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.

Heliosphere overviewWe 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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