E. P. Keath

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 Medium-Energy Particle Analyzer (MEPA) on the AMPTE CCE Spacecraft

The medium-energy particle analyzer (MEPA) was designed to measure the spectra and composition of magnetospheric particle populations from 10 keV per nucleon (for oxygen) to more than 6 MeV The instrument provides for high background rejection and a geometry tactor large enougn (10-2 cm2 * sr) to be sensitive to rare natural species and tracer ions beyond geosynchronous orbit, while having the ability to operate without saturation in the very high flux regions of the inner magnetosphere. The MEPA telescope measures time of flight and thus velocity of energetic ions from a thin front foil to a rear solid-state total-energy detector, determining the incident ion mass. The telescope is capable of isotopic resolution of hydrogen and helium, of elemental resolution up through oxygen, and can resolve major species and groups to beyond iron with 32-sector angular resolution and temporal resolution of 0.2-24 s. MEPA represents a new and extremely promising technology for space-particle instrumentation in that it has the capability of measuring low-energy heavy ions with high efficiency while discriminating against high natural fluxes of protons.

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.

Hot ions in Jupiter's magnetodisc : a model for Voyager 2 Low-Energy Charged Particle measurements

The Low-Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy-dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two-species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest-energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near-Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton-Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower-energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.

Hot plasma parameters of Jupiter's inner magnetosphere

The bulk parameters of the hot (>20 keV) plasmas of Jupiters inner magnetosphere, including the vicinity of the Io plasma torus, are presented for the first time (L = 5 to 20 RJ). The low-energy charged particle (LECP) instrument on Voyager 1 that obtained the data presented here was severely overdriven within the inner regions of Jupiters magnetosphere. On the basis of laboratory calibrations using a flight spare instrument, a Monte Carlo computer algorithm has been constructed that simulates the response of the LECP instrument to very high particle intensities. This algorithm has allowed for the extraction of the hot plasma parameters in the Jovian regions of interest. The hot plasma components discussed here dominate over other components with respect to such high-order moments as the plasma pressures and energy intensities. Our findings include the following items. (1) Radial pressure gradients change from positive (antiplanetward) to negative as one moves outward past about 7.3 RJ. While the observed hot plasma distributions will impede the radial transport, via centrifugal interchange, of iogenic plasmas throughout the Io plasma torus regions out to 8 RJ, the plasma impoundment concept of Siscoe et al. [1981] for explaining the so-called “ramp” in the flux shell content profile of iogenic plasmas (7.4–7.8 RJ [Bagenal, 1994]) is not supported. (2) We predict a radial ordering for the generation of the aurora, which translates into a latitudinal structure for auroral emissions. Planetward of about 12 RJ, intense aurora (10 ergs/(cm2 s) precipitation) can only be caused by ion precipitation, whereas outside of about 12 RJ such intense aurora can only be caused by electron precipitation. Uncertainties concerning the causes of Jovian aurora may stem in part from failures of some observations to resolve the latitudinal structure that is anticipated here and possibly from changes in the auroral configuration and/or charged particle spectral properties since the Voyager epoch.

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.

