Scientific Method / Science & Exploration

The laboratory at the end of the solar system

Last year, Voyager 2 crossed what is known as the termination shock, the …

Launched on August 20th, 1977, Voyager 2 was sent out to study the solar system and beyond. Just a hair over 30 years later, it reached what is known as the termination shock. The sun is constantly spewing out particles in all directions; as these particles move through the solar system, they are known as the solar wind. This wind pushes back against the interstellar plasma that exists throughout the galaxy.

At the end of the solar system, the solar wind finally begins to lose out and its speed drops below the speed of sound (relative to the interstellar medium), resulting in a roughly spherical shell known as the termination shock front. Almost 30 years to the day after it launched, between August 31st and September 1st of last year, Voyager 2 crossed the termination shock, and captured a great deal of information in the process.

This week's edition of Nature contains a series of five papers, plus a News and Views article, on the data sent back by Voyager 2 as it crossed the termination shock. When Voyager 1 crossed this region of space a few years prior, telemetry outages caused it to fail to send data back to Earth, leaving Voyager 2 as the only realistic hope for directly observing the physics that occur in this intermediate region of space (at least for many years to come). Each of the papers examined a slightly different type of measurement; since Voyager 2 had functional plasma sensors, magnetic field detectors, and a host of other equipment, it was able to directly gather a wealth of data to send back to Earth.

Given the difference in times and locations, it was expected that the two probes would encounter the termination shock at different radial locations. Before Voyager 2 crossed the termination shock, scientists observed interstellar hydrogen and helium flow directions; from these it was thought that the interstellar magnetic field is tilted about 60o relative to the flow direction. This should result in the southern portion of the termination shock being closer to the sun when compared to the northern portion of the shock. Voyager 2's crossing at about 45o to the south in heliolongitude terms confirmed this idea. The probe encountered the termination shock 10 AU closer to the sun than Voyager 1 did (one astronomical unit is the average distance between the Earth and the Sun). Voyager 2 crossed at 83.7 AU, while Voyager 1 crossed at 94.1 AU. When correcting for changes in solar wind pressure due to solar cycles, astronomers conclude that the termination shock has a seven to eight AU asymmetry in its shape.

Prior to the actual event, researchers had expected a fairly simple crossing, with the probe moving from a region where the solar wind was supersonic to one where it was subsonic. What they found instead was a "complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours." Instead of moving through the shock front once, Voyager 2 underwent five distinct crossings over a two day period. This was due to the dynamic nature of termination shock fluctuations; in fact, the time between crossings is on the order of magnitude expected for ripples propagating through the shock.

Various instruments searched for the force that is holding the shock front back, and it was found that energized "pickup ions" play a major role in the dynamics of the termination shock. These pickup ions are interstellar neutral atoms that are caught up in the solar wind and have become accelerated and ionized. One paper concluded that these ions account for more of the nonthermal partial pressure holding back the interstellar plasma than either the thermal plasma pressure or the pressure due to the magnetic field.

Other papers asked how similar this shock front is to other shock fronts in the solar system. Voyager 2 generated data that revealed the spectrum produced by the electric field of the plasma in the area near the termination shock. It was found to be qualitatively similar to the observed bow shocks upstream of Jupiter, Saturn, Uranus, and Neptune.

One difference between this set of observations and the data gather by Voyager 1 was that Voyager 1 saw anomalous cosmic rays as it neared the shock, and these did not peak when it crossed the shock. Voyager 2, on the other hand, saw no evidence of these anomalous cosmic rays, so their origin remains a mystery for now. Researchers feel that once both craft are clearly through to the heliosheath—the region beyond the termination shock—during the solar minimum, both can begin to measure the relative intensity gradient between the anomalous cosmic rays and more standard galactic cosmic rays.

Voyager 2's crossing of the termination shock represented a huge advance in our understanding of the geography of the solar system. Since no data was returned from Voyager 1, this represents the only realistic possibility for humanity to gather in situ data from this region of space for many years. While both probes may be beyond the influence of most of the solar wind, their time remaining in the heliopause will give them the chance to send back even more information that we already have.

Nature, 2008. DOI: 10.1038/nature07024
Nature, 2008. DOI: 10.1038/nature07030
Nature, 2008. DOI: 10.1038/nature07022
Nature, 2008. DOI: 10.1038/nature07029
Nature, 2008. DOI: 10.1038/nature07023

(DOI links will not go live until Thursday)

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