R. B. McKibben

Saturnian Trapped Radiation and Its Absorption by Satellites and Rings: The First Results from Pioneer 11

Electrons and protons accelerated and trapped in a Saturnian magnetic field have been found by the University of Chicago experiments on Pioneer 11 within 20 Saturn radii (Rs) of the planet. In the innermost regions, strong absorption effects due to satellites and ring material were observed, and from ∼ 4 Rs inwards to the outer edge of the A ring at 2.30 Rs (where the radiation is absorbed), the intensity distributions of protons (≥ 0.5 million electron volts) and electrons (2 to 20 million electron volts) were axially symmetric, consistent with a centered dipole aligned with the planetary rotation axis. The maximum fluxes observed for protons (> 35 million electron volts and for electrons < 3.4 million electron volts) were 3 x 104 and 3 x 106 per square centimeter per second, respectively. Absorption of radiation by Mimas provides a means of estimating the radial diffusion coefficient for charged particle transport. However, the rapid flux increases observed between absorption features raise new questions concerning the physics of charged particle transport and acceleration. An absorption feature near 2.5 Rs has led to the discovery of a previously unknown satellite with a diameter of ≈ 200 kilometers, semimajor axis of 2.51 Rs, and eccentricity of 0.013. Radiation absorption features that suggest a nonuniform distribution of matter around Saturn have also been found from 2.34 to 2.36 Rs, near the position of the F ring discovered by the Pioneer imaging experiment. Beneath the A, B, and C rings we continued to observe a low flux of high-energy electrons. We conclude that the inner Saturn magnetosphere, because of its near-axial symmetry and the many discrete radiation absorption regions, offers a unique opportunity to study the acceleration and transport of charged particles in a planetary magnetic field.

Impulsive bursts of relativistic electrons discovered during Ulysses' traversal of Jupiter's dusk-side magnetosphere

During its flyby of Jupiter in February 1992, the Ulysses spacecraft passed through the Southern Hemisphere dusk-side Jovian magnetosphere, a region not previously explored by spacecraft. Among the new findings in this region were numerous, sometimes periodic, bursts of high energy electrons with energies extending from less than 1.5 MeV to beyond 16 MeV. These bursts were discovered by the High Energy Telescope (HET) and the Kiel Electron Telescope (KET) of the COSPIN Consortium. In this paper we provide a detailed analysis of observations related to the bursts using HET measurements. At the onset of bursts, the intensity of > 16 MeV electrons often rose by a factor of > 100 within 1 min, and multiple, pulsed injections were sometimes observed. The electron energy spectrum also hardened significantly at the onset of a burst. In most bursts anisotropy measurements indicated initial strong outward streaming of electrons along magnetic field lines that connect to the southern polar regions of Jupiter, suggesting that the acceleration and/or injection region for the electrons lies at low altitudes near the South Pole. The initial strong outward anisotropies relaxed to strong field-aligned bidirectional anisotropies later in the events. The bursts sometimes appeared as isolated events, but at other times appeared in quasi-periodic series with a period of ∼ 40 min. For smaller events shorter periods of the order 2–3 min were also observed in a few cases. For large events, multiple injections were sometimes observed in the first few minutes of the event. Radio bursts identified by the Ulysses URAP experiment in the frequency range ∼ 1–50 kHz were correlated with many of the electron bursts, and comparison of the time-intensity profiles for radio and electrons shows that the radio emission typically started several minutes before the electron intensity increase was observed. For the strongest electron bursts, small increases in the low energy (> 0.3 MeV) proton counting rates were also observed. Using a computerized identification algorithm to pick out bursts from the data record using a consistent set of criteria, 121 events were identified as electron bursts during the outbound pass, compared to only three events that satisfied the same criteria during the inbound pass through the day-side magnetosphere. No similar electron burst events have been found outside the magnetopause. Estimates of the electron content of a typical large burst (>∼ 1027 electrons) suggest that these bursts may make significant contributions to the fluxes of electrons observed in Jupiters outer magnetosphere, and in interplanetary space.

