Heliospheric energetic particle observations during the October–November 2003 events

Abstract

The intense level of solar activity recorded from 19 October to 12 November 2003 led to unusually high energetic particle intensities observed throughout the heliosphere. The fleet of spacecraft distributed in the inner and outer heliosphere offers us the opportunity to study both the effects of these events in different regions of the heliosphere and the evolution of the energetic particle intensities measured at different heliocentric radial distances. Observations at 1 AU by the ACE and GOES‐11 spacecraft show multiple particle intensity enhancements associated with individual injections of solar energetic particles (SEPs) and with the arrival of shocks driven by coronal mass ejections (CMEs), resulting in a long time interval (∼40 days) of elevated low‐energy (<1 MeV) ion and near‐relativistic (<315 keV) electron intensities. Observations from the Ulysses spacecraft at 5.2 AU, 6° north of the ecliptic, and 120–90° west of the Earth also showed elevated low‐energy ion and near‐relativistic electron intensities for more than ∼40 days that were modulated by the effects of recurrent corotating interaction regions (CIRs) and the passage of a fast interplanetary coronal mass ejection (ICME). The Cassini spacecraft at 8.7 AU, 3° south of the ecliptic, and 75–35° west of the Earth saw an intense low‐energy ion and near‐relativistic electron event in association with the passage of an enhanced magnetic field structure formed by the compression of transient solar wind flows and CIRs. The prompt component of the SEP event at Cassini was largely reduced due to the modulating effect of intervening transient flows propagating between the Sun and the spacecraft. The Voyager‐2 spacecraft at 73 AU and 25° south of the ecliptic did not observe these events until April 2004. The arrival of a merged interaction region (MIR) at Voyager‐2 produced a ∼70‐day period with elevated <17 MeV proton and <60 keV electron intensities. Particle fluences computed over the duration of the events at each spacecraft show a radial dependence that decays more slowly than that expected from a simple model assuming adiabatic cooling of an isotropic particle population uniformly distributed in a shell symmetrically expanding at the solar wind speed. Although the SEP events were observed throughout the heliosphere, both (1) the solar particle injections occurring at different times and longitudes, and (2) the marked differences in the interplanetary stream structures propagating toward different longitudes resulted in distinct time‐intensity histories at each spacecraft, and therefore periods with equal particle intensities were not observed by this fleet of spacecraft.