Introduction
The heliosphere is a large region of space where the solar wind
interacts with the surrounding interstellar gas. The plasma (ionized)
component of the
interstellar medium interacts directly with the solar wind. The
physics of the heliosphere is made considerably more complex, however,
by
the presence of neutral hydrogen in the interstellar medium. It behaves
very differently from the plasma; both species, however, are weakly
coupled to each other. A nice introduction and review of the
heliosphere by Ian Axford and Steve Suess may be found
on
this web page, which is part of pages dedicated to the Voyager
Interstellar Mission.
We carry out large-scale computer
simulations to model this region. A typical simulation output is shown
here, with the Sun at the origin and the interstellar wind coming from
the right. Distances are in AU, the Sun-Earth distance; the orbit of
the farthest planet (Pluto) is outlined as dotted circle. The upper
panel shows the plasma temperature and reveals boundaries like the
termination shock and the interstellar bow shock. The fine structure
results from the solar wind varying over an 11 year period. The lower
panel is the density of the other species, neutral hydrogen. The
interaction with the plasma gives rise to an overdensity, the hydrogen
wall.
VOYAGER summary
Only a few spacecraft have left the solar system, among them Voyager 1
and 2 who are about to probe the first heliospheric boundary, the
termination shock. The Voyager trajectories out to the year 2030 have
been overlaid onto the plot.
Voyagers 1 and 2 were both launched in 1977. Voyager 1 encountered
Jupiter in 1979, and Saturn in 1980. Voyager 2 encountered Jupiter in
1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Both
spacecraft are now headed out of the solar system, but are still within
the supersonic solar wind. As of early 2004, Voyager 1 was 91 AU and
Voyager 2 was 73 AU from the sun, and Voyager 1 was likely close to the
termination shock for a period of 6 months in early 2003 before the
termination shock moved away (outward) from it. The Voyager 1 and 2
spacecraft should reach both these boundaries before they run out of
the hydrazine fuel used to stabilize the spacecraft. On the way out
these spacecraft are making new measurements of the solar wind
conditions far from the Sun.
General Structure
The Bow Shock is the line where the interstellar plasma is decelerated
from supersonic to subsonic velocities. Whether or not the interstellar
medium is
supersonic is an open question, and consequently is the existence of
the bow shock.
The supersonic Solar Wind terminates at the
Termination Shock where it becomes subsonic, shock-heated,
and starts flowing tailward
to the heliotail. A contact discontinuity, the Heliopause, separates
solar wind plasma and interstellar plasma.
The interaction of the protons of the plasma
with neutral hydrogen, to leading order, is charge exchange whereby the
neutral atom loses its electron to the proton, making the latter a
newly born
neutral hydrogen atom. Charge exchange
decelerates the neutrals, filters the incoming neutrals,
and creates a hydrogen wall, an accumulation of neutral hydrogen (lower
panel) that also is heated and decelerated as compared to the ISM.
Numerical Modeling
The global heliosphere is modeled numerically by solving a set of
differential equations (MHD) for the plasma. Solving for the neutrals
is a more complicated problem, as the charge exchange process occurs
frequently enough to make a difference in the heliosphere, resulting in
the hydrogen wall and other effects, but infrequent enough so that
overall neutral hydrogen is out of equilibrium in the heliosphere and
needs to be described not be a single fluid, but on a more fundamental
particle level. We carry out these large-scale simulations of the
global heliosphere, with multifluid codes or with kinetic particle
codes on MPI parallel platforms.
The simulation code is applied also to other cool stars (similar to
the Sun) interacting with their neighboring gas, giving rise to
astrospheres.
Astrospheres of nearby stars can be detected by a minute absorption
component in the stellar light due to the star's hydrogen wall.
This absorption component has been detected in a dozen high-resolution
UV spectra obtained by the Hubble Space Telescope.
The physics of neutral/plasma interaction will be applied to other
suitable astrophysical winds in the future.
The dynamical heliosphere
The solar wind is not steady, and its temporal variations are
introducing
important disturbances into the heliosphere and its boundaries. The
figure above is a time-snapshot from within one cycle. Example movies
that demonstrate the effect of a solar wind whose speed
varies sinusoidally with an 11 year cycle are shown on
this page.
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