COSMIC RAY MODULATION IN THE 3-D HELIOSPHERE

The basic physical processes that lead to the long-term modulation of cosmic rays by the solar wind have been known for many years. However our knowledge of the structure of the heliosphere, which determines which processes are most important for the modulation, and of the variation of this structure with time and solar activity level is still incomplete. Study of the modulation provides a tool for probing the scale and structure of the heliosphere. While the Pioneer and Voyager spacecraft are surveying the radial structure and extent of the heliosphere at modest heliographic latitudes, the Ulysses mission is the first to undertake a nearly complete scan of the latitudinal structure of the modulated cosmic ray intensity in the inner heliosphere (R<5.4 AU). Ulysses will reach latitudes of ~80°S in September 1994 and ~80°N in July 1995 during the approach to minimum activity in the 11 year solar cycle. We present a first report of measurements extending to latitudes of ~52°S, which show surprisingly little latitudinal effect in the modulated intensities and suggest that at this time modulation in the inner heliosphere may be much more spherically symmetric than had generally been believed based upon models and previous observations.

Three-Dimensional Solar Modulation of Cosmic Rays and Anomalous Components in the Inner Heliosphere

Our picture of modulation in the inner heliosphere has been greatly affected by observations from the Ulysses mission, which since 1992 has provided the first comprehensive exploration of modulation as a function of latitude from 80° S to 80° N heliographic latitude. Among the principal findings for the inner heliosphere are: a) the cosmic ray intensity depends only weakly on heliographic latitude; b) for the nuclear components, and especially for the anomalous components, the intensity increases towards the poles, qualitatively consistent with predictions of drift models for the current sign of the solar magnetic dipole; c) no change in the level of modulation was observed across the shear layer separating fast polar from slow equatorial solar wind near 1 AU; d) 26-day recurrent variations in the intensity persist to the highest latitudes, even in the absence of clearly correlated signatures in the solar wind and magnetic field; e) the surface of symmetry of the modulation in 1994-95 was offset about 10° south of the heliographic equator; f) the intensity of electrons and of low energy (< ∼ 100 MeV) protons showed essentially no dependence on heliographic latitude.

The latitude gradients of galactic cosmic ray and anomalous helium fluxes measured on Ulysses from the Sun's south polar region to the equator

Measurements from the Ulysses COSPIN (Cosmic Ray and Solar Particle INvestigations) High Energy Telescope between 80.2°S solar latitude and the suns equator from September 1994 to March 1995 confirm that the modulated fluxes of galactic cosmic rays and anomalous components depend only weakly on heliographic latitude in the inner heliosphere at this phase of the solar cycle. The new observations were made over a radial range of only ∼1 AU during a period of nearly constant modulation and thus require less correction for radial and temporal variations in modulation than measurments made during Ulysses climb to maximum latitude.

Simultaneous Observations of Solar Energetic Particle Events by imp 8 and the Ulysses Cospin High Energy Telescope at High Solar Latitudes

With Ulysses approaching the south solar polar latitudes during a period of high solar activity, it is for the first time possible to study the distribution of solar energetic particles (SEPs) in solar latitude as well as in radius and longitude. From July 1997 to August 2000, Ulysses moved from near the solar equator at ~5 AU to ~67° S latitude at ~3 AU. Using observations of > ~30 MeV protons from Ulysses and IMP-8 at Earth we find good correlation between large SEP increases observed at IMP and Ulysses, almost regardless of the relative locations of the spacecraft. The observations show that within a few days after injection of SEPs, the flux in the inner heliosphere is often almost uniform, depending only weakly on the position of the observer. No clear effect of the increasing solar latitude of Ulysses is evident. Since the typical latitudinal extent of CMEs, which most likely accelerate the SEPs, is only ~30°, this suggests that the enhanced cross-field propagation for cosmic rays and CIR-accelerated particles deduced from Ulysses’ high latitude studies near solar minimum is also true for SEPs near solar maximum.

Modulation of Cosmic Rays and Anomalous Components by CIRs

CIRs produce clearly visible recurrent modulation in the intensity of cosmic rays and anomalous components, but are not principally responsible for determining the overall global level of modulation. However, the localized variations imposed by CIRs in the parameters for propagation of energetic particles through the solar wind provide useful diagnostics for testing models of the propagation against observations. A principal result from Ulysses observations of CIR-induced variations is that the variations persist to very high latitudes, well beyond the range where CIRs are observed. This has driven theoretical models to provide for enhanced latitude transport of energetic particles. On the other hand, observations of Jovian electron intensities vs. latitude do not support enhanced latitude transport. This chapter contains a summary of the interaction between observations and models for the effects of CIRs, and its impact on the understanding of the physics of modulation.

