Impulsive near-relativistic solar electron events: Delayed injection with respect to solar electromagnetic emission

The time histories of near-relativistic scatter-free impulsive beamlike electron events (38-315 keV, 0.4 < v/c < 0.8) measured at 1 AU provide unique information on their solar acceleration and release. We have identified 79 such impulsive events from 1997 August through 2000 September. Detailed statistical analysis of the timing between the near-relativistic electron injection and the soft X-ray, microwave, chromospheric Halpha, and metric and decametric type III radio emission shows that the near-relativistic electrons measured by the Advanced Composition Explorer/Electron, Proton, and Alpha Monitor in orbit around the Earth's first Lagrangian point are associated with western hemisphere events and are injected with a median delay of similar to10 minutes after the start of the electromagnetic emissions (including metric and decametric type III events). The delayed injection, as well as there being only a weak statistical correlation between the intensities of the near-relativistic electrons and the characteristics of the electromagnetic emissions, indicates that the escaping near-relativistic electron populations are not directly related to those that generate the prompt flare-related emissions. The observations are consistent with acceleration of the escaping near-relativistic electrons by an outgoing coronal shock (V similar to 1000 km s(-1)) launched near the time of the prompt electromagnetic emissions.