C. Y. Fan

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.

Hot Plasma Environment at Jupiter : Voyager 2 Results

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 ∼ 30 Jupiter radii (RJ) by a hot (3 x 108 to 5 x 108 K) multicomponent plasma consisting primarily of hydrogen, oxygen, and sulfur ions. Outside ∼ 30 RJ the hot plasma exhibits ion densities from ∼ 10–1 to ∼ 10–6 per cubic centimeter and energy densities from ∼ 10–8 to 10–13 erg per cubic centimeter, suggesting a high β 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 ∼ 140 to 160 RJ on the nightside at ∼ 0300 local time. Beyond ∼ 150 RJ the hot plasma flow changes into a magnetospheric wind blowing away from Jupiter at an angle of ∼ 20� west of the sun-Jupiter line, characterized by a temperature of ∼ 3 x 108 K (26 kiloelectron volts), velocities ranging from ∼ 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 (≤ 1 million electron volts per nucleon) monotonically increase toward periapsis, while the carbon to helium ratio stays relatively constant; a significant amount of sodium (Na/O ∼ 0.05) has also been identified. The hydrogen to helium ratio ranges from ∼ 20 just outside the magnetosphere to values up to ∼ 300 inside; the modulation of this ratio suggests a discontinuity in the particle population at ∼ 50 to 60 RJ. Large fluctuations in energetic particle intensities were observed on the inbound trajectory as the spacecraft approached Ganymede, some of which suggest the presence of a wake. Five-and 10-hour periodicities were observed in the magnetosphere. Calculations of plasma flow velocities with the use of Compton-Getting formalism imply that plasma is mostly corotating to large radial distances from the planet. Thus the Jovian magnetosphere is confined by a plasma boundary (as was implied by the model of Brice and Ioannidis) rather than a conventional magnetopause. Inside the plasma boundary there exists a discontinuity at ∼ 50 to 60 RJ we have named the region inside this discontinuity the inner plasmasphere.

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.

Ionic charge states of N, Ne, Mg, Si and S in solar energetic particle events

Abstract The mean ionic charges of nitrogen, neon magnesium, silicon, and sulfur accelerated in solar flares have been determined for selected solar energetic particle events in the time period from September 1978 to September 1979. The observations were carried out with the Max-Planck-Institut / University of Maryland ULEZEQ sensor on the ISEE-3 satellite. These measurements complement the previously reported results for carbon, oxygen, and iron. Assuming that the ion charge states are established by equilibration in the hot plasma at the origin of the particles, the observed mean charge states are not compatible with a single temperature. The electron temperature of the plasma as calculated from ionization equilibrium tables range from ∼ 2×10 6 K for C, N, O, Si, and S to ∼ 4×10 6 K for Ne and Fe to ∼ 7×10 6 for Mg in a single flare.

INITIAL OBSERVATIONS OF LOW ENERGY CHARGED PARTICLES NEAR THE EARTH'S BOW SHOCK ON ISEE-1

We report initial measurements from the ULECA sensor of the Max-Planck-Institut/University of Maryland experiment on ISEE-1. ULECA is an electrostatic deflection - total energy sensor consisting of a collimator, deflection analyzer and an array of solid state detectors. The position of a given detector, which determines the energy per charge of an incident particle, together with the measured energy determine the particle’s charge state. We find that a rich variety of phenomena are operative in the transthermal energy regime (~10 keV/Q to ~100 keV/Q) covered by ULECA. Specifically, we present observations of locally accelerated protons, alpha particles, and heavier ions in the magnetosheath and upstream of the Earth’s bow shock. Preliminary analysis indicates that the behavior of these locally accelerated particles is most similar at the same energy per charge.

Direct observation of charge state abundances of energetic He, C, O, and Fe emitted in solar flares

Abstract The ionic charge states of helium, carbon, oxygen, and iron have been determined for three solar particle enhancements by an electrostatic deflection analyzer, which is combined with a thin window proportional counter dE/dX vs. E system. The observations are obtained during the periods September 23 to 29, 1978, June 6 to 8, 1979, and September 15 to 26, 1979, with an instrument onboard the ISEE-3 spacecraft. The mean ionic charge states for He, C, and oxygen exhibit a high degree of ionization with values of Q = 2, 6, and 7.2, respectively. The charge state of iron is near 13 charge units. Variations from flare to flare and within the September 23, 1978 flare are small. The most surprising feature of the charge state measurement is the observation of a small (∼10%) but finite contribution of singly ionized helium.