The Space Dust (SPADUS) instrument aboard the Earth-orbiting ARGOS spacecraft: I—instrument description

Abstract The Space Dust (SPADUS) instrument is being carried aboard the Advanced Research and Global Observation Satellite (ARGOS). ARGOS was launched into a circular, sun-synchronous polar orbit at ∼850 km altitude on February 23, 1999 on the Air Force ARGOS P91-1 Mission. The instrument provides time-resolved measurements of dust particle flux, mass distribution, and trajectories, as well as high time resolution measurements of energetic charged particles from the SPADUS Ancillary Diagnostic Sensor (ADS) subsystem, during the nominal three-year ARGOS mission. SPADUS uses Polyvinylidene Fluoride (PVDF) dust sensors developed at the University of Chicago. PVDF sensors have been used earlier on the Vega-1 and Vega-2 missions to Halleys Comet, and are currently being carried on experiments aboard the Cassini spacecraft to Saturn, as well as the Stardust spacecraft to Comet WILD-2. The SPADUS PVDF sensors have a total area of 576 cm 2 , and the SPADUS velocity/trajectory system permits distinction between orbital debris and cosmic (natural) dust, as well as a determination of the orbital elements for some of the impacting particles. The SPADUS instrument measures particle mass over the mass range ∼5×10 −11 g (3.3 μm diameter) to ∼1×10 −5 g (200 μm diameter), and also measures integral flux for particles of mass >∼1×10 −5 g .

The Space Dust (SPADUS) instrument aboard the Earth-orbiting ARGOS spacecraft: II–results from the first 16 months of flight

Abstract In a companion paper (Tuzzolino et al., Planet. Space Sci., 2001, to appear) hereafter called Part I, we present a detailed description of the Space Dust (SPADUS) instrument being carried aboard the Earth-orbiting Advanced Research and Global Observation Satellite (ARGOS). In this paper, we focus on examples of the flight data obtained by SPADUS during the first 16 months of the ARGOS mission. We present results obtained from the SPADUS polyvinylidene fluoride (PVDF) dust trajectory system, which measures dust particle flux, mass distribution, velocity and trajectory, as well as results obtained from the SPADUS Ancillary Diagnostic Sensor (ADS) subsystem, which measures energetic charged particles. Included are both raw data and reduced data for the time period from ARGOS launch (February 23, 1999) to June 8, 2000. The PVDF dust trajectory system detected a total of 258 dust impacts over this interval of approximately 470 days. Of these, 24 were D1–D2 type events—where particles impacted and penetrated a D1 sensor, then impacted a D2 rear array sensor—allowing for time-of-flight measurements. Of the 24 D1–D2 impacts on SPADUS, we identified 11 orbital debris particles, 7 interplanetary impactors, and 6 ambiguous impacts. Examples of particle orbits for both debris particles and interplanetary particles are detailed, and the results obtained for orbital debris flux and mass distribution are compared with predictions from an orbital debris model. We also describe transient particle streams detected by the SPADUS trajectory system resulting from the passage of ARGOS through streams of debris particles in Earth orbit.

The angle detecting inclined sensors (ADIS) system: measuring particle angles of incidence without position sensing detectors

Abstract We report on a novel system, the Angle Detecting Inclined Sensors (ADIS), for determining the angle of incidence of energetic charged particles. This system is particularly suited to space-based and balloon-borne instruments to study Solar Energetic Particles, Galactic Cosmic Rays and Anomalous Cosmic Rays. Such instruments are frequently constrained by limited resources in terms of mass, power and telemetry. At the same time, large detector area and acceptance angle, together with good elemental and isotopic resolution, can be critical for the required measurements. High-resolution particle identification requires that the angles of incidence of ion events in an instrument be determined. Conventional Position Sensing Detectors (PSDs) used in hodoscopes add significant complexity and require additional electronics, thus increasing instrument mass and power usage. The ADIS system overcomes many of these problems by using detector geometry in place of PSDs.