W. I. Axford

The plasma environment of Mars

When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.

Dynamical behavior of thermal protons in the mid-latitude ionosphere and magnetosphere

Abstract Satellite and other observations have shown that H+ densities in the mid-latitude topside ionosphere are greatly reduced during magnetic storms when the plasmapause and magnetic field convection move to relatively low L-values. In the recovery phase of the magnetic storm the convection region moves to higher L-values and replenishment of H+ in the empty magnetospheric field tubes begins. The upwards flow of H+, which arises from O+—H charge exchange, is initially supersonic. However, as the field tubes fill with plasma, a shock front moves downwards towards the ionosphere, eventually converting the upwards flow to subsonic speeds. The duration of this supersonic recovery depends strongly on the volume of the field tube; for example calculations indicate that for L = 5 the time is approximately 22 hours. The subsonic flow continues until diffusive equilibrium is reached or a new magnetic storm begins. Calculations of the density and flux profiles expected during the subsonic phase of the recovery show that diffusive equilibrium is still not reached after an elapsed time of 10 days and correspondingly there is still a net loss of plasma from the ionosphere to the magnetosphere at that time. This slow recovery of the H+ density and flux patterns, following magnetic storms, indicates that the mid-latitude topside ionosphere may be in a continual dynamic state if the storms occur sufficiently often.

THE LOW ENERGY CHARGED PARTICLE (LECP) EXPERIMENT ON THE VOYAGER SPACECRAFT

The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft is designed to provide comprehensive measurements of energetic particles in the Jovian, Saturnian, Uranian and interplanetary environments. These measurements will be used in establishing the morphology of the magnetospheres of Saturn and Uranus, including bow shock, magnetosheath, magnetotail, trapped radiation, and satellite-energetic particle interactions. The experiment consists of two subsystems, the Low Energy Magnetospheric Particle Analyzer (LEMPA) whose design is optimized for magnetospheric measurements, and the Low Energy Particle Telescope (LEPT) whose design is optimized for measurements in the distant magnetosphere and the interplanetary medium. The LEMPA covers the energy range from ∼10 keV to > 11 MeV for electrons and from ∼15 keV to ≳ 150 MeV for protons and heavier ions. The dynamic range is ∼0.1 to ≳ 1011 cm−2 sec−1 sr−1 overall, and extends to 1013 cm−2 sec−1 sr−1 in a current mode operation for some of the sensors. The LEPT covers the range ∼0.05 ≤ E ≳ 40 MeV/nucleon with good energy and species resolution, including separation of isotopes over a smaller energy range. Multi-dE/dx measurements extend the energy and species coverage to 300–500 MeV/nucleon but with reduced energy and species resolution. The LEPT employs a set of solid state detectors ranging in thickness from 2 to ∼2450 μ, and an arrangement of eight rectangular solid state detectors in an anticoincidence cup. Both subsystems are mounted on a stepping platform which rotates through eight angular sectors with rates ranging from 1 revolution per 48 min to 1 revolution per 48 sec. A ‘dome’ arrangement mounted on LEMPA allows acquisition of angular distribution data in the third dimension at low energies. The data system contains sixty-two 24-bit sealers accepting data from 88 separate channels with near 100% duty cycle, a redundant 256-channel pulse height analyzer (PHA), a priority system for selecting unique LEPT events for PHA analysis, a command and control system, and a fully redundant interface with the spacecraft. Other unique features of the LECP include logarithmic amplifiers, particle identifiers, fast (∼15 ns FWHM) pulse circuitry for some subsystems, inflight electronic and source calibration and several possible data modes.

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

Low-energy charged particle environment at Jupiter: A first look

The low-energy charged particle instrument on Voyager was designed to measure the hot plasma (electron and ion energies ≳ 15 and ≳ 30 kiloelectron volts, respectively) component of the Jovian magnetosphere. Protons, heavier ions, and electrons at these energies were detected nearly a third of an astronomical unit before encounter with the planet. The hot plasma near the magnetosphere boundary is predominantly composed of protons, oxygen, and sulfur in comparable proportions and a nonthermal power-law tail; its temperature is about 3 x 108 K, density about 5 x 10–3 per cubic centimeter, and energy density comparable to that of the magnetic field. The plasma appears to be corotating throughout the magnetosphere; no hot plasma outflow, as suggested by planetary wind theories, is observed. The main constituents of the energetic particle population (≳200 kiloelectron volts per nucleon) are protons, helium, oxygen, sulfur, and some sodium observed throughout the outer magnetosphere; it is probable that the sulfur, sodium, and possibly oxygen originate at 1o. Fluxes in the outbound trajectory appear to be enhancedfrom ∼90� to ∼130� longitude (System III). Consistent low-energy particle flux periodicities were not observed on the inbound trajectory; both 5-and 10-hour periodicities were observed on the outbound trajectory. Partial absorption of > 10 million electron volts electrons is observed in the vicinity of the Io flux tube.

The compton-getting effect

The differential current density and anisotropy seen by an observer moving relative to the frame of reference in which a flux of cosmic ray particles or photons is isotropic, is derived assuming that the observers speed is small. The results are applied to examples relevant to the theory of cosmic ray modulation and the expected anisotropies of photons originating outside our galaxy.

