At Venus the Energetic Particles Detector (EPD) on the Galileo spacecraft measured the differential energy spectra and angular distributions of ions >22 kiloelectron volts (keV) and electrons > 15 keV in energy. The only time particles were observed by EPD was in a series of episodic events [0546 to 0638 universal time (UT)] near closest approach (0559:03 UT). Angular distributions were highly anisotropic, ordered by the magnetic field, and showed ions arriving from the hemisphere containing Venus and its bow shock. The spectra showed a power law form with intensities observed into the 120- to 280-keV range. Comparisons with model bow shock calculations show that these energetic ions are associated with the venusian foreshock-bow shock region. Shock-drift acceleration in the venusian bow shock seems the most likely process responsible for the observed ions.

Using the University of Maryland/Max‐Planck‐Institut für Aeronomie charge‐energy‐mass (CHEM) spectrometer on the AMPTE Charge Composition Explorer (CCE) spacecraft, we have examined the nearly equatorial storm time energy spectra of four major magnetospheric ions, H+, O+, He+, and He++, over the energy range 1–300 keV/e in the L range 3–6. The data were obtained during the main and early recovery phases of all geomagnetic storms with minimum Dst less than −50 nT in the time period September 1984 to November 1985. When the spectra are organized by local time, certain features emerge. In particular, there is a dip in the spectra of all ions at 5–20 keV/e in the dawn‐to‐noon sector, while in the noon‐to‐dusk sector the proton phase space density drops off sharply below ∼5 keV. We have compared these spectra with those predicted by a model of ion drift and loss in the magnetosphere. The model calculates the drift paths in a Volland‐Stern electric field and dipole magnetic field and determines the losses due to charge exchange and strong pitch angle diffusion along the paths. We find that the spectra are most consistent with a Volland‐Stern electric field with γ = 2 and with a rotation of the nominal dawn‐to‐dusk electric field eastward by 2 hours local time. Charge exchange is found to be the dominant loss process during the main phase of the storm, producing qualitative agreement with the observed spectra for all species. There are some quantitative disagreements, particularly in the prenoon sector, which may be explained either by an additional loss process or by a modified drift model.

Observations with the University of Maryland (UMD) / Max-Planck-Institut für Aeronomie (MPAE) charge-energy-mass spectrometer (CHEM) onboard the AMPTE/CCE spacecraft were used to investigate variations of the ion distribution during substorm-associated dropouts of the particle intensity. The dropouts occurred on the nightside at 7 ≤ L ≤ 9 and were characterized by significantly different changes of the energy spectra of H+, He++, and O+ ions. It is suggested that locally induced transient electric fields play an important role in the dropout generation processes.

Data obtained by the AMPTE/CCE charge-energy-mass spectrometer are used to study the average spatial distributions of oxygen and carbon ions with charge states between 3 and 6. The O(6+) and C(6+) ion fluxes are found to increase with the drift shell parameter L up to a constant level at L of not less than 7. It is suggested that the diurnal variations noted are related to the shape of the L profiles. The results support a model in which the solar wind origin O(6+) and C(6+) ions and the terrestrial origin O(+) and O(2+) ions are transported from the tail towards the earth. Charge exchange processes near the earth produce the oxygen and carbon ions with charge states between 3 and 5.

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 largest geomagnetic storm in recent decades began with a sudden commencement on February 6, 1986, developed slowly over the next two days, and, with a rapid intensification late on February 8, reached a minimum Dst of −312 nT during the first hour of February 9. Initial recovery was rapid, but full recovery took more than a month. In this paper we follow the ring current development during the storm using particle measurements from the charge‐energy‐mass (CHEM) instrument on the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE spacecraft. We compare the energy content of the ring current ions with that expected from observed Dst values utilizing for the first time composition coverage over nearly the complete ring current energy range (1–310 keV/e). The ring current composition is followed for five days from prestorm quiet time to early recovery phase. Ions of both solar wind and ionospheric origin are important constituents of the storm time ring current. Although H+ carries the majority of the energy during most of the storm, O+ dominates near the storm's maximum phase, with 47% of the energy density compared with 36% in H+. This is in contrast with all of the more moderate storms which occurred during 1984–1985 in which H+ ions contained most of the energy density near storm maximum. The very rapid initial Dst recovery (τ ∼ 9.3 hours) in this storm results largely from the rapid loss of 75‐ to 100‐keV O+ via charge exchange in the inner portion of the ring current (L = 2.5–3.0). Since it has been long observed that initial Dst recovery is much more rapid in great storms than in moderate storms, we suggest that a major (>50%) O+ + N+ ring current component generally exists near the maximum phase of great storms.

The implanted ion spectrometer (IIS) is a dedicated instrument for the analysis of ionised cometary particles and their interaction with the solar wind. The particle identification principle used in IIS is based on ion deflection in an electrostatic analyser in combination with time-of-flight techniques. The constructed particle identifier function considers the ionic charge as a known quantity, since ionisation by solar UV radiation is the dominant process. The instrument uses novel principles to accommodate a low-power/low-weight state-of-the-art spectrometer. A multisensor system with miniature time-of-flight detectors together with the spacecraft spin motion provide a uniform, though not contiguous, angular coverage of the unit sphere. New detector multiplexing techniques permit the operation of IIS with maximum sensitivity without sacrificing time or angular resolution because of duty cycle effects. IIS is an integral but physically separate part of the solar wind ion analyser JPA in the Giotto payload. The sensor system and the operation of IIS are described. The flight performance is illustrated by observations in the undisturbed solar wind and near the comet.

