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
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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.
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