The Ulysses Cosmic Ray and Solar Particle Investigations (COSPIN)



(Adapted from Simpson et al., Astron. & Astrophys., 92, 365-399, 1992)



Summary

The Ulysses spacecraft, launched on 6 October 1990, is the first to undertake measurements in the heliosphere far from the ecliptic plane and over the polar regions of the Sun. The instrumentation carried by the spacecraft includes the COSPIN Consortium Experiment, a group of six charged particle telescopes to measure the energy, composition, intensity and anisotropy of nucleons in the energy range from ~0.5 MeV/nucleon to ~600 MeV/nucleon for elements in the range H to Ni. Isotopic abundances for nuclei H to Ni are obtained over a more limited energy range. Electron measurements extend from 4 to several hundred MeV. One set of telescopes will measure the three-dimensional anisotropies of protons and helium at low energies. A special high flux telescope provides measurements of protons and heavier particles ~ 0.2 to ~ 10 MeV/n with high azimuthal resolution. These instruments were prepared by the international COSPIN consortium to address a wide range of scientific objectives made possible by a mission to investigate the Sun and the heliosphere in three dimensions. Their program is called "Cosmic Ray and Solar Particle Investigation" or COSPIN.

Examples of the COSPIN scientific goals include:



Scientific Objectives

Investigations over the past 35 years have shown that the heliosphere is the largest structure so far directly accessible for spacecraft investigations of the electrodynamical interactions of plasmas, magnetic fields, and energetic charged particles on astrophysical scales. Studies made with the Pioneer 10 and 11, and Voyager 1 and 2 spacecraft out to > 50 A.U. have yielded discoveries vital for understanding phenomena such as collisionless shocks, charged particle acceleration mechanisms, galactic cosmic ray propagation and the interactions of the Sun with its environment mediated by the plasmas and magnetic fields in the solar wind. These phenomena, while studied locally in the heliosphere, are important in a wide variety of astrophysical settings. A limitation on heliospheric investigations to date is that observations have so far been available only from ground-based and spacecraft investigations restricted to the low solar latitudes in and near the ecliptic. Very little has been established regarding phenomena in the high latitude regions of the heliosphere or indeed, in the polar regions of the Sun itself, except through remote sensing techniques such as radio scintillations and the limited observations that can be made of the polar regions of the sun as a result of the seven degree tilt of the Sun's rotation axis with respect to the ecliptic.

The Ulysses Mission is the first mission, and the only mission in the foreseeable future, that will undertake measurements in regions of the heliosphere far from the ecliptic plane and over the polar regions of the sun. The instrumentation which it carries must be capable of being both exploratory, with the wide dynamic range of instrument response required to encompass unexpected phenomena and, simultaneously, definitive, with the sophistication and resolution necessary to completely characterize whatever phenomena are discovered. For these investigations the galactic cosmic radiation, the anomalous nuclear component, the galactic and jovian electron components, and the solar flare accelerated high energy particles are important charged particle test "probes" of large scale heliospheric phenomena and their changes with time. The measurement of a wide range of charged particle properties, including energies, anisotropies, spectra, and the chemical and isotopic composition of nucleons reveals specific acceleration mechanisms, propagation modes, and small scale transient phenomena, and provides information important for astrophysics beyond the solar system as well. These measurements, provided by the Cosmic Ray and Solar Particle Investigation (COSPIN), have a central role in achieving the scientific goals of the Ulysses Mission. For example, the COSPIN will measure at low energies (e.g., ~ 1 MeV) the acceleration of charged particles from solar flares, radial and corotating interplanetary shocks and, at higher energies (e.g., ~1 GeV), the modulation of the galactic cosmic ray spectrum resulting from large scale interplanetary dynamical phenomena. Thus, the primary experimental goals of the COSPIN are to address fundamental astrophysical questions concerning solar, heliospheric and galactic phenomena in the hitherto unexplored high latitude regions of the Sun and heliosphere.

With Ulysses plasma and magnetic field measurements, and in collaborative investigations with Pioneer-10, Voyager-1, and Voyager-2 in the distant heliosphere, we expect to be able to deduce the large scale, three-dimensional structure of the heliosphere and to investigate its changes with solar cycle activity.

Since Ulysses will reach its maximum latitude when the ~ 11 year solar cycle approaches a minimum, the modulation of cosmic rays will also be near a minimum. Thus, depending upon the topology of the heliospheric magnetic fields over the solar polar regions, we may have access to a lower energy portion of the interstellar spectrum than would be possible at low latitudes.

The following are some of the specific scientific goals of the COSPIN investigators for the Ulysses Mission during 1990-1996.

From the measurement of charged particles of solar origin we shall study their energy spectra, elemental and isotopic composition and anisotropy during propagation to high heliospheric latitudes to determine:

For the galactic cosmic ray investigations we shall:

With regard to the acceleration or modulation of charged particles in the interplanetary medium, we intend to:

During the Ulysses encounter with Jupiter (closest approach, 8 February 1992) our magnetospheric studies will include:



The COSPIN Measurements

To achieve the scientific goals discussed above, a degree of flexibility is needed from the instrumentation which can only be realized by employing a variety of sensor designs. A set of five telescope subsystems are combined within the COSPIN experiment to cover the charged-particle flux wide energy and nuclear charge intervals.

Each COSPIN sensor is designed to address a different range of the measurements required to characterize the cosmic rays and other energetic charged particles in the high latitude regions of the heliosphere. The development of the instrumentation concepts extended from 1975 to 1978 The instrumentation was designed, built and tested for space flight for the initially-planned STS launch by NASA in 1983. Because of the long duration of the Ulysses Mission, care has gone into the design of COSPIN to assure reliability and the ability to survive, to the maximum extent possible, failures in individual subsystems. A basic ground rule is that no single failure in any subsystem should result in complete loss of useful data from COSPIN. Thus, certain critical subsystems, such as the low voltage power converter and the central DPU, have been made completely redundant. The other subsystems have been designed to be isolated from each other so that a failure in one would cause a loss of data only from that subsystem.

The complete COSPIN instrumentation is packaged in five units mounted separately on the spacecraft platform and interconnected with one interface to the spacecraft. Within the Ulysses ESA and NASA Project nomenclature the five units are referred to as SIM-1, SIM- 2, etc. A tungsten shield protects the HET telescope from the intense gamma-radiation of the spacecraft Radio-isotope Thermal Generator (RTG) to minimize background in key data channels.

Many of the studies to be undertaken by the Ulysses mission in general and the COSPIN investigation in particular are greatly strengthened by the availability of comparable measurements from a stationary point in the heliosphere to permit separation of spatial from temporal variations, and to provide a baseline against which spatial variations may be measured. A 1 A.U. baseline for the Ulysses measurements is provided by instruments on the IMP-8 satellite which provide measurements of interplanetary charged particles, the solar wind, and magnetic fields .