS Spacecraft

This paper describes how data from the AMPTE UKS Spacecraft is acquired and processed by the United Kingdom Operations and Control Centre (UKOCC) at the Rutherford Appleton Laboratory. A review of UKOCC data handling is followed by a description of the systems used to analyze and display data in real time and to generate files and tapes of full-resolution and survey data post-pass. Examples are given of the survey data plots.

Electron acceleration by lower hybrid waves on auroral field lines

Particle measurements show that electron acceleration on auroral field lines is a statistical, velocity-dependent, process. It is proposed that the process is stochastic acceleration by waves, and demonstrated that lower hybrid waves seen on auroral field lines have the right properties to account for the electron acceleration. It is further shown that the lower hybrid wave power measured on auroral field lines can be generated by the streaming ions observed at the boundary of the plasma sheet, and that this wave power is sufficient to account for the electron power observed close to the atmosphere.