J. A. Lockwood

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

Forbush decreases in the cosmic radiation

The experimental observations of Forbush decreases in recent years are reviewed and related to different theoretical models which have been proposed. The observational data from both ground-based and spacecraft experiments were selected to illustrate the important characteristics of Forbush decreases. The form of the rigidity dependence of the cosmic-ray modulation during the decreases and effects of the geomagnetic field upon the magnitude of the decreases are discussed. Recent results to deduce the cosmic-ray flow patterns from the observed anisotropies during the decreases are presented. Other features such as differences in onset times, recovery times, precursory increases are discussed. In considering the theoretical models particular emphasis is placed upon the agreement of the predictions of the model with the experimental observations. A theoretical model is suggested which is not original but represents a synthesis of several models previously proposed. Future important measurements and analyses necessary to an understanding of Forbush decreases are outlined.

Solar Energetic Particles

In this brief review, we summarize the current state of knowledge of solar energetic particles. This includes energetic particles contained within the site of solar flares that are responsible for X-ray, γ-ray and neutron emission and particles accelerated at high coronal altitudes and in interplanetary space by travelling disturbances such as coronal mass ejections. Special emphasis is placed on those particles directly or indirectly associated with neutron monitor signals.

The Imaging Compton Telescope Comptel on the Gamma Ray Observatory

This instrument is based on a newly established concept of ¿-ray detection in the very difficult 1-30 MeV range. It employs the unique feature of a two-step interaction of the y-ray: a Compton scattering collision in a first detector followed by an interaction in a second detector element. COMPTEL has been designed to perform a very sensitive survey of the y-ray sky. Extreme care has been taken to minimize background so that the detection limits of COMPTEL will be dominated by source counting statistics. It combines a wide field of view (about 1 steradian) with a good angular resolution. The design criteria of COMPTEL and the perforrmance of a Science Model are described.

The rigidity dependence of forbush decreases observed at the Earth

The rigidity dependence of the large Forbush decreases occurring on July 23, 1981, July 11, 1982, and February 6, 1986, has been determined using neutron monitor and IMP spacecraft data which cover the energy range from about 50 MeV to 30 GeV. The contribution of solar flare protons to the lower-energy data from the IMP cosmic ray telescopes was carefully removed. We found that the rigidity dependences of the magnitudes of the July 1981, July 1982, and February 1986 Forbush decreases for P ≥ 2 GV were given by exp (−1/P0.75), exp (−1/P0.6), and exp (−1/P1.0), respectively. For 0.5 ≤ P ≤ 2 GV the magnitude of the Forbush decreases in July 1981 and July 1982 was rigidity independent. The February 1986 event also appeared to be rigidity independent below ∼1 GV. The characteristic recovery time of these Forbush decreases was found to be not strongly rigidity dependent. These results on the rigidity dependence of Forbush decreases for 0.5 < P < 20 GV are discussed in the context of proposed models.

Forbush decreases and interplanetary magnetic field disturbances : association with magnetic clouds

Magnetic clouds have been proposed as a mechanism to produce Forbush decreases in the cosmic radiation. We have examined the temporal association of magnetic clouds and Forbush decreases and find practically no association of the main phase of the Forbush decrease with the arrival of a magnetic cloud. On the other hand, Forbush decreases generally follow the strong interplanetary shocks which sometimes precede magnetic clouds. The main phase of the cosmic ray decrease occurs 2–5 hours after the shock and during the passage of the region in which the magnetic field is disturbed. It appears that a Forbush decrease is more likely to occur following a shock in which the magnetic field and plasma parameters are strongly enhanced. These results indicate that the decrease of the cosmic ray intensity may be produced by the smaller diffusion coefficient in the region behind the shock. The sweeping effect of the enhanced magnetic field associated with the fast shock also probably contributes to the rapid depression of the cosmic ray intensity seen in some decreases.

