ALBERT

Notes: Abstract Energetic particles, ejected from the Sun during solar flare events, may encounter interplanetary plasma/field conditions, which deviate considerably from the quiet time values used normally to describe the particle propagation. This is due to the presence of a hydromagnetic shock, which is emitted from the Sun at the time of the explosion. In a theoretical blast wave model, which incorporates the interaction with plane polarized Alfvén waves, we have analysed the changes in different terms of the Fokker-Planck equation, which describes energetic particle propagation. In this treatment, the shock influence on energy changes and on the transport coefficients are discussed.

Notes: Abstract The local interstellar medium can be probed in different ways: by analyzing low energy X-ray data in the range 0.1–0.4 keV, where the radiation is absorbed by the interstellar gas at column densities in excess of about 1020 cm-2 — and can therefore be regarded as ‘local’, by determining the absorption of stellar emission spectra from nearby stars along their lines of sight by intervening gas and by directin situ measurements of those components which penetrate the heliosphere sufficiently far, provided they can be distinguished from interplanetary material. The current status of these different investigations gives the following picture: the solar system is surrounded by a ‘bubble’ of hot gas (density 0.005cm-3, temperature 106 K) out to several tens of parsecs. More locally it is embedded in a small warm cloud of density 0.07cm-3, temperature 7000 K, column density ∼ 5 × 1017 cm-2 — which gives a mass of about 0.1M ⊙. The transition to the heliosphere is governed by solar UV ionization, snowploughing of the interstellar gas by the outwardly expanding solar wind and the bow shock. The heliosphere is the region inside the solar wind terminal shock. Classically it would be regarded as not yet affected by (or aware of) the obstacle ahead. Practically, the existence of the interstellar medium makes itself felt even far inside the heliosphere by the penetration of neutral gas, dust, plasma waves, shock accelerated particles and cosmic rays. These are the local ‘probes’ of the interstellar medium.

Notes: Abstract Various models are examined, which could give rise to point-like gamma-ray sources, at the present time indistinguishable, experimentally, from true point sources. These models involve energetic processes associated with interstellar clouds, e.g. supernova-cloud interactions, neutron star accretion inside interstellar clouds, cloud collisions, etc. The dynamical evolution of such systems is discussed and physical processes are described in a mathematical framework which can be solved. Statistical arguments are presented, where possible, on the likelihood that the scenarios may actually occur in our Galaxy. The visibility of the systems at other wavelengths, e.g. infrared, X-ray, radio etc., and further consequences, e.g. gamma-ray line emission, special radio emission line features, absorption features etc. are also discussed. Finally, a limited attempt at identification of some ‘gamma-ray objects’ is made based on the theoretical predictions.

Notes: Abstract A supernova remnant accelerates cosmic rays to energies somewhat above 105 GeV by the time that the free expansion phase of its evolution has come to an end. As the remnant's outer shock slows, these highest energy cosmic rays diffuse away from the shock along a magnetic flux tube with a radius comparable to that of the remnant at the end of its free expansion phase and which eventually (over a distance of the order of a kiloparsec) bends into the Galactic halo. A similarity solution exists for the temporal and spatial variations, in such a tube, of both the number density for these ∼ 105 GeV cosmic rays and the energy density of the waves on which they resonantly scatter. Wave-wave interactions probably do not dominate the evolution of the energy density of these lowest frequency waves, but we assume that they do establish a Kraichnan wave spectrum at higher wavenumber. Although we cannot rigorously justify this assumption, it does receive some support from the analysis of pulsar signals. There is a large body of observations to which such a model can be applied, yielding constraints that must be met. With the model that we develop here we obtain the following results: 1. The local intensity of ∼ 105 GeV cosmic rays implies that the flux tube which currently surrounds the Solar System last contained a remnant in the free expansion phase several times 107 years ago. We comment on the rough agreement between this age and that inferred from Be10 data. 2. The theoretical value of the cosmic ray diffusion coefficient at ∼ 1 GeV in the tube corresponding to that time is in harmony with the value of the diffusion coefficient inferred from cosmic ray composition and synchrotron measurements. In the light of our inhomogeneous cosmic ray acceleration/propagation model we re-examine our earlier work on the evidence for second order acceleration in a very old remnant. Such evidence is provided by the molecular compositions along several lines of sight to the Perseus OB2 association. We find as a third significant result that the model value of the diffusion coefficient at energies in the range of 1 MeV agrees within about an order of magnitude with that which we infer from the molecular data.

Notes: Abstract The energy spectra observed in a CsI crystal in the 20 keV-2 MeV range, due to the decay of radioactive isotopes produced in the crystal by bombardment with 155 MeV protons, are presented as a function of time after irradiation. It is shown that the large number of decay products produced by spallation can account for these spectra and that these spectra are in quantitative agreement with the predictions of a semi-empirical formula due to Rudstam, which gives the numbers of different isotopes produced. This formula is used to predict the spallation that would occur in such a crystal on board a satellite due to cosmic rays and passages through the South Atlantic Anomaly. Inspection shows that the spallation produced in the latter case is well approximated by that at 155 MeV. Hence the experimental results are used to explain previously observed background rates and to predict the background rates that would occur in the U.K.5 X-ray telescope of Imperial College. Using the Rudstam formula an estimate of cosmic ray induced background is also made. The relative importance of activity resulting from neutron interactions (atmospheric albedo and spacecraft secondaries) is considered. It is suggested that the Rudstam formula provides a general method of predicting induced radio-activity in satellite materials and that observed breaks in the diffuse cosmic X-ray spectrum could be due to inadequate allowance for this source of background.