The recent Voyager news, reported from the American Geophysical Union conference in San Francisco and recently discussed here, has drawn attention to the apparent asymmetry of our Solar System. Voyager 1 crossed the termination shock — where the solar wind first encounters the thin gas of the interstellar medium — some three years ago. But that was a crossing with a difference. Voyager 1 went through the termination shock just once. Voyager 2 has apparently crossed it five times and may encounter it again. Ahead in a decade or so: The heliopause, where the Sun’s influence effectively ends.
Thus we have a glimpse of how the solar wind varies with changes in the Sun’s activity level, pulsating as the solar cycle swings from solar flares into quiet periods, pushing the shock area out a bit farther, then contracting it. And while Voyager 1’s plasma science instrument had stopped working when it encountered the termination shock, Voyager 2’s is working well and making detailed measurements.
John Richardson, principal investigator for the instrument, calls the area Voyager 2 is now in “…a different kind of shockwave than we’ve seen anywhere else.” Among the new Voyager findings is the existence of an unexpectedly strong magnetic field in the surrounding interstellar region, one that is distorting the bubble of outflowing gases from the Sun. A second surprise: The temperature just outside the termination shock, hotter than inside it, was nonetheless found to be fully ten times cooler than had been expected. This may be the result of limitations in the plasma instrument, though other ideas may emerge.
J. Randy Jokipii (University of Arizona) is already looking farther out:
“The Holy Grail for me will be when the spacecraft begin traveling in pure interstellar space. That’s maybe 10 years away. Scientists now can start thinking about what they want to look for when the Voyagers break through the last barriers to true interstellar space.”
Yes, providing we’re still getting good data from our doughty spacecraft, which will probably be the case. Our first look at actual interstellar space, the medium through which, let’s hope, our interstellar probes will one day fly, will come as a triumphant vindication for the Voyagers, both of them built for a five year mission. John Belcher and Mark Bessette at MIT have put together several nifty animations showing the solar wind interacting with the interstellar medium. Keep them in mind as the Voyagers push on through the heliosheath on the way to their final exit of the system.