Electron Pitch Angle Distributions Research Articles

Synchrotron Radiation as the Source of Gamma‐Ray Burst Spectra

We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-absorption frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models.

Solar wind electron characteristics inside and outside coronal mass ejections

We present a study of 9 months of data from the solar wind plasma electron instrument on the ACE spacecraft. Electron pitch angle distributions were used to identify intervals of counterstreaming halo electrons, which were observed ∼16% of the time that the spacecraft was not magnetically connected to the Earth's bow shock. Counterstreaming electrons presumably indicate a closed magnetic field topology and thus indicate the passage of coronal mass ejections (CMEs) in the solar wind. In this study, we separately examine electron moments at times with and without counterstreaming electrons, including both magnetic clouds and noncloud CMEs. The properties of both the core and halo electron populations were nearly identical at times with and without counterstreaming electrons. Both low and high electron densities and temperatures were observed in either type of event. In contrast, magnetic clouds, on average, showed increased densities and reduced temperatures. The core/halo density ratio and the total electron density were anticorrelated in all cases, indicating that the halo contributes more to the total electron density at low density times, regardless of magnetic topology. Total electron temperature and density were anticorrelated in all types of events. This consistent anticorrelation implies that such single‐spacecraft measurements of temperature and density cannot be used to determine a polytropic index for solar wind electrons. Instead, the anticorrelation may be due to pressure balance of the solar wind plasma or may be a remnant of coronal conditions.

Energetic trapped electron measurements from the Galileo Jupiter probe

Energetic trapped electrons were measured with the Energetic Particle Instrument (EPI) on the Galileo Jupiter Probe. Samples were taken from inside Io's orbit to just above Jupiter's atmosphere. Under the assumption of simple power law spectra the energetic electron fluxes and spectra agree well with earlier results from the Pioneer spacecraft, at those locations where comparisons can be made. New features from this Galileo data include measurements of the inner boundary of the trappped particles, inside 1.6 RJ, direct observations of the electron pitch angle distributions, and energy spectral softening, both as the atmosphere is approached, and at smaller pitch angles at each measurement location.

An investigation into the roles of ECH and whistler mode waves in the formation of “pancake” electron distributions using data from the CRRES satellite

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOS-1 and GEOS-2 spacecraft in 1977/8, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by Electron Cyclotron Harmonic (ECH) waves. Here we use observations by instruments on board the CRRES spacecraft to study these distributions in more detail. The pancake distributions are now seen to develop from injected distributions that are nearly isotropic in velocity space, on a time scale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling

We investigate Interball‐Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different Kp levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high‐energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball‐Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.

Energetic electron beams and trapped electrons at Io

We present results from a continuing study of the magnetic field‐aligned energetic electron beams and accompanying trapped electron distributions discovered during the Galileo satellite's passage through Io's cold, dense, low‐speed wake on December 7, 1995 [Williams et al., 1996]. A companion paper by Thorne et al. [this issue] presents an analysis and explanation of the evolution of the electron pitch angle distributions measured on approach to Io and describes the resulting energetic electron flow paths and adiabatically “forbidden” regions expected to exist around Io. In Io's wake, only bidirectional, field‐aligned electron beams are seen; no ion beams are observed. At energies &gt;∼15 keV the measured beams represent an energy flow of ∼0.03 erg cm−2 s−1, and if they penetrate Jupiter's atmosphere, they can provide an energy deposition of ∼15 erg cm−2 s−1 at the foot of the Io flux tube. This is sufficient to stimulate observable aurora in Jupiter's atmosphere. Extrapolating the measured spectra to lower energies yields much higher values (e.g., ∼104 ergs cm−2 s−1 at 0.25 keV). The angular width of the measured trapped‐like electron distributions is independent of energy and varies across Io's wake in a manner consistent with the measured magnetic field variation. We conclude that these electrons are trapped in Io's magnetic field configuration. The narrowness of the beams and the simultaneous existence of an apparently unaccelerated trapped electron population provide evidence that the source region for the beams is close to Jupiter. A deconvolution of the detector response to the beams gives a beam angular half width of ∼6°, placing the formation of the beams at an altitude of ∼0.6–0.7 RJ. The slight energy dependence of the beam width provides a rough upper limit estimate of &lt;∼1.8 (10)8 cm−3 for Io's neutral SO2 atmosphere in the flyby region. No proposed acceleration mechanism operating close to Io (neither double layers nor Fermi acceleration via propagating Alfven waves) is able to reproduce measured beam characteristics.

Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi‐adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E−n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low‐speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field‐aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the “trapped” electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest “trapped” electron population to be detected above the background level for energies up to 200 keV.

“Pancake” electron distributions in the outer radiation belts

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV &lt; E &lt; 500 eV by the GEOSl and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV &lt; E &lt; 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV &lt; E &lt; 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

Relativistic theory of wave‐particle resonant diffusion with application to electron acceleration in the magnetosphere

Resonant diffusion curves for electron cyclotron resonance with field‐aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single‐ion plasma, and a numerical scheme is developed for the more realistic case of a multi‐ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth's magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler (R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere.

Properties of Ganymede's magnetosphere as revealed by energetic particle observations

Energetic particle observations made during the Galileo satellite's close encounters with Jupiter's moon Ganymede provide a measure of many characteristics of Ganymede's magnetosphere, a magnetosphere larger than that of Mercury. Changes in energetic ion anisotropy signatures, caused by Jupiter's corotating magnetic field, show that Ganymede's magnetosphere significantly slows down the ambient nearly corotating Jovian plasma. The data further indicate that this convection slowdown apparently extends at least several Jupiter radii away from Ganymede along Jovian field lines connected to the moon. The locations of these anisotropy signature changes occur at the locations of magnetometer and plasma wave magnetopause identifications that match surprisingly well the predictions of a simple model comprised of an intrinsic dipole field superimposed on Jupiter's ambient field. Loss cones measured throughout Ganymede's magnetosphere provide a quantitative estimate of the moon's surface magnetic field along the subsatellite track that agrees well with model field predictions and provides verification of Ganymede's intrinsic magnetic field. On Jovian field lines connected to Ganymede, electrons are trapped between the moon and a near‐Jupiter mirror point for several bounces as they convect across Ganymede's magnetosphere. This unique geometry allows a measurement of the amount of electron pitch angle scattering occurring in a single bounce between the Ganymede and near‐Jupiter mirror points. The energy‐dependent pitch angle diffusion coefficient and scattering lifetimes have been extracted from these observations. The consistency of these results from encounter to encounter demonstrates the stability of these processes, at least over the several month interval between encounters. Evidence for loss cone signatures established one Jovian rotation (∼10 hours) earlier indicates that the entire Jovian L shell region traversed by Ganymede provides an electron scattering environment similar to that measured on Jupiter‐Ganymede field lines. Measurements from Galileo's last close encounter with Ganymede show electron pitch angle distributions characteristic of particles trapped on closed magnetic field lines in a distorted magnetospheric configuration. Thus with its distorted magnetic field and apparent radiation belt, Ganymede takes its place as a unique member of the solar system family of magnetospheres.

On synchrotron radiation energy losses of trapped magnetospheric electrons

Abstract Using an energy balance equation it is explicitly demonstrated that synchrotron radiation (SR) significantly decelerates several MeV electrons confined on geomagnetic L -shells below 2 during their radial diffusive transport into inner magnetosphere. SR losses from the ambient electrons are found to be comparable in importance with Coulomb friction energy losses, a well-known and much modeled cause of energy degradation in the radiation belts. It may also be true that, under some geophysical conditions and at some electron energies, the effective electron lifetime due to wave-induced pitch-angle scattering can be shorter than both the Coulomb and the SR lifetimes. But even when this holds true, the effect of the competing SR energy loss process can substantially influence the outcome of model computations of the ambient inner zone electron flux, pitch angle, spatial and energy distributions. We find that in the inner radiation zone of the Earth, care must be exercised to include the simultaneous effects of all three competing electron loss mechanisms.

An investigation of the spatial variations of the energetic trapped electrons at low altitudes

We have analyzed the trapped electron data (0.19–3.2 MeV) taken by the Japanese OHZORA satellite operated at 350–850 km altitude in polar orbit during 1984–1987 near solar minimum. The electron observations reveal all the global attributes of the quiet-time electron radiation belts, such as the South Atlantic Anomaly, the electron “slot”, and the outer radiation belt regions. The electron data are in general agreement with the NASA AE-8 electron model, but there are differences, particularly with respect to distinctive local-time variations in the slot region. In this paper, we present results from analyses of variations of the electron pitch angle distributions with local time, L-shell and altitude.

