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| Titel |
The heliospheric modulation of cosmic ray boron and carbon |
| VerfasserIn |
M. S. Potgieter, U. W. Langner |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 22, no. 10 ; Nr. 22, no. 10 (2004-11-03), S.3729-3740 |
| Datensatznummer |
250015044
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| Publikation (Nr.) |
 copernicus.org/angeo-22-3729-2004.pdf |
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| Zusammenfassung |
| The observed boron to carbon ratio (B/C) at Earth provides a
good measure of the overall secondary to primary ratio of galactic cosmic
rays. This makes B/C an important constraint and test for the validity and
general applicability of theoretical and numerical models of galactic
propagation and heliospheric modulation. For this purpose, the modulation of
boron and carbon in the heliosphere must be understood in greater detail.
The latest approach to heliospheric modulation, using a numerical model
containing a termination shock, a heliosheath and particle drifts, is used
to the study the modulation of the two species. This model also includes a
more comprehensive set of diffusion coefficients. From this and previous
work follows that the model is compatible with a variety of observations, for
seven species, i.e. protons, anti-protons, electrons, positrons, helium,
boron, and carbon, with the same set of parameters for both solar magnetic
polarity cycles. Despite the rather flat interstellar spectrum for carbon
below 100MeV/nuc, the modulated spectra at 1AU look very similar for boron
and carbon, caused by adiabatic energy losses, implying that the carbon
modulation should have a much larger radial gradient in the outer
heliosphere below ~200-500MeV/nuc than boron. Significant modulation
can be caused by the heliosheath but it is strongly dependent on energy and
on the field polarity, with almost no effect at high energies to the largest
effect at low energies. The solar wind termination shock has an important
effect on the B to C ratio in the heliosphere, although small at Earth,
during the A<0 cycle, with E<~600MeV/nuc, but it seems less
significant for the A>0 cycle and with increasing tilt angles. Drift
models produce different spectra for consecutive solar minimum conditions
which may account for the modulation level differences between observations
around 100MeV/nuc compared to around 500MeV/nuc. All factors taken into
account, heliospheric modeling indicates that the interstellar spectra for B
and C need further refinement around 1GeV/nuc, in order to fit observations
over a wide energy range at Earth and that this refinement probably has to
take into account the proposed contribution of a local interstellar carbon
component. These results confirm that this numerical model with a TS can
reasonably reproduce the B and C modulation between the outer boundary and
Earth, making it a reasonable approximation for both polarity cycles from
solar minimum to moderate solar activity. |
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