Energetic Particle Observations by the Cassini Spacecraft during Its Heliospheric Cruise to Saturn

Abstract

We present energetic particle observations performed by the Cassini spacecraft during its heliospheric cruise to Saturn. We cover three years of data (20002002) during the maximum of the solar cycle 23. Cassini's helio centric radial distance ranged from 2.5 to 8.0 AU. Energetic particle intensity enhancements at these distances are associated with either Solar Energetic Particle (SEP) events or Corotating Interaction Regions (CIRs). High-energy (>25 MeV) ion intensity increases are associated with intense SEP events. The largest low-energy (<8 MeV) ion intensities occur at the passage of transient interplanetary shocks associated with SEP events. Comparison with energetic particle data at 1 AU shows that, in general, intense SEP events near Earth are also observed at Cassini. However, when a new injection of SEPs occurs after a period of intense solar activity, it may produce a new SEP event at 1 AU but not at the larger helio centric radial distances where Cassini is. Recurrent CIR events are also observed during solar maximum. In particular, we study the recurrence of four CIR events first seen near Earth and later at Cassini (at ∼7 AU).