C. Lopate

Cosmic Ray and Solar Particle Investigations Over the South Polar Regions of the Sun

Observations of galactic cosmic radiation and anomalous component nuclei with charged particle sensors on the Ulysses spacecraft showed that heliospheric magnetic field structure over the south solar pole does not permit substantially more direct access to the local interstellar cosmic ray spectrum than is possible in the equatorial zone. Fluxes of galactic cosmic rays and the anomalous component increased as a result of latitude gradients by less than 50% from the equator to -80�. Thus, the modulated cosmic ray nucleon, electron, and anomalous component fluxes are nearly spherically symmetric in the inner solar system. The cosmic rays and the anomalous nuclear component underwent a continuous, -26 day recurrent modulation to -80.2�, whereas all recurring magnetic field compressions and recurring streams in the solar wind disappeared above ∼55�S latitude.

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.

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.

Further indications of a ∼140 day recurrence in energetic particle fluxes at 1 and 5 AU from the Sun

A possible ∼140 day recurrence in the fluxes and anisotropies of interplanetary MeV energy protons has previously been identified in data from the Anisotropy Telescopes (ATs) on board Ulysses. This measurement was made during 1998–1999 at a distance from the Sun of ∼5 AU. Earlier reports of a 154-day periodicity in the occurrence rate of solar flares and other solar activity indicators exist, mainly for cycle 21 data. In this paper we compare the ATs measurement, made for protons in the 1.3–2.2 MeV energy range, with fluxes of relativistic electrons (∼4–10 MeV) and protons in the range ∼30–100 MeV. We consider 5-AU data from the Ulysses Kiel Electron Telescope (KET) instrument and 1-AU measurements from the IMP 8 Cosmic Ray Nuclear Composition (CRNC) telescope. We find that at these higher energies significant fluxes at 5 AU are detected only in correspondence with a specific feature in the ATs recurrent pattern, thus supporting the hypothesis of recurrent behavior. We look for signatures of these events at 1 AU in the CRNC data and find for all of them an associated long-duration particle event, regardless of the longitudinal separation between Ulysses and IMP 8. Possible solar sources of the events are discussed.

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.