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University of Maryland Department of Physics and Astronomy PROPAGATION OF COSMIC RAYS IN THE GALAXY R. R. Daniel Tata Institute of Fundamental Research Homi Bhabha Road, Bombay, India and S. A. Stephens* Department of Physics & Astronomy, University of Maryland and Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. Technical Report No. 75-028 UNIVERSITY OF MARYLAND DEPARTMENT OF PHYSICS AND ASTRONOMY COLLEGE PARK, MARYLAND (NASA-CR-141190) PROPAGATION OF COSMIC RAYS N75-15580 IN THE GALAXY (Maryland Univ.) 208 p HC $7.25 CSCL 03B Unclas G3/93 05074 This is a preprint of research carried out at the University of Maryland. In order to promote the active exchange of research results, individuals and groups at your institution are encouraged to send their preprints to PREPRINT LIBRARY DEPARTMENT 'OF PHYSICS AND ASTRONOMY UNIVERSITY OF MARYLAND COLLEGE PARK, MARYLAND 20742 U.S.A. PROPAGATION OF COSMIC RAYS IN THE GALAXY R. R. Daniel Tata Institute of Fundamental Research Homi Bhabha Road, Bombay, India and S. A. Stephens* Department of Physics & Astronomy, University of Maryland and Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. Technical Report No. 75-028 To appear in Space Science Reviews *on leave from Tata Institute of Fundamental Research, Bombay, India. CONTENTS 1. Introduction 1.1. Models of cosmic ray confinement 1.2. The galactic model for cosmic ray confinement 1.3. Scope of the present review 2. General description of the Galaxy 2.1. Dimensions and density 2.2. Structure 2.3. Interstellar gas 2.4. Magnetic fields 2.5. Radiation fields 3. Cosmic ray propagation in the Galaxy: Theoretical aspects 3.1. Quasi-stationary state and spatial homogenity of cosmic rays 3.2. Models of diffusion of cosmic rays 3.2.1. Motion of charged particles in a magnetic field and their diffusion 3.2.2. Self-scattering of cosmic rays 3.2.3. Instability model 3.3. Diffusion process and isotropy of cosmic rays 3.4. General formulation of the propagation of cosmic rays in interstellar space 3.5. Quasi-equilibrium models 3.5.1. Nucleonic components 3.5.2. Electronic components ii 4. Interpretation of the observed data 4.1. Matter traversed by cosmic rays 4.1.1. The effect of ionization 4.1.2. Fragmentation of nuclei 4.1.3. Charge composition of cosmic ray electrons 4.1.4. Gamma-ray component of the galactic background radiation 4.1.5. Summary 4.2. The chemical composition at the source 4.3. Modification of energy spectrum at low energies 4.3.1. L/M ratio 4.3.2. 2 H/4 He ratio 4.3.3. 3He/4He ratio 4.3.4. He/M ratio 4.3.5. He/LH ratio 4.3.6. He/VH ratio 4.3.7. Summary 4.4. Modification of energy spectrum at high energies 4.4.1. Observed change in the abundances of secondary nuclei 4.4.2. Energy dependence in the abundance of primary nuclei 4.4.3. Theoretical models to explain the change in composition 4.5. Propagation of the electron component and its source characteristics 4.5.1. Secondary electrons in the Galaxy 4.5.2. Directly accelerated electrons 4.5.3. The origin of cosmic ray electrons 4.5.4. Interpretation of high energy electron spectrum 4.6. Leakage lifetime of cosmic rays 4.6.1. Mean matter traversed by cosmic rays and the residence time 4.6.2. Leakage lifetime from long-lived radio nuclei in cosmic rays 4.7. Anti-nuclei in cosmic rays 4.7.1. Anti-nuclei of secondary origin 4.7.2. Anti-nuclei of primary origin 5. Cosmogenic electromagnetic radiations in the Galaxy 5.1. Cosmic ray interaction with interstellar gas 5.1.1. Pionic gamma rays 5.1.2. Bremsstrahlung radiation 5.2. Cosmic ray interaction with radiation fields 5.3. Synchrotron radiation 5.3.1. Observational data 5.3.2. The energy spectrum of electrons in interstellar space 5.3.3. Magnetic fields in the Galaxy 5.3.4. Mean density of cosmic rays in different regions of the Galaxy 5.3.5. Some inferenceson the properties of interstellar medium 5.3.6. The galactic halo 6. The role of cosmic rays in galactic dynamics 6.1. Hydrostatic equilibrium of the gaseous component of the Galaxy 6.2. Stability of self-gravitating gas and the formation of clouds 6.3. Thermal equilibrium of interstellar gas I. Introduction As early as the mid-nineteen twenties, when scientists were just beginning to accept the concept of the extraterrestrial origin of the radiation which discharged electroscopes carried aloft in manned balloons, and even when knowledge of the cosmos was still quite vague and unsettled, James Jeans (1925) speculated that the newly discovered cosmic radiation filled the entire universe. After two decades and a half, Richtmeyer and Teller (1949) proposed the other extreme kind of model wherein cosmic rays are supposed to be confined to the solar neighborhood; this model was masterfully developed later by