Positive charge prevalence in cosmic rays: Room for dark matter in the positron spectrum

The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100–300 GeV range. The data indicate that this ratio, after a decline between 0.5 and 8 GeV, rises steadily with a trend towards saturation at 200–400 GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a single supernova remnant (SNR) shock. We argue that eþ=e− ratio can still be explained by the diffusive shock acceleration if positrons are accelerated in a subset of SNR shocks which (i) propagate in clumpy gas media and (ii) are modified by accelerated cosmic ray protons. The protons penetrate into the dense gas clumps upstream to produce positrons and charge the clumps positively. The induced electric field expels positrons into the upstream plasma where they are shock accelerated. Since the shock is modified, these positrons develop a harder spectrum than that of the cosmic ray electrons accelerated in other SNRs. Mixing these populations explains the increase in the eþ=e− ratio at E > 8 GeV. It decreases at E < 8 GeV because of a subshock weakening which also results from the shock modification. Contrary to the expelled positrons, most of the antiprotons, electrons, and heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the 100–300 GeVAMS-02 fraction exceeding the model prediction, including, but not limited to, possible dark matter contribution, are also discussed.