Acceleration mechanisms 2: force-free reconnection

Abstract

We suggest an unconventional view of the origin of most cosmic rays (CRs) in the universe. We propose that nearly every accelerated CR was part of the parallel current that maintains all force-free (f-f) magnetic fields. Charged particles are accelerated by the electric field E ∥ produced by reconnection parallel to the magnetic field B . The inferred total energy in extragalactic cosmic rays is ∼ 10 60 ergs per galaxy spacing volume, provided that the assumed acceleration mechanisms do not preferentially only accelerate ultra high energy cosmic rays (UHECRs). This total energy is quite large, about 10 5 times the parent galactic CR or magnetic energy. We argue that the formation energy of supermassive black holes (SMBHs) at galaxy centers, ∼ 10 62 ergs, becomes the only feasible source. We propose an efficient dynamo process which converts gravitational free energy into magnetic energy in an accretion disk around a SMBH. Aided by Keplerian winding, this dynamo converts a poloidal seed field into f-f fields, which are transported into the general intergalactic medium (IGM) eventually. This magnetic energy must also have been efficiently converted into particle energies, as evidenced by the radiation from energetic particles. In this view CRs of the IGM are the result of the continuing dissipation, in a Hubble time, of this free energy, by acceleration in situ within the f-f fields confined within the super-galactic walls and filaments of large scale structures. In addition, UHECRs are diffusively lost to the galactic voids at time scales below the GZK attenuation time, ∼ 10 8 years. Similarly, within the galaxy we expect that the winding by the disk rotation of the galaxy, by the rotation energy of magnetized neutron stars, and by the Keplerian winding of star formation disks are efficient sources of f-f magnetic field energy and hence the sources of galactic CR acceleration. To cite this article: S.A. Colgate, H. Li, C. R. Physique 5 (2004).