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Ann. Geophys., 21, 1315-1330, 2003
www.ann-geophys.net/21/1315/2003/
© European Geosciences Union 2003
Imprints from the solar cycle on the helium atom and helium pickup ion distributions
D. Rucinski1,†, M. Bzowski2, and H. J. Fahr2
1Space Research Centre PAS, Bartycka 18A, 00-716 Warsaw, Poland
2Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
†deceased in February 2002
Abstract. Neutral interstellar
helium atoms penetrate into the solar system almost unaffected by gas–plasma
interactions in the heliospheric interface region, and thus can be considered
as carriers of original information on the basic parameters (like density,
temperature, bulk velocity) of the Very Local Interstellar Medium (VLISM). Such
information can nowadays be derived from analysis of data obtained from
different experimental methods: in situ measurements of He atoms (Ulysses),
observations of the solar backscattered He 584 A radiation (EUVE), in situ
measurements of He + pickup ions (AMPTE, Ulysses, Wind, SOHO, ACE). In view of
the current coordinated international ISSI campaign devoted to the study of the
helium focusing cone structure and its evolution, we analyze expected
variations of neutral He density, of He + pickup fluxes and of their phase
space distributions at various phases of the solar activity cycle based on a
realistic time-dependent modelling of the neutral helium and He + pickup ion
distributions, which reflect solar cycle-induced variations of the
photoionization rate. We show that the neutral helium density values are
generally anticorrelated with the solar activity phase and in extreme cases
(near the downwind axis) the maximum-to-minimum density ratio may even exceed
factors of ~ 3 at 1 AU. We also demonstrate that in the upwind hemisphere (at 1
AU and beyond) the He + fluxes are correlated with the solar cycle activity,
whereas on the downwind side the maximum of the expected flux up to distances
of ~ 3 AU occurs around solar minimum epoch, and only further away does the
correlation with solar activity become positive. Finally, we present the
response of the phase space distribution spectra of He + pickup ions (in the
solar wind frame) for different epochs of the solar cycle and heliocentric
distances from 1 to 5 AU covering the range of Ulysses, Wind and ACE
observations.
Key words. Solar physics,
astrophysics and astronomy (ultraviolet emissions) – Space plasma physics (ionization
processes; numerical simulation studies)
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