The possibility of accelerated protons in solar flares having a sharp change in their spectral index is discussed. The analysis is based on the Tsytovich (1982, 1984, 1987a, b, c) acceleration model by MHD turbulence, which is shown to have different resonant conditions for non-relativistic and relativistic particles. The different resonant condition is shown to result in a sharp change in the accelerated proton spectral index, even in the absence of any peculiarity in the spectra of the MHD turbulence. Time scales for accelerated protons to relativistic energies are also derived, and shown to be consistent with observations. We also show that the threshold energy for electron acceleration by low frequency MHD turbulence is much greater than for proton acceleration. The turbulence therefore preferentially accelerates protons.

One of the main experiments of the Active Magnetospheric Particle Explorers (AMPTE) satellite experiments was the release of neutral atoms which ionized by photoionization, creating artificial expanding plasmas in the solar wind. In the experiments carried out diamagnetic cavities were produced which interrupted the solar wind flow. In this paper we examine how the newly created expanding plasma couples to the solar wind by means of electromagnetic instabilities driven by unstable velocity space ion distributions. In particular we show how lower-hybrid waves are generated from the unstable ion distributions and how these waves can produce high energy electron tails. The development of the diamagnetic cavity and the barium cloud was also analysed using a 2-D hybrid code. The experimental results will be compared with theory and a computer simulation of the electron acceleration and heating in the microturbulence.

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.

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.

The AMPTE barium ion release in the solar wind at 1232 UT on 27 December 1984 produced strong electric component plasma wave noise at frequencies below 3.9 kHz (amongst other emissions) which was detected at both the AMPTE-UKS and AMPTE-IRM spacecraft. In the region outside the magnetic cavity, this noise appears to be correlated with electron heating and changes sin the magnetic field. This paper compares data from the two spacecraft at high temporal resolution in order to identify the waves and the associated processes.

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.

Measurements with the Magnetospheric Ion Composition Spectrometer (MICS) onboard the polar orbiting Swedish satellite VIKING clearly show a strong asymmetry between the dusk‐ and dawnside ion distributions obtained after the onset of a magnetic storm on May 2‐3, 1986. The eveningside of the inner ring current region is quickly filled by protons with energies of some tens of keV. The morningside continues for several hours to show the quiet‐time inner ring current proton spectra in which protons below ∼100 keV have been removed due to charge‐exchange processes. The data obtained at high latitudes will be compared with measurements of the CHEM instrument on the AMPTE/CCE spacecraft near the equatorial plane. The observations are discussed in context of the convection of injected ions into the inner magnetosphere during magnetically active periods.

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.

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.

Preliminary results from the JPA instrument on Giotto indicate that Comet Halley, even on the flanks, has a bow shock which moves backwards and forwards over the spacecraft. To understand the structure properly will require more detailed investigation of the relationships between three particle populations, cometary ions, solar wind ions and electrons.

The process of mass loading of the solar wind by cometary ions, which forms comet tails, has been observed throughout the coma of comet Halley. Three distinct regimes were found where the nature of the energy and momentum coupling between solar wind and cometary ions is different. Outside the bow shock, where there is little angular scattering of the freshly ionised particles, the coupling is described by the simple pickup trajectory and the energy is controlled by the angle between the flow and the magnetic field. Just inside the bow shock, there is considerable scattering accompanied by another acceleration process which raises some particle energies well above the straightforward pickup value. Finally, closer to the nucleus, the amount of scattering decreases and the coupling is once more controlled by the magnetic field direction.