Solar Release Times of Energetic Particles in Ground-Level Events

We study the onset times of energetic particles of various species and velocities, v, in large solar energetic particle events with sufficiently hard spectra that are seen by neutron monitors at ground level. Observations of He, O, and Fe from the Wind spacecraft provide especially well-defined sequences of onset times, and data from IMP-8, GOES, and neutron monitors contribute importantly at higher energies. Plotting onset times versus v -1 yields a line with the initial solar particle release (SPR) time as the intercept and the magnetic path length as the slope. We find consistent results for 13 of the 16 ground-level events that occurred from 1994 to 2007, in solar cycle 23. Path lengths vary from 1.1 to 2.2 AU in the 13 events. In all of the events, SPR times occur after the onset of the shock wave-induced type II radio emission. Events with well-defined SPR times are found over a wide span of solar longitude, suggesting that all ion species and energies are released together, even far from the source longitude, with no evidence of energy- or rigidity-dependent coronal transport. If the SPR time is converted to a radial distance of the source shock wave from the Sun and plotted against longitude, acceleration for well-connected events is found to begin at 2-4 solar radii over a longitude span of ~ 100° and to rise to greater heights only at longitudes more distant from the source, as would be expected from shock-acceleration models.