Trapped electrons in Ganymede's magnetic field

The NASA Galileo satellite encountered the Jovian moon Ganymede on 7 May 1997. The Energetic Particles Detector (EPD) measured energetic electron pitch angle distributions characteristic of closed magnetic field lines. Significantly different from distributions observed during other encounters with Ganymede, they displayed loss cone features near both 0° and 180° pitch angles and an additional minimum near 90° pitch angle. These double loss cone, butterfly distributions are characteristic signatures of particles drifting in a distorted magnetic field configuration. Distortions due to the flow of Jovian plasma past Ganymede qualitatively could account for the observed distributions. Electron injection could then occur either through the equatorial downstream hemisphere or at low Ganymede altitudes on high‐latitude Ganymede‐Jovian field lines adjacent to the closed field line region. The data indicate maximum injection efficiences of 1–10%.

Synchrotron evidence for Amalthea's influence on Jupiter's electron radiation belt

High‐resolution radio images of Jupiter at wavelength λ = 20 cm obtained with the Very Large Array (VLA) in June 1994 (a few weeks before comet Shoemaker‐Levy 9 collided with the planet) are compared with detailed model calculations. All major features of the radio emission can be explained or simulated through model calculations. In particular, we infer how the pitch angle distribution of high‐energy electrons varies with L value. The electron pitch angle distribution seems to undergo a dramatic change at Amalthea's orbit: A fraction of the electron population is redistributed in pitch angle (isotropized) there, so that fewer electrons mirror near the magnetic equator and more electrons mirror off the equator at L ≲ 2.65 than beyond. The isotropic component leads to the high‐latitude emission regions, while the decreased number of equatorially mirroring electrons results in a “shoulder” or flattening in the radio intensity pattern at L ∼ 2.5, as is observed. Perhaps Amalthea's motion through Jupiter's magnetic field induces Alfvén or whistler wings or electrostatic high‐frequency waves which lead to the “observed” pitch angle scattering. Jupiter's ring absorbs 80%–100% of electrons with small pitch angles that diffuse through the region it occupies. The observed effect of this absorption is that the high‐latitude emission peaks remain distinct from the equatorial maximum. Ring absorption causes nearly all electrons at L ≲ 2 to be narrowly confined to the magnetic equator, a distribution which accounts for the east‐west asymmetry, which is very prominent at certain central meridian longitudes. The azimuthal variation (east‐west asymmetry) over a Jovian rotation is completely determined by the magnetic field configuration, as was suspected by many researchers in the past but never modeled succesfully before. We infer, however, that Connerney's O6 magnetic field model from 1992–1993 is slightly oversimplified, since the radiation characteristics cannot be completely matched at all Jovian longitudes: Deviations appear in particular at longitudes λcml ∼ 140°–180° and λcml ∼ 300°–340° (corresponding to λIII ∼ 30°–90° and λIII ∼ 210°–270° in Jovicentric coordinates).

Energetic electron precipitation from the inner zone

Energetic electron precipitation flux from the Earth's inner radiation belt (L≤2) is usually below the detection threshold of current satellite instrumentation. Theoretical estimates of the longitudinally and interhemispherically averaged rate of precipitation have been obtained by evaluating the equilibrium electron pitch angle distribution in the loss cone due to pitch angle scattering. The results are normalized to CRRES MEA measurements of the trapped flux. An energy deposition code is employed to evaluate ion production rates in the middle atmosphere. We demonstrate that inner zone electron precipitation is an important mesospheric energy source for L≥1.35 and that it can be the dominant ionization source at altitudes between 60 and 80 km. For E≳1MeV the energetic electron deposition declines monotonically with decreasing L, in agreement with the trend implied by observations at L≥2.

