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Measurements of the hot (electron and ion energies >/=20 and >/= 28 kiloelectron volts, respectively) plasma environment at Jupiter by the low-energy charged particle (LECP) instrument on Voyager 2 have revealed several new and unusual aspects of the Jovian magnetosphere. The magnetosphere is populated from its outer edge into a distance of at least approximately 30 Jupiter radii (R(J)) by a hot (3 x 10(8) to 5 x 10(8) K) multicomponent plasma consisting primarily of hydrogen, oxygen, and sulfur ions. Outside approximately 30 R(J) the hot plasma exhibits ion densities from approximately 10(-1) to approximately 10(-6) per cubic centimeter and energy densities from approximately 10(-8) to 10(-13) erg per cubic centimeter, suggesting a high beta plasma throughout the region. The plasma is flowing in the corotation direction to the edge of the magnetosphere on the dayside, where it is confined by solar wind pressure, and to a distance of approximately 140 to 160 R(J) on the nightside at approximately 0300 local time. Beyond approximately 150 R(J) the hot plasma flow changes into a "magnetospheric wind" blowing away from Jupiter at an angle of approximately 20 degrees west of the sun-Jupiter line, characterized by a temperature of approximately 3 x 10(8) K (26 kiloelectron volts), velocities ranging from approximately 300 to > 1000 kilometers per second, and composition similar to that observed in the inner magnetosphere. The radial profiles of the ratios of oxygen to helium and sulfur to helium (

The low-energy charged particle instrument on Voyager was designed to measure the hot plasma (electron and ion energies greater, similar 15 and greater, similar 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 10(8) 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 ( greater, similar200 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 approximately 90 degrees to approximately 130 degrees 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.

Carbon, oxygen, and heavier nuclei have been observed below 8 MeV per nucleon during solar quiet times. We find that the C/O abundance ratio is 0.8±0.4, the N/O ratio is 0.4±0.25, and the differential energy spectra below 1 MeV per nucleon have the form KE~A* d±0 ' 3 . We infer from this spectral form that most of these particles are likely to be of solar origin. The large errors on the abundance ratio do not allow a decisive answer to the likely origin. Although extensive satellite and balloon measurements 1 " 5 have provided abundant information over most of the present solar cycle about the modulated galactic cosmic rays above roughly 15 MeV per nucleon, the extension of observations during solar quiet times below this energy has been limited to protons and a particles. 6 ' 7 The continuous presence of low-energy protons and helium nuclei during solar quiet times was first established by Fan and co-workers 6 ' 8 who also found unexpected upturns in the spectra below about 20 MeV and surprisingly little variation in the flux or spectral shapes during quiet periods from 1964 to 1967. Kinsey's 7 careful analysis of data from the Goddard dE/dxversus-£ experiments on IMP-3 and IMP-4, while revealing a continuous presence of protons and a particles down to 5 MeV per nucleon, showed substantial variability of these low-energy components over the period from May 1967 to August 1968. He concluded that during most of the time period particles below about 10 MeV per nucleon were substantially of solar origin.In this Letter we present for the first time measurements of low-energy (<8 MeV per nucleon) heavy cosmic rays in interplanetary space during relatively quiet time periods. The data were obtained in October 1972 using a newly designed ultralow energy telescope (ULET) on board the IMP-7 (Explorer 47) satellite which was launched on 22 September 1972 into a nearly circular, 240 000-km apogee orbit. ULET operated successfully until 19 November 1972 when the thin window of the proportional counter ruptured. Because of this only a limited amount of quiet-time data could be collected. In this Letter we restrict ourselves to particles with a nuclear charge Z ^ 6.The detector system 9 makes use of the dE/dxversus-£ method for particle identification and energy determination. To extend to 200 keV per nucleon the energy range over which two-parameter analysis can be made, we use a thin-window proportional counter (D x ) for the "dE/dx" measurement and a conventional, fully depleted, 700jum-thick surface-barrier silicon detector (D 2 ) for the "E" determination. A plastic scintillator cylindrical cup anticoincidence detector S, which surrounds D 1 and D 2 , is used to reject background and penetrating particles. The total thickness of material in front of the solid-state detector is 330 /ug/cm 2 , 140 /ig/cm 2 of which is due to the isobutane counter gas. The geometrical factor of the telescope is 1.0 cm 2 sr. To obtain preferential an-650