Energetic Particle Fluences from Solar Wind to GCR Energies
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).
For further information, see "Solar and Galactic Composition" by R. A.
Mewaldt et al. in
This figure compares the long-term fluences of He, O, and Fe nuclei.
(Fluence is defined to be the total number of particles per cm2sr-MeV/nuc
integrated over this 33-month time period.). All three species clearly have
a common spectral shape. Note that this period starts with solar-minimum
conditions in 1997 and ends with solar-maximum conditions in 2000. Given
the large number of contributing sources, it is somewhat surprising that
all three spectra are so similar
These fluence data were obtained by summing hourly-average fluxes within a
large number of separate energy intervals, taking into account the measured
instrument live times and geometry factors. (Such hourly-average fluxes are
a routine product of ACE Level-2 data processing, and are available at
http://www.srl.caltech.edu/ACE/ASC/).
Table 1: Sources of Fluence Data
These data are published in a paper entitled "Long-Term Fluences of
Energetic Particles in the Heliosphere" by R. A. Mewaldt, G. M. Mason, G.
Gloeckler, E. R. Christian, C. M. S. Cohen, A. C. Cummings, A. J. Davis, J.
R. Dwyer, R. E. Gold, S. M. Krimigis, R. A. Leske, J. E. Mazur, E. C.
Stone, T. T. von Rosenvinge, M. E. Wiedenbeck, and T. H. Zurbuchen. The
paper appears in Solar and Galactic Composition, AIP Conference
Proceedings #598, R. F. Wimmer, editor, American Institute of Physics,
Melville, NY, p. 165-170, 2001.
Click here for hi-res version (~400k)
Click here for hi-res version (~400k)
SWICS 0.0005 to 0.030 MeV/nuc
ULEIS 0.040 to ~5 MeV/nuc
SIS ~8 to ~100 MeV/nuc
CRIS ~80 to ~400 MeV/nuc
Contributed Data Page
Last modified: Tue May 13 09:21:11 PDT 2003