Charged Cosmic Rays: a Review of Balloon and Space Borne Measurements

Abstract

Current experimental data on cosmic-ray charged leptons are briefly reviewed including measurements of the positron fraction, electron and positron spectra and inclusive electron+positron data. Precision measurements by PAMELA and AMS-02 of the positron spectrum and its striking difference from the electron spectrum have prompted several theoretical speculations. In order to discriminate among different interpretations of the 'positron anomaly', a major step is needed to achieve an accurate direct measurement of the shape of the inclusive electron spectrum beyond 1 TeV. Ongoing efforts along this direction by instruments already in orbit and by the recently launched CALET and DAMPE missions are described. A challenging experimental scenario, at variance with the standard paradigm of a single power law spectrum, emerges for the cosmic-ray charged hadrons after the discovery of a discrepant hardening in the rigidity spectra of protons and He in the 200 to 300 GV region (CREAM, PAMELA, AMS-02) and the observation by AMS-02 of a possible break in the Li spectrum. An additional puzzle comes from the apparent violation of the universality of spectral indices whereby He and proton spectra are progressively hardening above ~100 GV with a similar rigidity dependence, but the He spectrum is harder than proton's. Secondary-to-primary ratios and isotope flux ratios provide insight into the subtleties of propagation mechanism(s) and test the internal consistency of the models of acceleration and propagation of cosmic rays in the galaxy. Important complementary information on the abundance of nuclei heavier than iron comes from dedicated balloon-borne instruments as SuperTIGER or space-based missions as ACE at L1 that provided the first measurement of a primary "cosmic-ray clock". A brief overview of future missions is given at the end of the paper.