Interstellar Matter and the Boundary Conditions of the Heliosphere |
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Authors: | Frisch Priscilla C |
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Institution: | (1) Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, 60637, U.S.A. |
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Abstract: | The interstellar cloud surrounding the solar system regulates the galactic environment of the Sun, and determines the boundary
conditions of the heliosphere. Both the Sun and interstellar clouds move through space, so these boundary conditions change
with time. Data and theoretical models now support densities in the cloud surrounding the solar system of n(H0)=0.22±0.06
cm−3, and n(e−)∼0.1 cm−3, with larger values allowed for n(H0) by radiative transfer considerations. Ulysses and Extreme Ultraviolet
Explorer satellite He0 data yield a cloud temperature of 6400 K. Nearby interstellar gas appears to be structured and inhomogeneous.
The interstellar gas in the Local Fluff cloud complex exhibits elemental abundance patterns in which refractory elements are
enhanced over the depleted abundances found in cold disk gas. Within a few parsecs of the Sun, inconclusive evidence for factors
of 2–5 variation in Mg+ and Fe+ gas phase abundances is found, providing evidence for variable grain destruction. In principle,
photoionization calculations for the surrounding cloud can be compared with elemental abundances found in the pickup ion and
anomalous cosmic-ray populations to model cloud properties, including ionization, reference abundances, and radiation field.
Observations of the hydrogen pile up at the nose of the heliosphere are consistent with a barely subsonic motion of the heliosphere
with respect to the surrounding interstellar cloud. Uncertainties on the velocity vector of the cloud that surrounds the solar
system indicate that it is uncertain as to whether the Sun and α Cen are or are not immersed in the same interstellar cloud.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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