Solar Energetic Particle Propagation in 1997-99: Observations from ACE, Ulysses, and Voyagers 1 and 2
R.B. Decker, E.C. Roelof, and S.M. Krimigis
All at: The Johns Hopkins Univ. Applied Physics Lab., Laurel, Maryland, USA

The intercomparison of directional intensities of a given particle species measured at comparable energies and at widely separated positions within the heliosphere has proven invaluable for investigating energetic particle propagation. The Low Energy Charged Particle (LECP) instruments on the Voyager 1 (V1) and 2 (V2) spacecraft in the distant heliosphere each possess a subset of energetic particle channels well-matched with selected channels from instruments on key spacecraft in the inner heliosphere, including the EPAM instrument on ACE and the HI-SCALE instrument on Ulysses. Beginning in late 1997 and continuing to the present, solar activity producing large flares and CMEs has been markedly enhanced. Solar energetic particles (SEPs) injected during intense activity in April-May 1998 were observed at high intensities at both ACE (1 AU) and Ulysses (5 AU), where HI-SCALE detected a strong anti-sunward streaming along the interplanetary magnetic field (IMF). SEP protons from this activity have also been identified in a 0.5-1.5 MeV proton channel at V2 (56 AU, 18 deg S) and V1 (72 AU, 33 deg N). A shell of 1 MeV protons ~60 days wide reached peak intensity at V2 on 1998.7 and at V1 on 1998.8. These peaks were midway between moderate but abrupt decreases in cosmic ray intensities, observed ~110 days apart at each Voyager, evidently caused by solar wind disturbances (with radial speeds ~500-600 km/s) from the Nov. 1997 and April-May 1998 solar activity. We interpret these results in terms of weak-scattering propagation from 1 to 72 AU and over ~50 deg in latitude. For example, SEP protons with energies above ~1 MeV travel at speeds in excess of 0.3 AU/hr and must follow the spiral IMF. Between 5 and 70 AU, this spiral path winds around the Sun 10 times and has a length of over 2000 AU. Despite such long paths and energy losses suffered during this 2000 AU journey, the SEPs manage to outrun the solar wind disturbance from the April-May 1998 solar activity, and arrive at Voyager 1 nearly 1.5 months before passage of the SWD and 6 months after they left the Sun.