Relativistic Proton Production During the 2001 April 15 Solar Event

Extreme solar processes at the Sun on 2001 April 15 accelerated protons to relativistic energies, resulting in a groundlevel enhancement in cosmic rays observed at Earth. The GOES 10 spacecraft measured the proton flux directly, and ground-based observatories measured the secondary responses to higher energy protons. We have modeled the ground-level response to this event using a technique that deduces the spectrum, arrival direction, and anisotropy of the high-energy protons. To investigate the acceleration process(es), we have employed theoretical shock and stochastic acceleration spectral forms in our fits to spacecraft and neutron-monitor data. In the case of stochastic acceleration, we use two different spectral forms. The first (model A) incorporates an idealized preacceleration step through monoenergetic injection. The second (model B) incorporates a more realistic injection function using preacceleration by DC electric fields in a reconnecting neutral current sheet. We find that at the rise, peak, and decline phases of the event (14:20, 14:30, and 14:45 UT, respectively), the spectrum between 120 MeV and 10 GeV is best fitted by a shock acceleration spectral form. This implies that protons were accelerated to relativistic energies either by a coronal shock or at the bow shock of the 2001 April 15 coronal mass ejection. In addition, reinvestigation of the 2000 July 14 solar event using the more realistic injection function (model B) further supports our earlier findings that MHD turbulence arising from magnetic reconnection from a dissipating neutral current sheet was important in relativistic proton production for that event. © 2007. The American Astronomical Society. All rights reserved.