E. Kirsch

The solar wind and suprathermal ion composition investigation on the wind spacecraft

The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms−1 (protons) to 1280 kms−1 (Fe+8), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He+, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.

Direct observation of lunar pick-up ions near the Moon

Measurements made with the Suprathermal Ion Spectrometer (STICS) on the WIND spacecraft during 17 lunar fly-bys which took place between 1995 and 1997 allow the direct observations of lunar pick-up ions (PUIs) on the earthwards side of the Moon as close as 17 lunar radii. The composition measurements reveal an O+, an Al+, Si+ and possibly a P+ component.

First identification in energetic particles of characteristic plasma boundaries at Mars and an account of various energetic particle populations close to the planet

Abstract Signatures of characteristic boundaries, interpreted to be the bow shock and magnetopause with, between them, the magnetosheath, have been recorded for the first time in energetic particles ( 3.2 MeV) in the downstream nightside Martian environment by the SLED instrument aboard Phobos 2. Also, energetic particles, interpreted to be oxygen ions, have been recorded by SLED at four distinct locations close to Mars. These include (a) anisotropic fluxes at the terminator shocks with energies of up to at least 72 keV; (b) anisotropic fluxes with energies of up to at least 225 keV inside the magnetopause at a height above the planet of approximately 900 km in the subsolar part of the magnetosphere; (c) fluxes with energies of up to at least 3.2 MeV in the flanks of the magnetosheath displaying quasiperiodic variations (period approximately 45 min) which are synchronous across the recorded energy spectrum and correlated in time with changes in the local magnetic field; (d) beams of oxygen ions with energies of up to at least 55 keV travelling out along open field lines in the magnetotail with, in some cases, a suggestion of confinement close to the neutral sheet. A preliminary discussion is provided concerning the energization of the various populations of particles identified.

Energetic particle studies at Mars by SLED on Phobos 2

A preliminary overview of particle records obtained by the SLED instrument on Phobos 2, February–March, 1989 during Mars encounter, is presented. Data obtained while in close elliptical orbit around the planet (pericenter < 900 km), in both spin and three axis stabilised mode, display evidence of energy related particle shadowing by the body of Mars. This effect was also observed, under favourable conditions, in certain circular orbits (altitude 6330 km above the planet). Flux enhancements, inside the magnetopause, in the approximate range 30–350 keV, recorded in the same general location at < 900 km above Mars over an 8 day period during three consecutive elliptical orbits, are described. Possible explanations of these enhancements include the presence of quasi-trapped radiation at the planet and the detection of the propagation of accelerated particles along the boundary of the magnetopause from the day to the night side of Mars. Large anisotropic ion flux increases (1–1.5 orders of magnitude) in the approximate range 30–200 keV recorded in front of the bow shock (inbound and outbound) during certain circular orbits, provide evidence that the spacecraft traversed strongly anisotopic jets of energetic particles. These are suggested to have constituted O+ ions. The pickup process would have been sufficient to accelerate such ions to their observed energies in the prevailing solar wind conditions. Alternatively, they might have comprised particles that had leaked from inside the magnetopause, perhaps undergoing shock drift acceleration in the process. Significant flux enhancements were also sometimes identified in the magnetotail (approximate energy range 30–50 keV). These are suggested to represent the signatures of O+ beams, impelled by acceleration processes similar to those associated with terrestrial ion beams.

An overview of energetic particles (from 55 keV to > 30 MeV) recorded in the close Martian environment, and their energization in local and external processes

Abstract Observations made by the SLED particle detector on Phobos-2 in the close Martian environment from 29 January to 27 March, 1989 during the early rising phase of Solar Cycle 22, show the frequent presence close to the planet, under reasonably “quiet” interplanetary conditions, of particles with energies ( E ) in the range from several tens of, to several hundred, keV. Under disturbed interplanetary circumstances, particles reaching energies of several tens of MeV were recorded close to Mars. Those particles in the keV range were observed at well-defined locations, i.e. at the Terminator Shocks ( E up to ≈600 keV); just inside the subsolar Planetopause ( E up to ≈225 keV), and travelling down the Tail, E ≥55 keV. These three populations are herein suggested, instancing various candidate mechanisms, to have been energized by processes local to the planet. Since the seed particles for ions accelerated at the Terminator Shocks may comprise ambient, pre-accelerated, solar particles, the energies of ions detected by SLED during Bow Shock transits was observed (during two months) to vary between ≈50 keV and ≈600 keV. Particles with energies up to several tens of MeV which were found to suffuse the close planetary environment over extended periods, are interpreted to have been produced in association with solar processes external to Mars (Co-rotating Interaction Regions; Gradual and Impulsive Solar Events). Particle enhancements in the keV range recorded by SLED (under favourable magnetic conditions) during Bow Shock traversals, provide topographical information concerning the location of the Martian subsolar and distant shock surfaces. These observations constitute a new data set, complementary to those determinations of key boundaries derived from plasma and magnetic field measurements made aboard various American and Russian spacecraft at Mars which, for more than thirty years now, have been generally used in modelling the Solar Wind interaction with the planet. Three-dimensional measurements made at low altitudes over long dwell times are presently required to provide further insights into those local processes whereby populations of keV particles discovered in SLED data, close to the inbound Planetopause and travelling down the Tail, are individually energized.

