1. California Institute of Technology, Pasadena, California 91125, USA
2. Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
3. NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
4. Department of Physics and Astronomy, New Mexico State University, Las Cruces, New Mexico 88003, USA

Correspondence to: Edward C. Stone¹ Correspondence and requests for materials should be addressed to E.C.S. (Email: ecs@srl.caltech.edu).

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, 10 au closer to the Sun than found by Voyager 1 in the north^{1, 2, 3, 4}. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field^{5, 6}, from transient-induced shock motion⁷, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma⁸, magnetic field⁹, plasma-wave¹⁰ and lower energy particle¹¹ observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock^{12, 13, 14} or in the heliosheath^{15, 16, 17, 18, 19}. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath²⁰ or the local interstellar Galactic cosmic ray intensity is lower than expected²¹.

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