Response functions of organic scintillators to high energy neutrons

Abstract Two cylindrical liquid scintillators of dimensions 5 cm × 5 cm and 12.5 cm × 12.5 cm, filled with NE213, were calibrated with high energy neutrons from E n = 3 MeV to E n = 75 MeV at the time-of-flight facility associated with the Michigan State University Cyclotron. Pulse shape discrimination was used on each detector to separate the protons and alphas produced by neutron interactions from the electrons produced by gamma rays. Response functions for monoenergetic neutrons from about 2 MeV to 75 MeV have been determined. These response functions are very different from the calculated response using the Monte Carlo method. The implications of these calibrations for measurements on high energy neutrons using liquid scintillators are discussed.

Neutron measurements in space

The experimental measurements of the neutron flux and energy spectrum in space since 1964 are reviewed and related to the theoretical predictions. A discussion of the neutron sources is presented. The difficulties associated with neutron measurements of both the atmospheric neutron leakage flux and solar neutrons are included. Particular emphasis is placed upon the neutron leakage flux and energy measurements at energies greater than about 1 MeV. The possibilities of CRAND as a source for the energetic trapped protons are discussed in light of recent measurements of the 10–100 MeV neutron flux. The current status of the solar neutron flux observations is also presented.The primary purposes of neutron measurements in space have been to determine the neutron leakage flux from the atmosphere of the Earth and the solar neutron flux. As a consequence of the inefficient methods for neutron detection and the difficulties of conducting the measurements in the presence of the galactic and solar cosmic-ray backgrounds, the experimental results are very conflicting. It is the purpose of this review to interpret and discuss recent neutron measurements. In order to understand these results the theoretical predictions of the neutron fluxes and energy spectra from possible neutron sources will be briefly presented. Since comparisons of the different neutron measurements depend critically upon the experimental techniques, we will briefly discuss neutron detection methods applicable to space measurements. The emphasis will be upon measurements since 1964 made outside the Earths atmosphere, but considerable reference will be made to high energy neutron experiments conducted within the Earths atmosphere at < 10g cm-2 altitude. A review of earlier neutron measurements of terrestrial and solar neutrons has been made by Haymes (1965).

Differences in the maximum intensities and the intensity‐time profiles of cosmic rays in alternate solar magnetic field polarities

We have found that there is a significant difference in the intensity-time profiles of the galactic cosmic rays with energies from 100 MeV to 50 GeV around the solar activity minima in the five alternate solar magnetic field polarities from 1954 to 2000. This difference in the appearance of the two halves of the 22-year solar magnetic cycle, peaked and flat-topped, supports the role of drifts near the cosmic ray intensity maxima. The intensity of cosmic rays at the solar activity minima also depends on the solar magnetic field polarity, and it is less at neutron monitor energies when the solar magnetic field polarity is positive than when it is negative. At energies of ∼500 MeV and lower the opposite is the case: The values of the intensity maxima are less when the solar magnetic field polarity is negative. This intensity difference arises from the role of drifts in the cosmic ray propagation, together with possibly a change in the value of the diffusion coefficient in the two polarity cycles, that produces changes in the shape of the proton and helium energy spectra. The rigidity dependence of the ratios of the maximum intensities of both protons and helium nuclei below 1 and 2 GV, respectively, in alternate solar magnetic cycles (e.g., 1987/1977) is given by P(0.45±0.05), but the ratios are shifted by a factor of 2 in rigidity. This shift does not appear to be predicted by current modulation theories that include cosmic ray drift effects.

GLOBAL PROCESSES THAT DETERMINE COSMIC RAY MODULATION

The global processes that determine cosmic ray modulation are reviewed. The essential elements of the theory which describes cosmic ray behavior in the heliosphere are summarized, and a series of discussions is presented which compare the expectations of this theory with observations of the spatial and temporal behavior of both galactic cosmic rays and the anomalous component; the behavior of cosmic ray electrons and ions; and the 26-day variations in cosmic rays as a function of heliographic latitude. The general conclusion is that the current theory is essentially correct. There is clear evidence, in solar minimum conditions, that the cosmic rays and the anomalous component behave as is expected from theory, with strong effects of gradient and curvature drifts. There is strong evidence of considerable latitude transport of the cosmic rays, at all energies, but the mechanism by which this occurs is unclear. Despite the apparent success of the theory, there is no single choice for the parameters which describe cosmic ray behavior, which can account for all of the observed temporal and spatial variations, spectra, and electron vs. ion behavior.