Galactic Cosmic Ray Composition, Spectra, and Time Variations

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Galactic Cosmic Ray Composition, Spectra,
andTime Variations

• Mark E. Wiedenbeck
• Jet Propulsion Laboratory,
• California Institute of Technology
• mark.e.wiedenbeck_at_jpl.nasa.gov

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With contributions from Dick Mewaldt Alan
Cummings Jeff George Andrew Davis and the
entire ACE/CRIS science team.
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(No Transcript)
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Where the measurements fall along the cosmic ray
spectrum observed near Earth. The peak of the
intensity spectrum has been relatively well
covered.
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Where the measurements fall in the periodic
table. All elements up through the iron peak
have been well measured. Abundances above Z30
fall so rapidly that they contribute little to
the dose, but could might occasionally be an
issue for electronics upsets. Measurements in
this region were made on HEAO-3 and Ariel 6 in
the early 1980s.
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Were the measurements fall in time. Only in the
most-recent solar-cycle has good coverage of Z
and E/M been available. A particularly-complete
set of measurements exist for mid-1998. IMP-8
measurements of H, He cover 3 cycles. Neutron
monitor data extend back over 4 cycles.
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Energy spectra from all major GCR species are
available for in mid-1998. H and He from AMS
(BESS data also available). Zgt2 data from
ACE/CRIS. Spectra on relatively-short time
scales available from late 1997 to
present. Extensions to lower energies can be
made using ACE/SIS (not shown) when not obscured
by solar particles.
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Even isotopes!
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Quality Factor changes relative importance of
cosmic rays of different energies.
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Although GCR fluences are dominated by H and He,
heavy ions are critical for ionization and
biological effects. Over a wide range of
energies Fe is the most important element from
the point of view of equivalent dose.
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Modulation and adiabatic deceleration prevent
GCRs that have less than a few hundred MeV per
nucleon in the local interstellar medium from
reaching Earth. Present-day cosmic ray
observations would not be sensitive to very
significant low-energy turn-ups in the
interstellar spectra.
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Solar modulation changes GCR intensities at low
energies by up to an order of magnitude between
solar min and solar max. HEAO-3 data can be
scaled to fill in the higher energy portions of
the spectra. Spectral shape changes are
reasonably-well accounted for by a single
modulation parameter
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Scaling of measurements from diverse times in the
solar cycle to a common level of modulation can
produce a universal spectrum for each element.
J. George et al.
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Modulation parameter values can be derived on
time scales as short as a solar rotation from the
low-energy roll off of the spectra. Resulting
values are consistent with those derived from
energy changes of spectral features in the
spectra of electron capture isotopes
(boxes). Parameterization of modulation is
largely empirical and ignores a lot of physics
which could be important.
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Similar results are obtained from studies of the
time dependence of low energy H and He. Here
data from a series of balloon flights of the
Japanese BESS experiment between solar minimum
and solar maximum have been used to derive the
modulation parameter.
Shikaze et al. 2003
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More-physical models of the heliospheric
modulation of cosmic rays predict more-complex
time dependences. The tilt of the heliospheric
current sheet plays an important role. Drift
effects change the bulk flow patterns between
cycles with opposite polarity of the solar
magnetic field. Such effects are not included in
the simple calculations commonly used to fit
observed energy spectra.
Jokipii Thomas 1981
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Evidence for these effects are seen in the
different shapes of successive GCR intensity
maxima, e.g. in neutron monitor data.
Usoskin
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Now that the Voyagers are going beyond the solar
winds termination shock it is becoming clear
that not all of the GCR modulation occurs inside
this boundary as was once thought.
Usoskin
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Since Voyager 1 crossed the termination shock in
December 2004 it has continued to observe an
increase in the GCR intensity. This increase is
seen at Voyager 2 as well. Apparently
significant modulation is occurring in the
heliosheath.
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Direct knowledge of GCR intensities is limited to
just a few solar cycles. Since solar activity
drives the modulation of GCRs, one must be
concerned that records of significantly different
solar activity in the past may mean significantly
different (higher!) GCR intensities as
well. Changes apparently can occur on short
relatively-short time scales.
Space Age
Maunder Minimum
Usoskin
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• Summary
• Composition and spectra have been well measured
  over nearly a full solar cycle.
• There are now good measurements of the heavy ion
  component, which is a major contributor to the
  biological risks of radiation.
• Interstellar spectra below a few hundred MeV/nuc
  are essentially unknown.
• Direct measurements of intensity variations in
  the heliosheath by the Voyagers are critical to
  understanding modulation and constraining
  interstellar spectra.
• Longer-term intensity variations could be much
  larger than have been observed in the space age.