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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.
Abstract
Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈0.2 to ≈0.4 nanotesla, and B remained near 0.4 nanotesla until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 mega–electron volt per nuclear particle. The direction of B did not change significantly across any of the five boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath (the heliosheath depletion region), rather than the interstellar medium.
