The high-energy impulsive ground-level enhancement
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We have studied short-lived (21 minute average duration), highly anisotropic pulses of cosmic rays that constitute the first phase of 10 large ground-level enhancements (GLEs), and which extend to rigidities in the range 5-20 GV. We provide a set of constraints that must be met by any putative acceleration mechanism for this type of solar-energetic-particle (SEP) event. The pulses usually have very short rise-times (three to five minutes) at all rigidities, and exhibit the remarkable feature that the intensity drops precipitously by 50% to 70% from the maximum within another three to five minutes. Both the rising and falling phases exhibit velocity dispersion, which indicates that there are particles with rigidities in the range 1 < P (GV) < 3 in the beam, and the evidence is that there is little scattering en route from the Sun. We name these events the high-energy impulsive ground-level enhancement (HEI GLE). We argue that the time-dependence observed at Earth at ∼5 GV is a close approximation to that of the SEP pulse injected into the open heliospheric magnetic field in the vicinity of the Sun. We conclude that the temporal characteristics of the HEI GLE impose nine constraints on any putative acceleration process. Two of the HEI GLEs are preceded by short-lived, fast-rising neutron and >90 MeV gamma-ray bursts, indicating that freshly accelerated SEPs had impinged on higher-density matter in the chromosphere prior to the departure of the SEP pulse for Earth. This study was based on an updated archive of the 71 GLEs in the historic record, which is now available for public use.