Characteristics of hot plasma in the Jovian magnetosphere: Results from the Voyager spacecraft

The low-energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, angular distributions, and composition of ions (30 keV ≲E ≲150 MeV) and electrons (14 keV ≲E ≲10 MeV) during encounters with the Jovian magnetosphere in 1979. Detailed analysis of the multicomponent (H, He, O, S) low-energy (∼30 keV to ∼4 MeV) ion population reveals the Jovian environment to be dominated by magnetospheric ions to distances ≳200 RJ upstream and ≳350 RJ downstream from the planet. Inside the magnetosphere, ions move generally in the sense of corotation to the dayside magnetopause, and on the nightside to distances of ∼130–150 Rj, beyond this distance, but inside the magnetopause, ion flow abruptly changes to an antisunward, anti-Jupiter direction and continues to large (>350 RJ) radial distances outside the magnetosphere. The ion particle spectrum is characterized by a nonthermal power law (E−γ) component for E ≳200 keV, and a convected Maxwellian for E ≲200 with characteristic temperatures (kT) of ∼20-45 keV. Temperature maxima generally coincide with crossings of the Jovian plasma sheet, while at higher energies spectra become softer at the equator. The ion spectra and composition are affected strongly by convective flows in all parts of the magnetosphere. By using the observed spectra and angular distributions, density and pressure profiles are produced for ions measured above the lowest LECP detector threshold (E ≳30 keV) and are compared with reported ambient total electron densities and magnetic field pressures. The particle pressures are found to be comparable to magnetic field pressures to at least ∼10 RJ, i.e., Jovian magnetosphere dynamics are determined by pressure variations in a high β plasma. Energetic ion densities are found to be comparable with the total electron densities in the outer (≳40 RJ) dayside magnetosphere but are generally lower at smaller radial distances and exhibit substantial variability. We interpret the hot plasma outflow on the nightside of Jupiter as a ‘magnetospheric wind' and estimate the mass and energy loss through this region at ∼2 × 1027 ions/s and ∼2 × 1020 ergs/s, respectively. We find the plasma source from the active volcanoes on Io to be adequate for supplying the mass outflow; only the rotational kinetic energy of Jupiter is sufficient to provide the energy in the flow, although energy from the solar wind interaction can perhaps contribute a significant fraction. A phenomenological model of Jupiter's magnetosphere is presented which accounts for the observations.

[1]  C. Kennel,et al.  Possible origins of time variability in Jupiter's outer magnetosphere, 2. Variations in solar wind magnetic field , 1977 .

[2]  G. Gloeckler,et al.  Time-of-flight technique for particle identification at energies from 2–400 keV/nucleon , 1979 .

[3]  M. Kivelson,et al.  High β plasma in the dynamic Jovian current sheet , 1978 .

[4]  J. Carbary.  Periodicities in the Jovian magnetosphere: Magnetodisc models after Voyager , 1980 .

[5]  W. I. Axford,et al.  Low-Energy Charged Particle Environment at Jupiter: A First Look , 1979, Science.

[6]  H. Bridge,et al.  The low energy plasma in the Jovian magnetosphere , 1980 .

[7]  J. Blamont,et al.  Extreme Ultraviolet Observations from Voyager 1 Encounter with Jupiter , 1979, Science.

[8]  Norbert Sckopke,et al.  A general relation between the energy of trapped particles and the disturbance field near the Earth , 1966 .

[9]  W. Axford.  On the Origin of Radiation Belt and Auroral Primary Ions , 1970 .

[10]  P. Sturrock,et al.  Centrifugal instability of the jovian magnetosphere and its interaction with the solar wind , 1974 .

[11]  George Gloeckler,et al.  Compositions of energetic particle populations in interplanetary space , 1979 .

[12]  W. I. Axford,et al.  Ion anisotropies in the outer Jovian magnetosphere , 1981 .

[13]  S. Krimigis,et al.  Intensity variations in plasma flow at the dawn magnetopause , 1980 .

[14]  C. Kennel,et al.  Possible origins of time variability in Jupiter's outer magnetosphere, 1. Variations in solar wind dynamic pressure , 1977 .

[15]  A. W. Schardt,et al.  Energetic protons in the Jovian magnetosphere. [0. 2 to 20 MeV] , 1979 .

[16]  R. Hoffman,et al.  Ring current particle distributions during the magnetic storms of December 16–18, 1971 , 1973 .

[17]  G. Siscoe,et al.  Sunward flow in Jupiter's magnetosheath , 1980 .

[18]  S. Collins,et al.  Discovery of Currently Active Extraterrestrial Volcanism , 1979, Science.