Cassini as a heliospheric probe - the potential of pick-up ion measurements during its cruise phase

Abstract Measurements of the spatial distributions of pick-up ions can be utilized to infer important information about the structure of the heliosphere. First, a determination of the symmetry axis of flux distributions of pick-up ions not experiencing any significant filtration of their neutral parent atoms in the heliospheric interface represents a new method to derive the orientation of the upwind-downwind axis of the heliosphere from observation. Second, a comparison of pick-up ion flux distributions resulting from neutrals both filtered and not filtered during their passage through the heliospheric interface might provide insight into the geometry of the interface and, subsequently, give information about the strength and orientation of the local interstellar magnetic field. A feasibility study for carrying out such measurements with the Cassini spacecraft is presented, and the most promising pick-up ion candidates are identified.

Characteristic boundary transitions in energetic particle data (60– ≥ 260 keV) recorded at comets P/Grigg-Skjellerup and P/Halley by the EPONA instrument on Giotto

Abstract An overview of published magnetic field, low energy electron and low to intermediate energy ion data recorded on board Giotto during an encounter with P/Grigg-Skjellerup (G-S) on 10 July, 1992 is presented and these measurements compared with particle data secured by the onboard energetic particles experiment EPONA ( E = 60 to ≥260 keV). It is shown that, in general, the same cometary boundaries (Bow Wave, Bow Shock, Mystery Boundary Transitions (MBTs)) were sensed in energetic particles as by the other experiments. Elevated fluxes of accelerated particles, interpreted to be ions of species M = 16–18, were identified in the G-S cometosheath. It is inferred that heavy cometary ions of species M = 28–33 were present in the innermost region of the comet (as viewed in the sunward direction). Energetic particles recorded by EPONA on Giotto at P/Halley are considered relative to other particles and fields measurements obtained onboard. Evidence based on pitch angle data indicates that, inbound, the MBTs at G-S and at Halley, were spatially associated with changes from trapped to “flowing” distributions of water group ions, suggesting that mirroring of these particles was taking place during encounter in the magnetic field piled up around each comet. Downstream of the MBTs, outbound, flowing distributions were, in each case, identified. Evidence of MBTs in energetic particle data are, herein, reported for the first time. The roles of various mechanisms in accelerating cometary ions in association with Bow Wave/Bow Shock boundaries at G-S and at Halley are inferred from comparisons between experimental and theoretical spectra pertinent to each comet. Similarities and differences between the signatures of characteristic boundaries recorded in each case are discussed in the light of basic differences between the comets themselves, and between the ambient interplanetary circumstances pertaining during each cometary encounter.

Comparison of lunar and terrestrial ion measurements obtained by the WIND and GEOTAIL spacecraft outside and inside the Earth's magnetosphere

Abstract The WIND spacecraft, launched on November 1, 1994, undergoes prior to its halo orbit insertion several lunar swing-by maneuvers. The GEOTAIL spacecraft, skimming around the dayside magnetopause (Dec. 1994), passes also several times behind the moon. The HEP-LD spectrometer onboard of GEOTAIL and the Solar Wind and Suprathermal Ion Composition (SMS) Experiment onboard of the WIND spacecraft are well positioned for observing ions in the mass range between II and Fe in the direct vicinity of the moon. We report first observations of lunar pickup-ions in the magnetotail at a distance of ≈ 80 R e .

Overview of recent analysis of the energetic particle observations recorded in situ by the EPONA instrument on the Giotto mission to Comet Halley

Abstract Highlights of recent results obtained through analyzing data recorded by the energetic particles experiment EPONA on the Giotto Mission to Comet Halley (March 12–15, 1986) from ∼7 × 10 6 km upstream of the bow shock (inbound) to ∼6 × 10 6 km upstream of the bow shock (outbound) are presented. The identities of the ions recorded, which displayed energies in excess of those attributable to the pickup process acting alone, are discussed and evidence is presented to indicate that these ions were of the water group. Indications of the local influence of various acceleration processes in energizing particles along different sections of the Giotto Encounter trajectory are provided. Spin modulation of the outgassing rate of the nucleus may have been responsible for quasiperiodic variations in the flux levels observed inbound and outbound. A dramatic burst of energetic particles deemed to be of cometary origin, recorded while the spacecraft traversed the magnetic pileup region and the magnetic cavity, is an unexpected feature of the in situ observations, uniquely recorded by the EPONA instrument.

Energetic ions upstream (7.5 × 106 km) of comet Halley—What are they and how did they get there?

The EPONA experiment on board the Giotto spacecraft measures high intensities of ions out to distances of 7.5 Gm (≡ 7.5 × 106 km) from comet Halley. Although it is not possible to determine directly the mass of these ions, the most likely candidates from consideration of the energetics of the pick-up process are water group ions, H2O+, OH+, and O+. This is confirmed indirectly by spectral analysis, which is more consistent with a mass of the order of 16 than 1 amu. A difficulty with this identication is that the parent neutral atoms are expected to have speeds of about 1 km s−1 relative to the comet nucleus and an ionization time constant of about 106 s. This would mean that the ion intensity should fall off more sharply with distance than is actually observed. This problem is overcome by re-doing the calculation with Kepler orbits for the neutral particle trajectories, and by showing that the resultant theoretical distance profile for water group ions matches in general terms that of the measured data.