The acceleration of particles in the vicinity of comets

Abstract The various processes capable of accelerating ions in the vicinity of comets as part of the comet/ solar wind interaction are considered. It is concluded that the most important are ion pick-up, adiabatic compression and second order Fermi acceleration in the turbulence associated with the interaction. Model particle spectra are calculated on the assumption that the turbulence is dominated by Alfven waves, and it is shown that the peak associated with ion pick-up remains a distinct feature of the spectrum despite spreading to higher and lower energies. Diffusive shock acceleration and magnetic field reconnection are shown to be relatively unimportant in producing energetic ions.

Hot Plasma and Energetic Particles in Neptune's Magnetosphere

The low-energy charged particle (LECP) instrument on Voyager 2 measured within the magnetosphere of Neptune energetic electrons (22 kiloelectron volts ≤ E ≤ 20 megaelectron volts) and ions (28 keV ≤ E ≤ 150 MeV) in several energy channels, including compositional information at higher (≥0.5 MeV per nucleon) energies, using an array of solid-state detectors in various configurations. The results obtained so far may be summarized as follows: (i) A variety of intensity, spectral, and anisotropy features suggest that the satellite Triton is important in controlling the outer regions of the Neptunian magnetosphere. These features include the absence of higher energy (≥150 keV) ions or electrons outside 14.4 RN (where RN = radius of Neptune), a relative peak in the spectral index of low-energy electrons at Tritons radial distance, and a change of the proton spectrum from a power law with γ ≥ 3.8 outside, to a hot Maxwellian (kT [unknown] 55 keV) inside the satellites orbit. (ii) Intensities decrease sharply at all energies near the time of closest approach, the decreases being most extended in time at the highest energies, reminiscent of a spacecrafts traversal of Earths polar regions at low altitudes; simultaneously, several spikes of spectrally soft electrons and protons were seen (power input ≈ 5 x 10-4 ergs cm-2 s-1) suggestive of auroral processes at Neptune. (iii) Composition measurements revealed the presence of H, H2, and He4, with relative abundances of 1300:1:0.1, suggesting a Neptunian ionospheric source for the trapped particle population. (iv) Plasma pressures at E ≥ 28 keV are maximum at the magnetic equator with β ≈ 0.2, suggestive of a relatively empty magnetosphere, similar to that of Uranus. (v) A potential signature of satellite 1989N1 was seen, both inbound and outbound; other possible signatures of the moons and rings are evident in the data but cannot be positively identified in the absence of an accurate magnetic-field model close to the planet. Other results indude the absence of upstream ion increases or energetic neutrals [particle intensity (j) < 2.8 x 10-3 cm-2 s-1 keV-1 near 35 keV, at ∼40 RN] implying an upper limit to the volume-averaged atomic H density at R ≤ 6 RN of ≤ 20 cm-3; and an estimate of the rate of darkening of methane ice at the location of 1989N1 ranging from ∼105 years (1-micrometer depth) to ∼2 x 106 years (10-micrometers depth). Finally, the electron fluxes at the orbit of Triton represent a power input of ∼109 W into its atmosphere, apparently accounting for the observed ultraviolet auroral emission; by contrast, the precipitating electron (>22 keV) input on Neptune is ∼3 x 107 W, surprisingly small when compared to energy input into the atmosphere of Jupiter, Saturn, and Uranus.

The acceleration of minor ion species in the solar wind

This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Suns rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be ‘trapped’ between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.

Low energy hot plasma and particles in Saturn's magnetosphere

The low-energy charged particle instrument on Voyager 2 measured low-energy electrons and ions (energies ≳ 22 and ≳ 28 kiloelectron volts, respectively) in Saturns magnetosphere. The magnetosphere structure and particle population were modified from those observed during the Voyager 1 encounter in November 1980 but in a manner consistent with the same global morphology. Major results include the following. (i) A region containing an extremely hot ( ∼ 30 to 50 kiloelectron volts) plasma was identified and extends from the orbit of Tethys outward past the orbit of Rhea. (ii) The low-energy ion mantle found by Voyager 1 to extend ∼ 7 Saturn radii inside the dayside magnetosphere was again observed on Voyager 2, but it was considerably hotter ( ∼ 30 kiloelectron volts), and there was an indication of a cooler ( < 20 kiloelectron volts) ion mantle on the nightside. (iii) At energies ≳ 200 kiloelectron volts per nucleon, H1, H2, and H3 (molecular hydrogen), helium, carbon, and oxygen are important constituents in the Saturnian magnetosphere. The presence of both H2 and H3 suggests that the Saturnian ionosphere feeds plasma into the magnetosphere, but relative abundances of the energetic helium, carbon, and oxygen ions are consistent with a solar wind origin. (iv) Low-energy ( ∼ 22 to ∼ 60 kiloelectron volts) electron flux enhancements observed between the L shells of Rhea and Tethys by Voyager 2 on the dayside were absent during the Voyager 1 encounter. (v) Persistent asymmetric pitch-angle distributions of electrons of 60 to 200 kiloelectron volts occur in the outer magnetosphere in conjunction with the hot ion plasma torus. (vi) The spacecraft passed within ∼ 1.1� in longitude of the Tethys flux tube outbound and observed it to be empty of energetic ions and electrons; the microsignature of Enceladus inbound was also observed. (vii) There are large fluxes of electrons of ∼ 1.5 million electron volts and smaller fluxes of electrons of ∼ 10 million electron volts and of protons ≳ 54 million electron volts inside the orbits of Enceladus and Mimas; all were sharply peaked perpendicular to the local magnetic field. (viii) In general, observed satellite absorption signatures were not located at positions predicted on the basis of dipole magnetic field models.