Ion acceleration near CME-driven interplanetary shocks

Abstract

We have surveyed properties of the magnetic field power spectral densities and energetic ions and compared them with the shock normal angles of 74 CME-driven IP shocks observed at ACE and Wind during solar cycle 23. We searched for events that exhibited clear signatures of first-order Fermi acceleration at quasi-parallel shocks and shock-drift acceleration at quasi-perpendicular shocks as predicted by the diffusive shock acceleration theory. Our results show that events with clear signatures of either shock-drift or first-order Fermi acceleration at 1 AU are rare, with 64 of the 74 IP shocks (~87%) exhibiting mixed signatures. We classify the remaining ten events as follows. (1) Four quasi-perpendicular shocks with θBn>70° exhibit no enhancements in the magnetic field power spectrum around the proton gyro-frequency and a slight hardening or no change in the ~80–300 keV/nucleon CNO spectral index across the shocks, indicating the absence of upstream wave activity and the re-acceleration of a pre-existing suprathermal seed spectrum. (2) Six quasi-parallel or oblique IP shocks with θBn<70° exhibit significant enhancements in the power spectral densities around the proton gyro-frequency and are accompanied by unfolding (softening) of the ~80–300 keV/nucleon CNO spectral index across the shocks, indicating the acceleration and efficient trapping of <300 keV/nucleon CNO ions by the Alfvén waves that were most likely excited by the accelerated protons as they streamed away from the shocks. In this paper, we present contrasting energetic particle and magnetic field observations near 2 IP shocks at 1 AU to highlight the complex signatures associated with the two distinct types of shock acceleration mechanisms.