Multispacecraft Observations of Few-MeV SEP Ion Event Onsets Near Earth

Few-MeV-energy SEP proton flux onsets observed in interplanetary space near Earth are used to investigate the effectiveness of a single spacecraft stationed at the sunward L1 libration point ( ∼230 Re) to provide complete early warning information about Solar Energetic Particle (SEP) events. Knowledge of the behavior and distribution of few-MeV-energy protons at onset aids development of asset protection, predictive capabilities, and understanding of magnetospheric disturbances related to SEP appearance at Earth. Widely separated spacecraft, ACE near L1 and GOES-8, Geotail, and IMP-8 nearer to Earth ( ∼6.6-44 Re), provided simultaneous SEP observations from late 1997 to the present. A number of cases of spatial-temporal intensity variations between L1 and nearer-Earth observations are presented. Cases presented demonstrate large-scale transport characteristics both consistent and inconsistent with simple outward radial propagation (i.e., convection with the solar wind) of a locally planar feature over the ∼230 Re ( ∼0.01 AU) maximum separation distance.