Energetic electron pitch angle distribution parameters at 6.6 Re, as deduced from GOES X-ray observations

X-ray sensors that measure the Sun's radiant output in two soft X-ray channels, 1–8 and 0.5–4 Å, are carried on all GOES geostationary equatorial weather satellites. A comparison of X-ray measurements from two co-operational GOES reveals a systematic difference signal that shows periodic diurnal and seasonal variations. These effects are seen during geomagnetically quiet times as well as disturbed times and are most noticeable when solar activity is low to moderate. The GOES orbit lies just above the main outer electron belt of the van Allen radiation belts but it falls inside the region containing >2MeV trapped electrons; thus the local particle environment includes electrons of sufficient energy to cause significant Bremsstrahlung on the walls of the ion chamber as well as direct deposition of energy through the entrance aperture. These background effects occur despite passive shielding of the ion chambers and in-orbit electronic suppression of the spurious particle contribution. However, because of the regularity of the difference signal it is possible to exploit this X-ray contaminant to infer certain properties of the energetic electron pitch angle distribution in anisotropy and in local time, on the assumption that these energetic electrons are responsible for the spurious X-ray detector response. The basic attributes of the observed diurnal and seasonal effects can be re-created in a model that incorporates a tilted dipole magnetosphere and local-time-dependent, generic pitch angle distributions. It is possible to infer the anisotropy index, n, for dayside sin n (α) distributions and the anisotropy index, m, for nightside sin m (2α) butterfly distributions as well as the local times where these distributions convert from normal loss-cone to butterfly in the afternoon and return to normal loss-cone in the morning. Examples of the diurnal and seasonal variations in the observed X-ray difference signal are shown, and these waveforms are re-created by a model simulation.

Observation by Ulysses of hot (∼270 keV) coronal particles at 32° south heliolatitude and 4.6 AU

An unusual event of streaming 60 keV‐2 MeV ions (with energy spectrum peaked ∼270 keV) and of 42–315 keV electrons occurred during the passage of a coronal mass ejection (CME) over the Ulysses spacecraft June 9–13, 1993, located at helioradius 4.6 AU and heliolatitude 32° south. The topology of the interplanetary magnetic field (IMF) within the CME has been identified as a helical magnetic flux rope by Gosling et al. [1994]. The ion and electron pitch angle distributions (PADs) had a bidirectional component in the outer (large‐pitch) regions of the flux rope, while there were strongly unidirectional (antisunward) beams in the inner (small‐pitch) core of the structure, where the electron PADs also displayed a distinctive depletion of electrons moving inward (sunward). Because the core ion beam was narrow, we can associate the observed energy spectrum in the peak direction of the beam (characterized by a Maxwellian with kT = 270 keV) directly with the spectrum injected in the inner heliosphere. The well‐defined spatial structure of the event and the absence of any clear signatures of local interplanetary shock acceleration during the period June 9–17 implies that the injection source could have been a long‐lived hot coronal ion population. The “weak scattering” electron PAD implies that the other (antisunward) end of the core of the flux rope was magnetically connected, not back to the sun, but rather to the outer heliosphere.

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening MLT sector. Season‐dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely‐directed, field‐aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies &gt;5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near‐Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field‐aligned currents and conductivity gradients. The generation of field‐aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.

A study of quasi-periodic ELF-VLF emissions at three Antarctic stations: evidence for off-equatorial generation?

Abstract. The spatial extent and temporal behaviour of quasi-periodic (QP) intensity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a comparative study of data collected at the Antarctic stations of South Pole (L=14), Halley (L=4), and Siple (L=4). Frequently, the waveforms of ELF-VLF signals simultaneously received at each site were identical. Although of similar frequency structure, the waveforms of the accompanying Pc3 magnetic pulsations did not show a one-to-one association. Whereas both are dayside phenomena, QP emissions occur over a smaller range of local times, and have a maximum of occurrence later in the day closer to local noon. QP emissions are identified with the periodic modulation of the electron pitch-angle distribution by the propagation of ULF compressional fast-mode waves through a region. However, contrary to previous ideas, rising-tone emissions do not represent the frequency-time signatures of such waves. In addition to generation close to the equatorial plane, we propose an additional high-latitude source of QP emissions. These emissions are associated with regions of minimum B produced by the dayside compression of the magnetosphere close to the magnetopause. Model magnetic field calculations of these minimum-B regions as a function of magnetic local time and invariant latitude are presented.

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas ( E ≳ 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies ≳ 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos 25 α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of ∼ 10 −4 μAm −2. The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.