Observations improve understanding of the heliosphere

The outer regions of the heliosphere, a giant bubble around the solar system created by solar wind, are difficult to study directly. The Interstellar Boundary Explorer (IBEX) mission, which maps emission of energetic neutral atoms from the boundary between the heliosphere and the interstellar medium, has greatly contributed to understanding of the heliosphere and even discovered some unexpected features. In particular, IBEX maps showed a strange “ribbon” of enhanced energetic neutral atom emission that was not predicted by any theory. The origins of this ribbon, which stretches more than 300° across the sky, remain unknown. McComas et al. review IBEX observations, current understanding of the heliosphere, hypotheses that have been proposed to explain the ribbon, and directions for future research. (Geophysical Research Letters, doi:10.1029/2011GL048763, 2011)