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.

Investigations of the average spatial distributions of the ionospheric ions O+ and O2+ and of the solar wind ions O6+ and C6+ provide a possibility to estimate the relative contributions of the ionosphere and the solar wind to the energetic ion population in the magnetosphere. We used measurements with the charge energy mass spectrometer (CHEM) on board the AMPTE CCE spacecraft to determine the relative fluxes of these ions near the equatorial plane as a function of the drift shell parameter L, the magnetic activity index Kp, and the local time LT. The O+ and O2+ ions have radial profiles with maxima at about L = 5 and diurnal variations with a broad maximum on the dayside. The O6+ and C6+ ion fluxes increase with L between L = 5 and L = 7 and level off farther out. The diurnal variations of the relative O6+ and C6+ fluxes exhibit a pronounced minimum on the dayside. The observations can be interpreted in terms of ion convection from the tail onto quasi‐trapped drift orbits and further radial diffusion onto closed drift shells. The maximum of the O+ and O2+ fluxes as well as the minimum of the O6+ and C6+ fluxes on the dayside can be explained by drift shell splitting. The inward transport is associated with ion losses, mainly by charge exchange. O6+ and C6+ ions are removed already at larger distances than O+ and O2+. As a result, the ionosphere contributes most of the oxygen ions in the magnetosphere. Important contributions from the solar wind are encountered in the outer magnetosphere (L > 7).

The interaction of comets with the solar wind depends on the ionization of the heavy cometary neutrals (mostly H2O and its dissociation products O, OH) which flow out from the nucleus, and the coupling of these newly produced cometary ions with the solar wind through its embedded magnetic field. The 'pick-up' of these heavy ions slows the solar wind such that a shock may form. As expected, the structure of such a shock transition is highly complex because the gyroradius of a heavy cometary ion is much larger than that of a solar wind proton. Here we present a comparative study of the solar wind electrons and protons and the cometary pick-up ions measured by Giotto at the inbound crossing of the bow shock at comet Halley. We find a highly structured shock transition starting with a cometary ion 'foot' seen at a distance on the order of the cometary ion gyroradius upstream from a sharp decrease in the solar wind proton speed. The total solar wind thermal and magnetic pressure is dominated by the relatively small population of cometary ions throughout the shock region.

The Charge‐Energy‐Mass Spectrometer CHEM on the AMPTE/CCE spacecraft provides measurements of oxygen ions with charge states between 1 and 6 in the magnetosphere close to the equatorial plane. The first investigations of the spatial distribution of O 3+ , O 4+ , and O 5+ ions revealed that these ions are continuously present in the magnetosphere. Their L profiles exhibit broad maxima that are located at 6 ⩽ L ⩽ 7 for O 3+ and O 4+ and at 7 ⩽ L ⩽ 8 for O 5+ ions during periods of low to medium geomagnetic activity (Kp < 4). With increasing Kp the flux versus L distributions extend further earthward and the maxima move to slightly lower L values. While the flux decrease earthward of the maximum is very steep, the gradual decrease outward of the maximum becomes flatter with increasing charge state. Comparison of these profiles to those of O + , O 2+ , and O 6+ ions suggests that O 3+ , O 4+ , and O 5+ ions are produced predominantly by charge exchange processes, and that the O 4+ and O 5+ ions are produced from the solar wind origin O 6+ . Our measurements of O 3+ indicate that both the solar wind O 6+ and the terrestrial ions O + and O 2+ contribute to the production of this ion species.

Gaseous material expanding form the nucleus of comet Halley into space form the neutral coma around the comet. Ionisation in the solar UV radiation removes particles from the coma and injects them into the solar wind plasma. These freshly created ions are accelerated by the interplanetary electric field on cycloidal trajectories with gyrocenters moving with the speed of the magnetic field lines. In the solar wind frame of reference these particles move along the magnetic field lines with a fixed pitchangle. Pitchangle scattering and energy diffusion reduce quickly the initial energy anisotropy which is associated with the narrow pick-up structures. First observations of heavy cometary pick-up ions (water group ions) at the bowshock are presented. The evolution of the distribution function in the vicinity of the shock and radial density profiles are discussed.

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.

The global magnetospheric compression on 13 July 1982 following the arrival of an interplanetary shock was observed by a number of spacecraft. Energetic electron and ion data as well as magnetometer measurements on three geostationary spacecraft operating on the frontside of the magnetosphere are used to obtain information about the shape of the magnetopause during times of enhanced solar wind dynamic pressure. The deviation from the average shape of the uncompressed magnetopause to a more circular shape found occasionally during this event is discussed in terms of the unusually high thermal pressure in addition to the enhanced kinetic pressure of the streaming solar wind plasma. Unidirectional distributions of energetic (up to ∼300 keV) ions found at the magnetopause outside the last closed field line for a period of more than 1 h are interpreted as field aligned streaming of ions indicative of the depletion of newly reconnected field lines at the frontside of the magnetosphere.