Exiting the heliosphere

As the Sun plows through the local interstellar medium (ISM), the wind of supersonic charged particles continually emanating from its surface envelops the solar system in hot, low-density plasma. Called the heliosphere, this plasma bubble is presumed to have a distinct aspherical boundary with the ISM’s much cooler, higher-density plasma that comes within a few times 10¹⁰ kilometers of the Sun. (The orbital radius of Uranus is 5 × 10⁹ km.) To considerable acclaim, NASA reported in September that its Voyager 1 spacecraft, launched toward the outer planets 35 years ago, crossed that boundary “on or about” 25 August 2012, at a distance of 1.8 × 10¹⁰ km from the Sun. The principal evidence for the spacecraft’s emergence into the ISM, the first by any human artifact, is the expected abrupt 40-fold increase in plasma density, measured by the rise of the plasma’s density-dependent oscillation frequency. Simultaneous with the density step, Voyager’s cosmic-ray detectors recorded the abrupt disappearance of energetic charged particles from sources inside the heliosphere and a concomitant rise in the flux of cosmic rays from the rest of the galaxy. There was, however, one surprise. Straightforward models of the heliosphere posit that its magnetic field, generated by the Sun, is quite independent of the galactic magnetic field beyond the bubble. But Voyager’s magnetometer found no evidence of a change in the magnetic field’s direction. The paper, authored by the team that monitors the spacecraft’s plasma-wave detector, speculates that “the interstellar magnetic field may be linked to the Sun’s magnetic field by some mechanism … not completely understood.” (D. A. Gurnett et al., Science 341, 1489, 2013.)