Observations of the solar wind and interstellar pick-up ion populations in the heliosphere with ULYSSES
Abstract
The solar wind (SW) and interstellar pick-up ions (IPI) are the dominant ion populations in the heliosphere. While SW ions are by far the most abundant at low thermal energies and closer to the sun, IPI ions with thermal speeds exceeding the SW bulk speed become dynamically more and more important beyond a few AU. Measurements of the characteristics (composition, charge states and distribution functions) of these ion populations provide essential information about (1) solar abundances, acceleration of the solar wind and conditions in the corona, (2) local heliospheric processes that shape the distribution functions of SW ions and IPI, (3) conditions in the outer heliosphere, including acceleration processes that produce the anomalous cosmic rays, and (4) characteristics and composition of the local interstellar cloud. The orbit and timing of Ulysses is particularly well suited for these observations: Polar passes of Ulysses at solar minimum allow us to study the solar wind from polar coronal holes under the simplest conditions. Extended time periods at heliocentric distances between ~3 and 5 AU make it possible to observe in addition to He^+ a number of new IPI species (H^+, O^+, N^+, Ne^+, and He^++). We will discuss a number of results based on ion composition and energy spectra measured from 0.6 to 60 keV/e with the time-of-flight vs. energy Solar Wind Ion Composition Spectrometer (SWICS). We find that (a) the composition of the solar wind from coronal holes is least biased compared to the solar photospheric composition and (b) the composition in the interstellar medium deduced from IPI measurements resembles to better than a factor of two solar system abundances. These observations show that SW and IPI are a relatively unbiased samples of the solar and interstellar material respectively which can be used to study the isotopic and chemical compositions in these remote regions.
- Publication:
-
Advances in Space Research
- Pub Date:
- August 1995
- DOI:
- 10.1016/0273-1177(95)00360-Q
- Bibcode:
- 1995AdSpR..16i.343G
- Keywords:
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- Coronal Holes;
- Heliosphere;
- Ion Density (Concentration);
- Solar Wind;
- Ulysses Mission;
- Abundance;
- Distribution Functions;
- Energy Spectra;
- Solar Wind Velocity;
- Thermal Energy;
- Solar Physics