[19]  S. Krimigis,et al.  Composition of nonthermal ions in the Jovian magnetosphere , 1981 .

[20]  Donald J. Williams.  Ring Current Composition and Sources , 1979 .

[21]  G. Ioannidis,et al.  The magnetospheres of Jupiter and Earth , 1970 .

[22]  W. Axford.  An Analytic Model Illustrating the Effects of Rotation on a Magnetosphere , 1976 .

[23]  N. Gehrels,et al.  Voyager 1: Energetic Ions and Electrons in the Jovian Magnetosphere , 1979, Science.

[24]  D. Gurnett,et al.  Plasma Wave Observations Near Jupiter: Initial Results from Voyager 2 , 1979, Science.

[25]  W. I. Axford,et al.  Hot Plasma Environment at Jupiter: Voyager 2 Results , 1979, Science.

[26]  W. I. Axford,et al.  Measurements of plasma flow at the dawn magnetopause by Voyager 1 , 1979 .

[27]  D. Gurnett,et al.  Measurements of plasma wave spectra in Jupiter's magnetosphere , 1981 .

[28]  T. Hill,et al.  High‐order magnetic multipoles as a source of gross asymmetry in the distant Jovian magnetosphere , 1975 .

[29]  S. Atreya,et al.  Model ionospheres of Jupiter , 1976 .

[30]  S. Krimigis,et al.  Statics of the nightside Jovian plasma sheet , 1980 .

[31]  E. Roelof,et al.  Fluxes of =50‐keV protons and =30‐keV electrons at ∼35 RE , 1. Velocity anisotropies and plasma flow in the magnetotail , 1976 .

[32]  A. Cheng,et al.  Effects of Io's volcanos on the plasma torus and Jupiter's magnetosphere , 1980 .

[33]  M. Acuna,et al.  Magnetic Field Studies at Jupiter by Voyager 2: Preliminary Results , 1979, Science.

[34]  R. Brown.  Heavy ions in Jupiter's environment , 1981 .

[35]  N. Gehrels,et al.  Voyager 2: Energetic Ions and Electrons in the Jovian Magnetosphere , 1979, Science.

[36]  S. Krimigis,et al.  Ion and electron angular distributions in the Io torus region of the Jovian magnetosphere , 1981 .

[37]  M. Acuna,et al.  Magnetic Field Studies at Jupiter by Voyager 1: Preliminary Results , 1979, Science.

[38]  D. Gurnett,et al.  Determination of Jupiter's electron density profile from plasma wave observations , 1981 .

[39]  W. I. Axford,et al.  Energetic (∼ 100‐keV) tailward‐directed ion beam outside the Jovian plasma boundary , 1980 .

[40]  W. I. Axford,et al.  A weak interaction model for Io and the jovian magnetosphere , 1980, Nature.

[41]  S. Krimigis,et al.  Energetic particle events (≥30 keV) of Jovian origin observed by Voyager 1 and 2 in interplanetary space , 1981 .

[42]  W. I. Axford,et al.  The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft , 1977 .

[43]  E. Smith,et al.  Jupiter's magnetic field and magnetosphere , 1976 .

[44]  D. Shemansky.  Radiative cooling efficiencies and predicted spectra of species of the IO plasma torus , 1980 .

[45]  W. I. Axford,et al.  Detection of energetic hydrogen molecules in Jupiter's magnetosphere by Voyager 2: Evidence for an ionospheric plasma source , 1980 .

[46]  E. Roelof,et al.  Energetic particles in the Jovian magnetosphere , 1974 .

[47]  F. Ipavich.  The Compton-Getting Effect for low energy particles , 1974 .

[48]  W. Fillius.  The trapped radiation belts of Jupiter , 1976 .

[49]  D. Gurnett,et al.  Jupiter Plasma Wave Observations: An Initial Voyager 1 Overview , 1979, Science.

[50]  F. Mcdonald,et al.  Observations of energetic Jovian electrons and protons. , 1976 .

[51]  Travis W. Hill,et al.  Configuration of the Jovian magnetosphere , 1974 .

[52]  S. Krimigis,et al.  Low‐energy charged particle observations in the 5–20 RJ region of the Jovian magnetosphere , 1981 .

[53]  D. Gurnett,et al.  The structure of the Jovian magnetotail from plasma wave observations , 1980 .

[54]  R. Brown.  The Jupiter hot plasma torus - Observed electron temperature and energy flows , 1981 .

[55]  R. McNutt,et al.  Plasma Observations Near Jupiter: Initial Results from Voyager 1 , 1979, Science.