2022ApJ...926L...4L

The dissipation effects of primordial magnetic fields on the primordial elemental abundances were investigated. When a magnetic field reconnects, its energy is converted to the kinetic energy of charged particles, as observed for solar energetic particles arriving on Earth. This accelerates the cosmic background nuclei and energetic nuclei induce nonthermal reactions. A constraint on the dissipation is derived from a theoretical calculation of the nonthermal reactions during Big Bang nucleosynthesis. We found that observations of the Li and D abundances can be explained if 0.01%-0.1% of the cosmic energy density was utilized for nuclear acceleration after the electron-positron annihilation epoch. Reconnections of such amplitudes of magnetic fields generate outgoing jets, the bulk velocity of which evolves to values appropriate for cosmic-ray (CR) nuclear energies of 0.1-1 MeV necessary for successful CR nucleosynthesis. Therefore, acceleration of cosmic background nuclei during the dissipation of primordial magnetic fields is a possible generation mechanism of soft CRs that has been suggested as a solution to the cosmic Li problem. Among the solutions suggested without exotic physics, only the dissipating magnetic field model suggested here explains observations of both low Li and high D abundances. Our results demonstrate that signatures of strong magnetic fields in the early universe have been observed in primordial elemental abundances.