Solar System bent out of shape

The Solar System is not round, but has an asymmetric, squashed shape, according to recent data streamed back from the Voyager 2 spacecraft. The results are reported as part of a series of papers in this week’s Nature analyzing recent observations from the outer limits of the Solar System, and help build a picture of how the Sun interacts with the rest of the Galaxy.

Launched in 1977, the Voyager spacecraft were originally sent to fly by and observe Jupiter and Saturn. The two probes eventually continued their mission into the outer Solar System, with Voyager 1 becoming the most distant man-made object in space in the 1990s. Operating in remote, cold and dark conditions, and powered by long-life nuclear batteries in the absence of solar energy, the probes are still transmitting data back to Earth, over thirty years after they were first launched and long after their original missions ended.

The current mission of both spacecraft is to reach and study the outer limits of the heliosphere — a magnetic ‘bubble’ around the Solar System created by the solar wind. The transition zone between the heliosphere and the rest of interstellar space is known as the ‘termination shock’ and marks the boundary for the slowing of the solar wind from supersonic to subsonic speeds.

Voyager 2 crossed this boundary much closer than Voyager 1 and experienced very different conditions: this difference between the crossings has provided the research teams with some valuable comparisons, particularly concerning the behavior of ‘pickup’ ions and other energetic particles in the solar winds. These energetic particles are found in high intensity on either side of the termination shock and this series of papers reveals much about their role in dissipating the energy of the solar wind as it passes through the boundary.

Edward Stone from the California Institute of Technology and his colleagues report that Voyager 2 crossed the termination shock closer to the Sun than expected, suggesting that the heliosphere in this region is dented, or pushed in, closer to the Sun by a local magnetic field.

Companion papers explore more details of Voyager 2’s crossing of the termination shock, analyzing the plasma, magnetic field, plasma-wave and lower-energy particle observations.

John Richardson from the Massachusetts Institute of Technology and Chinese Academy of Sciences and colleagues report an unexpected finding: most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.

Leonard Burlaga from NASA and his team describe magnetic field observations revealing a complex shock of moderate strength that reorganizes on a scale of a few hours, rather than the expected days.

Donald Gurnett and William Kurth from the University of Iowa report that they have detected intense plasma-wave electric fields at the solar wind termination shock. Electric fields of these waves help to dissipate energy in the shock and drive the pickup ions back towards thermal equilibrium. They also observed that these waves were similar in size to those observed at other shock sites such as those upstream of Jupiter, Saturn, Uranus and Neptune.

Robert Decker from The Johns Hopkins University Applied Physics Laboratory and his team study the changes in electron and ion intensity observed by Voyager 2. They suggest that acceleration of ions extracts a large portion of bulk-flow kinetic energy from the solar wind.

Finally, Linghua Wang from the University of California and colleagues report the detection and mapping of energetic neutral atoms produced by charge exchange between ions and neutral atoms. These energetic neutral atoms disperse much of the energy associated with the shock wave of the solar wind hitting the boundary.

Commenting in a related ‘News and Views’ article, J.R. Jokipii from the University of Arizona notes that the termination shock is very complex, and many ambiguities remain. “It will be a long time before we receive more in situ data, but — fortunately — remote observations from the inner heliosphere should fill some gaps,” he says.