Alfven (1949, 1950) when little was yet known of the nature of this radiation. Between then and now, there has been a spate of astounding and far reaching observational information on cosmic rays and astronomy. This in turn has stimulated the proposal and development of a variety of models and theories for the confinement and propagation of cosmic rays. For any model and theory of propagation of cosmic rays to be acceptable, it should be able, among other things, to explain satisfactorily the rele- vant observational data on cosmic rays. These include: (i) the chemical and isotopic composition of cosmic rays as a function of energy; (ii) the flux and energy spectrum of the individual nucleonic components; (iii) the flux and energy spectrum of the electronic component; (iv) the cosmic ray prehistory; and (v) the degree of isotropy in their arrival directions as a function of energy. The model should also be able to lead naturally and quantitatively to the observed intensities and spectra of electromagnetic radiations arising from interactions of cosmic rays with particles and fields in the space traversed by them such as pionic gamma rays from 2 nuclear interactions of cosmic rays with matter and synchrotron radiation emitted by electrons spiralling along the feeble magnetic fields. As at present the model which has been able to bring to pass the greatest measure of success in this respect is the galactic confinement model. Proposals have also been made that cosmic rays may be confined to a cluster of galaxies of which our own, the Milky Way, is a member. 1.1 Models of Cosmic Ray Confinement Of the various.possibilities listed above, the solar model can be ruled out as untenable because of the many insuperable obstacles it en- counters. Among others they include: (i) the anticorrelation between the cosmic ray intensity observed near the Earth and the 11-year solar activity cycle; (ii) the disagreement between the elemental abundance of cosmic rays and the solar composition; (iii) the significant difference between the steep spectral shape of particles emitted at times of solar flares and the relatively flat energy spectrum associated with cosmic rays at quiet times; (iv) the existence in cosmic rays of a continuation of the power law energy spectrum from a few GeV up to 1012 eV and extending even many orders beyond; (v) the difficulty of explaining simultaneously the few g.cm- 2 of matter that cosmic rays are known to have traversed before reaching us and the spectral shape of electrons; and (vi) the fact that we know from studies of the diffuse galactic radio noise, that the intensity of energe- tic cosmic ray electrons in interstellar space is of the same magnitude as is sampled near the Earth. Under extragalactic confinement models there are three important varia- tions: (a) the universal model (Jeans, 1925; Lamaitre, 1931; Burbidge and 3 Hoyle, 1964); (b) the super cluster model (Burbidge, 1962); and (c) the local cluster model (Sciama, 1962). By their very nature these models are associated with considerable amount of speculation arising from the severe lack of information on objects and space beyond our galaxy. On the other hand these models derive their strength from recent observations on the frequent occurrence of violent extragalactic events releasing colossal amounts of energy in energetic particles and hard quanta. The most impor- tant arguments against extragalactic models are the following: (i) The flux of hard quanta expected to result from the inverse Compton scattering of cosmic ray electrons residing within the confinement region (having an intensity same as in near-interstellar space), with photons associated with visible light and the universal black body radiation at 2.70 K, can be cal- culated as a function of the dimension of the confinement region. This can then be compared with the total observed flux of the isotropic compo- nent of gamma rays of energy in excess of 100 MeV (Fichtel et al., 1973; Kraushaar et al., 1972) to set an upper limit for the radiating distance which is found to be only ,1023 cm (Daniel and Stephens, 1970); this dimen- sion may be compared with the diameter of our own galaxy which is %1023 cm, the dimension of u1024 cm for the local cluster of galaxies and of ,1027 cm for the supercluster. (ii) In a similar fashion if one attributes the entire flux of the observed isotropic X-rays (Metzger et al., 1964) to the inverse Compton scattering of electrons filling all space, with pho- tons of the universal black body radiation (Felten and Morrison, 1966), one notes that the flux of electrons needed is a factor of 40 or perhaps even 103 smaller (Longair, 1970) than what is known to exist in interstel- lar space.
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