Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions

Unexpected Magnetic Highway The heliopause is thought to separate the heliosphere (the bubble of plasma and magnetic field originating at the Sun) from interstellar plasma and magnetic field. In August last year, the Voyager 1 spacecraft, which was launched 35 years ago, was 18.5 billion kilometers away from the Sun, close to the expected location of the heliopause. Krimigis et al. (p. 144, published online 27 June) report observations of energetic ions and electrons by Voyager 1 that suggest that a sharp and distinct boundary was crossed five times over ∼30 days. Burlaga et al. (p. 147, published online 27 June) found that the magnetic field direction did not change across any of the boundary crossings, indicating that Voyager 1 had not crossed the heliopause but had entered a region in the heliosphere that serves as a magnetic highway along which low-energy ions from inside stream away and galactic cosmic rays flow in from interstellar space. Stone et al. (p. 150, published online 27 June) report the spectra of low-energy galactic cosmic rays in this unexpected region. The Voyager 1 spacecraft entered an unexpected region of the heliosphere at the boundary with interstellar space. On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the previous 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 106 electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 106 to 40 × 106 electron volts per nucleon and an increasing galactic cosmic-ray electron intensity down to ~10 × 106 electron volts.