| ACE News Archives | ACE News #54 - June 29, 2001 |
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ACE Science Nuggets |
Energetic particles in the heliosphere originate from a number of separate sources and acceleration processes, and are reasonably variable in intensity and composition. It is therefore somewhat surprising that the spectrum of oxygen nuclei, integrated over a 3-year period and extending over six decades in energy, exhibits a reasonably high degree of organization.
The figure above shows the time-integrated intensity (fluence) of oxygen from ~300 eV/nucleon to ~300 MeV/nucleon. Energetic particle data with >30 keV/nucleon were obtained by the ULEIS, SIS, and CRIS instruments on ACE from September 1997 to June 2000, a 33-month period including both solar-minimum and solar-maximum conditions. The lower-energy fluences were obtained from the SWICS instrument on ACE during the first 11 months of 1999 and multiplied by x3 to correspond to the longer time period. The peak at ~800 eV/nucleon corresponds to ~400 km/sec solar wind. Occasional higher-speed streams with up to ~1000 km/sec produce a shoulder on the solar wind distribution. At ~10 keV/nucleon a region begins that extends to ~10 MeV/nucleon with a power-law slope of -2. Above ~50 MeV/nucleon, galactic cosmic rays dominate, continuing on for many decades in energy.
The region from ~100 keV/nucleon to ~30 MeV/nucleon includes contributions from a number of sources, some of which are indicated above. From ~3 to 30 MeV/nucleon the steady contribution from anomalous and galactic cosmic rays (ACRs and GCRs) is overwhelmed by that from gradual solar energetic particle (SEP) events that occur ~10 times a year at solar maximum. From ~0.1 to 1 MeV/nucleon there are contributions from impulsive solar flares and particles accelerated in corotating interaction regions (CIRs), and >100 separate events can be identified during this period. The suprathermal region from ~10 keV/nucleon to ~100 keV/nucleon is relatively unexplored, and the origin of the most important contributions is not yet identified. These are the first spectra to extend continuously from solar wind to cosmic ray energies. He and Fe spectra from this time period are almost identical except for overall intensity differences. Given the variable composition and spectra of the contributing sources, the relatively smooth nature of these spectra is surprising.
Contributed by Richard Mewaldt (Caltech) and by George Gloeckler and Glenn Mason (University of Maryland).
Last modified 29 June 2001, by
Andrew Davis