The Alpha Magnetic Spectrometer (AMS-02), which is scheduled to be deployed onboard the International Space Station later this year, will be capable of measuring the composition and spectra of GeV-TeV cosmic rays with unprecedented precision. In this paper, we study how the projected measurements from AMS-02 of stable secondary-to-primary and unstable ratios (such as boron-to-carbon and beryllium-10-to-beryllium-9) can constrain the models used to describe the propagation of cosmic rays throughout the Milky Way. We find that within the context of fairly simple propagation models, all of the model parameters can be determined with high precision from the projected AMS-02 data. Such measurements are less constraining in more complex scenarios, however, which allow for departures from a power-law form for the diffusion coefficient, for example, or for inhomogeneity or stochasticity in the distribution and chemical abundances of cosmic ray sources.
Despite nearly a century of observational and theoretical progress, the origin of the cosmic ray spectrum remains a major puzzle of modern astrophysics. The task of identifying the sources of these particles is complicated by the non-trivial processes involved in cosmic ray propagation. Although cosmic ray composition and spectrum measurements have taught us a great deal about the acceleration and propagation of cosmic rays, we still lack a detailed and self-consistent understanding of how these particles are produced, and how they travel through and interact with the interstellar medium.
Below the spectral feature known as the knee (E ∼ 10 15 eV), the bulk of the cosmic ray spectrum is believed to be of galactic origin. Non-relativistic shocks occurring in supernova remnants seem to be likely sources, and are predicted to accelerate cosmic rays with a power-law injection spectrum Q ∝ E -γ with γ ∼ 2 [1]. At higher energies, even less is known about the origin of the cosmic ray spectrum. In this work, we focus solely on galactic cosmic rays at energies well below the knee.
Our understanding of how cosmic rays propagate through the Milky Way is informed largely by measurements of the spectra of various cosmic ray species as observed at Earth. In particular, by comparing the spectrum of particles produced in cosmic ray accelera-tors (primaries) to those that are produced by inelastic processes during propagation of primary particles (secondaries), we can learn about the mechanisms involved in cosmic ray propagation. While stable secondary-toprimary ratios (such as boron-to-carbon and antiprotonto-proton) provide information that can be used to constrain the effective column density cosmic rays pass through before reaching the Solar System, unstable ratios (such as beryllium-10-to-beryllium-9) are useful in constraining the time interval since spallation. Combinations of such observations make it possible to constrain the basic properties of relatively simple cosmic ray propagation models.
To date, some of the most precise cosmic ray measurements over the GeV-TeV energy range have been made by the CREAM (boron-to-carbon) [2], PAMELA (antiproton-to-proton) [3], ISOMAX ( 10 Be-to-9 Be) [4], and HEAO-3 (Subiron-to-iron, boron-to-carbon) [5] experiments. These measurements have been used to place fairly stringent constraints on the parameters of the underlying cosmic ray propagation model [6,7]. In this article, we extend this approach to include data anticipated from the Alpha Magnetic Spectrometer (AMS-02) experiment, which is scheduled to be deployed on the International Space Station in 2010. With its greater acceptance and superior particle identification relative to previous experiments, measurements from AMS-02 are expected to dramatically improve our understanding of the processes involved in galactic cosmic ray propagation.
Once injected from their sources into the interstellar medium, charged cosmic rays -unlike photons or neutrinos -undergo a number of processes potentially capable of significantly altering their spectra (for a recent review, see Ref. [8]). The Galactic Magnetic Field, in particular, is responsible for deflecting charged particles, leading them to diffuse gradually throughout the Galaxy, following paths resembling a random walk. Particles with greater energy, and therefore rigidity, diffuse more efficiently and tend to escape the Galaxy more quickly, whereas less energetic cosmic rays are typically confined by the Galactic Magnetic Field for a greater duration.
An essentially inevitable consequence of high energy particle scattering in the turbulent magnetic field is stochastic acceleration, also known as diffusive reacceleration [9]. This mechanism gives rise to diffusion in momentum space with a diffusion coefficient determined by the spatial diffusion coefficient and the Alfvén velocity, which represents the typical velocity at which magnetic irregularities propagate in the interstellar medium.
Other potentially important effects to consider include galactic winds, which may result in the convection of particles away from the Galactic Plane, as well as various energy loss processes. Such energy losses occur as a result of the cosmic rays traversing the galactic medium, which is permeated with gas, radiation fields, and magnetic fields. In the case of nuclei Coulomb and ionization energy losses exist, but they play only a minor role in their propagation. On the other hand, GeV electrons and positrons lose significant quantities of energy through inverse Compton and synchrotron processes; at lower energies, ionization, Coulomb interactions, and bremsstrahlung processes may also be relevant. Furthermore, the decays of unstable, radioactive species must be taken into account, including the introduction of any relevant decay products. Lastly, cosmic ray spallation on the interstellar medium can lead to the extinction of the incident particle and to the creation of a secondary flux c
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