Pick-up Ion Measurements in the Heliosphere – A Review |
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| Authors: | R Kallenbach J Geiss G Gloeckler R von Steiger |
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| Institution: | (1) International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland;(2) International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland;(3) Dept. of Physics and IPST, University of Maryland, College Park, MD, 20742, U.S.A.;(4) Dept. of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, 48109, U.S.A. |
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| Abstract: | Measurements of the composition and spatial distribution of pick-up ions inside the heliosphere are reviewed. The first interstellar
4He+pick-up ions were detected with the SULEICA instrument on the AMPTE spacecraft near Earth's orbit. Most data on pick-up ions
were taken in the solar-wind and suprathermal energy range of SWICS on Ulysses while the spacecraft cruised from 1.4 to 5.4
AU and explored the high-latitude heliosphere and solar wind from the ecliptic to ± 80° heliolatitude. This includes the discovery of H+, 4He++, 3He+, N+,O+, and Ne+ pick-up ions that originate from the interstellar neutralgas penetrating the heliosphere. From their fluxes properties of
the interaction region between the heliosphere and the Local Interstellar Cloud such as the limits on filtration and the strength
of the interstellar magnetic field have been revealed. Detailed analysis of the velocity distributions of pick-up ions led
to 1) the discovery of a new distinct source, the so-called Inner Source, consisting of atoms released from interstellar and
interplanetary dust inside the heliosphere, 2) the determination of pick-up ion transport parameters such as the long mean
free path for pitch-angle scattering of order1 AU, and 3) detailed knowledge on the very preferential injection and acceleration
of pick-up ions during interplanetary energetic particle events such as Co-rotating Interaction Regions and Coronal Mass Ejections.
SWICS measurements have fully confirmed the theory of Fisk, Koslovsky, and Ramaty that pick-up ions derived from the interstellar
gas are the dominant source of the Anomalous Cosmic Rays; they are pre-accelerated inside the heliosphere and re-accelerated
at the solar-wind Termination Shock according to Pesses, Eichler, and Jokipii. The data indicate that the Inner Source of
pick-up ionsis largely responsible for the occurence of C+ in the Anomalous Cosmic Rays. The abundances of recently discovered Inner-Source Mg+ and Si+ are solar-wind like and consistent with their abundances in the energetic particles associated with Co-rotating Interaction
Regions. Knowledge on the injection and acceleration processes in Co-rotating Interaction Regions is applied to discuss the
current observational evidence for the Interplanetary Focusing Cone of the interstellar neutral gas due to the Sun's gravitational
force. The 25–150 keV/amu suprathermal 4He+ pick-up ion fluxes measured by CELIAS/STOF on board SOHO over 360° of ecliptic longitude represent a `local' ionization and acceleration of interstellar atoms at 1 AU or smaller heliocentric
distances. Completing the first limited data set of SULEICA/AMPTE on 4He+ pick-up ions they indicate a density enhancement in the Interplanetary Focusing Cone which is confirmed by recent SWICS/ACE
data. Clear evidence for signatures in ecliptic longitude are found in the data on energetic neutral H fluxes observed with
the CELIAS/HSTOF sensor on board SOHO. These fluxes are enhanced in the upstream and downstream directions of the interstellar
wind. Detection of energetic H atoms, which propagate unaffected by the Heliospheric Magnetic Field, provided for the first
time a diagnostic tool for observations near Earth to analyze the structure in ecliptic longitude of the interface region
between the heliosphere and the Local Interstellar Cloud.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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