Interstellar Matter and the Boundary Conditions of the Heliosphere

by P. C. Frisch

This talk was presented at the ACE science meeting (ACE=Advanced composition Explorer) held January, 1997, at Caltech in Pasadena, California.

The paper reference number of this paper on the Los Alamos server is: astro-ph/9804009

Postscript files:

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gzipped Text (0.13 Mb; on a unix machine unpack this with "gzip ace.ps")

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(HI)=0.22+/-0.06 cm^-3, and n(e-)~0.1 cm-3, with larger values allowed for n(HI) by radiative transfer considerations. Ulysses and Extreme Ultraviolet Explorer satellite HeI data yield a cloud temperature of 6,400 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 MgII and FeII 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 alpha Cen are or are not immersed in the same interstellar cloud. Please email me if downloading down't work

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