Spatial distribution of Z ⩾ 2 ions in the outer radiation belt during quiet conditions

Abstract During selected periods of quiet magnetospheric conditions (between October 24, 1977 and January 26, 1978) the distribution of Z ≥ 2 ions (He, C, O) in the B, L-space have been investigated with the heavy ion instrument ULEZEO on the ISEE-1 satellite. The equatorially mirroring ion distribution in L below 1 MeV/nucleon exhibits a maximum around L = 3.2. The equatorial pitch angle distribution, if fitted to a sin m α law, falls of sharply down to ∼ 39° pitch angle ( B B O ∼ 2.5) with values of m ≈ 16 for C and O at ∼ 0.5 Mev/nucleon. The locally mirroring flux above B B O ∼ 3 is subjected to large temporal variations even during quiet conditions.

The composition of heavy ions in solar energetic particle events

We review recent advances in determining the elemental, charge-state, and isotopic composition of ≲ 1 to ≲ 20 MeV per nucleon ions in solar energetic particle (SEP) events and outline our current understanding of the nature of solar and interplanetary processes which may explain the observations.The composition within individual SEP events may vary both with time and energy, and will in general be different from that in other SEP events. Average values of relative abundances measured in a large number of SEP events, however, are found to be roughly energy independent in the ∼ 1 to ∼ 20 MeV per nucleon range, and show a systematic deviation from photospheric abundances which seems to be organized in terms of the first ionization potential of the ion.Direct measurements of the charge states of SEPs have revealed the surprisingly common presence of energetic He+ along with heavy ions with typically coronal ionization states. High-resolution measurements of isotopic abundance ratios in a small number of SEP events show these to be consistent with the universal composition except for the puzzling overabundance of the SEP 22Ne/20Ne relative to this isotopes ratio in the solar wind. The broad spectrum of observed elemental abundance variations, which in their extreme result in composition anomalies characteristic of 3He-rich, heavy-ion rich and carbon-poor SEP events, along with direct measurements of the ionization states of SEPs provide essential information on the physical characteristics of, and conditions in the source regions, as well as important constraints to possible models for SEP production.It is concluded that SEP acceleration is a two-step process, beginning with plasma-wave heating of the ambient plasma in the lower corona, which may include pockets of cold material, and followed by acceleration to the observed energies by either flare-generated coronal shocks or Fermi-type processes in the corona. Interplanetary propagation as well as acceleration by interplanetary propagating shock will often further modify the composition of SEP events, especially at lower energies.

LOW-ENERGY HEAVY-ION OBSERVATIONS ON THE IMP 7 SATELLITE

H, He, and CNO ion pulses have been detected by an electrostatic deflection spectrometer on the IMP 7 satellite in the magnetotail, the magnetosheath, and in the region up-stream from the Earth’s bow shock. The spatial distribution of the pulses indicates that the ions are accelerated in the up-stream region and in the near-Earth region of the neutral sheet

On compositional variations of heavy ions during solar particle events

Abstract Intensity-time profiles of protons, alpha particles, and heavy ions (C, O, Fe) in the MeV/nucleon energy range have been analyzed for one solar particle event following the solar flare on September 23, 1978. The data have been obtained with the wide angle double dE/dx-E sensor of the Max-Planck-Institut/University of Maryland experiment onboard ISEE-3. We found time variations in the iron to helium ratio of up to 2 orders of magnitude and a significant variation of the O/He ratio during this event, whereas the C/O-ratio at the same energy/nucleon appears to be time independent. We investigated the influence of a rigidity dependent mean free path in interplanetary space and of rigidity dependent coronal propagation on heavy ion ratios during solar particle events. We found that both the magnitude and time scale of the ratio changes during the September 23 event cannot be explained by rigidity dependent interplanetary or coronal propagation alone. These ratio changes are probably caused by multiple injection at the sun.