The magnetosphere of Neptune - Hot plasmas and energetic particles

A comprehensive overview is provided of the hot plasmas and energetic particles (≳keV) observed in the vicinity of Neptune by the low energy charged particle (LECP) experiment on the Voyager 2 spacecraft. The LECP data are ordered with respect to magnetic field data and models derived from the Voyager magnetometer experiment. The findings include the following: (1) Weakly enhanced ion and electron fluxes were observed at the position of the subsolar bow shock. (2) Magnetic-field-aligned, antiplanetward streaming ions and electrons were sporadically observed within the inbound (subsolar) and outbound (tail flank) magnetosheaths, and within the unique “pole-on” cusp region encountered during the inbound trajectory. (3) Tangential ion streaming was observed at the positions of both the inbound (dawnward streaming) and outbound (tailward streaming) magnctopauscs. (4) A distinct “trans-Triton” ion population outside the minimumL shell of Triton is characterized by large angular anisotropics that show that heavy ions (presumably N+) are a likely constituent This population is at least partially corotating with Neptune out to at least L = 27 RNand is also characterized at times by cigar-shaped (field-aligned) pitch angle distributions, possibly indicative of an interaction with a neutral torus. (5) Within the middle magnetospheric regions (inside Triton), pitch angle distributions have well-developed trapped or “pancake” shapes. Also, in contrast to Uranus, flux profiles show no evidence of substorm-generated azimuthal asymmetries. (6) Triton (and/or Triton-generated neutral gas) controls the outer bounds of the hot plasmas and energetic particles, although the mechanism of that control is unclear. Also, there are clear charged particle signatures of satellite 1989NI and of ring 1989N3R. However, the large number of calculated criticalLshell positions associated with all of the rings and satellites renders impractical at this time the unique determination of causal relationships between the many observed particle signatures and known material bodies. (7) Concerning the bulk (integral) and spectral parameters of the not plasmas, if it is assumed that the trans-Triton population is dominated by N+, the plasma β parameter reaches ∼1 within the near-planet magnetotail (L ∼28 RN; in conjunction with a magnetic field depression “tail event”), having only reached ∼0.2 in the more planetward regions. Integral electron energy intensities are such that the more localized Neptune UV aurora can be explained if loss cone intensities are ≲1% of trapped intensities. In contrast to the Uranian magnetosphere, the lower-energy electron distributions appear generally to be at least as well characterized by hot Maxwellian distributions (kT = 10 to 30 keV) as by power law distributions inside L ∼20 RN, a characteristic generally exhibited at the other planets by the ions. At Neptune the ions have kT = 12 to 100 keV, and kTis strongly correlated with position relative to Tritons L shell. (8) Within the Neptunian magnetotail, planetward, magnetic-field-aligned streaming of ions and electrons is observed within the distant (∼67RN) plasma sheet and within a closer region thought to be a detached or striated portion of the plasma sheet population. Within the near-planet magnetotail (L ∼28 RN), where the spacecraft crossed from the plasma sheet to the tail lobes, cigarlike electron distributions are observed, suggestive of shell-splitting/magneto pause-sweeping effects. Consistent with the middle magnetospheric observations, and in sharp contrast to the Uranian magnetotail, the Neptunian magnetotail shows no evidence of substorm processes.

A convected K distribution model for hot ions in the Jovian magnetodisc

Hot ion angular anisotropies measured by the Low Energy Charged Particle (LECP) instrument during the Voyager 2 encounter with the Jovian dayside outer magnetosphere (60–30 RJ) have been fitted to a 2 species convected K distribution function using a non-linear least squares technique. The resulting parameters are well constrained by the data. The heavy ion species was assumed to be either sulfur or oxygen of unknown charge. The light species was assumed to be protons. The bulk flow speeds deduced from the model were found, contrary to some theories, to increase with increasing radial distance from Jupiter within the radial region addressed, remaining a substantial fraction (∼0.6) of the rigid corotation speed. Agreement with the averaged Voyager Plasma Science (PLS) results was obtained near 30 RJ. The core Maxwellian temperature of the heavy ion distribution functions (∼30–100 keV) increased with increasing radial distance, following the trend anticipated from the corotation pickup of heavy ions. The proton temperature (∼20 keV) remained nearly constant.

A nebula of gases from Io surrounding Jupiter

Several planetary missions have reported the presence of substantial numbers of energetic ions and electrons surrounding Jupiter; relativistic electrons are observable up to several astronomical units (au) from the planet. A population of energetic (>30 keV) neutral particles also has been reported, but the instrumentation was not able to determine the mass or charge state of the particles, which were subsequently labelled energetic neutral atoms. Although images showing the presence of the trace element sodium were obtained, the source and identity of the neutral atoms—and their overall significance relative to the loss of charged particles from Jupiters magnetosphere—were unknown. Here we report the discovery by the Cassini spacecraft of a fast (>103 km s-1) and hot magnetospheric neutral wind extending more than 0.5 au from Jupiter, and the presence of energetic neutral atoms (both hot and cold) that have been accelerated by the electric field in the solar wind. We suggest that these atoms originate in volcanic gases from Io, undergo significant evolution through various electromagnetic interactions, escape Jupiters magnetosphere and then populate the environment around the planet. Thus a ‘nebula’ is created that extends outwards over hundreds of jovian radii.

INCA: the ion neutral camera for energetic neutral atom imaging of the Saturnian magnetosphere

Techniques developed for the detection and characterization of energetic (>20 keV) ions in space plasmas have been modified to include imaging so that energetic neutral atoms at Saturn may be used to form images of the Saturnian magnetosphere and its interaction with the atmosphere of the moon Titan. The basic elements of the ion-neutral camera head on the magnetospheric imaging instrument for the Cassini mission are described, with emphasis on developmental detection techniques and components. In particular, pulse-height analysis of the microchannel plate responses to different mass neutrals is used for rough composition determination, and deflection plates in the aperture as well as time-of-flight measurements allow imaging of neutral atoms from within regions of moderate intensity ambient ion and electron fluxes.