SEP Onset Variability Near Earth

We investigate the spatial-temporal variability of near-to-several-MeV energy proton flux onsets in a number of impulsive Solar Energetic Particle (SEP) events in interplanetary space near Earth using instruments on the ACE, IMP-8, Geotail, and GOES spacecraft. At some times and locations, a portion of the approaching SEP particle population is propagating along the average interplanetary magnetic field, and at other times and locations, a portion is transported with escaping solar plasma. ACE is stationed at L1, about 230 Re upstream of Earth in the solar wind. ACE presently serves as the primary early warning monitor of approaching interplanetary disturbance flux, both at solar wind and SEP energies. The GOES geosynchronous orbit (6.6 Re) satellites serve as our indicators of geoeffective SEP events, defined in this study as those events that produce hard energy spectrum proton flux enhancements at geosynchronous orbit. IMP-8 and Geotail orbit Earth at 25-44 and 10-30 Re geocentric, respectively, providing a diverse array of spatial configurations for measuring spatial-temporal variability in the near-Earth interplanetary medium. It is of considerable practical interest to determine the effectiveness of a single spacecraft monitor at L1 in detecting deleterious particle fluxes before they reach the Earth (as in the Living With A Star Program, for example) and of general scientific interest to determine the spatial and temporal variability of SEP onsets. At the leading edges of SEP populations, one might not necessarily anticipate a locally uniform structure, but rather a structured, variable, and anisotropic structure. Both might occur simultaneously, but at different locations. This study is a first step in determining how variable those structures are. The purpose of an early warning from L1 is to anticipate danger and help protect assets at geosynchronous orbit, such as communications satellites, and at lower altitudes, such as at the International Space Station and polar flying aircraft, for example. We present several examples of onsets in which the local spatial structure appears to be variable.