Revealing Ribbon Riddles: From Ibex’s Discovery to What We Know Now

Abstract:

The first sky maps of energetic neutral atoms (ENAs) from the outer heliosphere, measured by the Interstellar Boundary Explorer (IBEX), were published in a 2009 special issue of Science. These maps revealed a bright, narrow Ribbon of ENAs encircling the sky that appears to be organized by the local interstellar magnetic field. Remarkably, this feature was totally unexpected and unpredicted by any prior theory or model. Since its discovery, over a dozen theories have been proposed to explain what physical phenomena might cause this remarkable feature. These possible explanations span from sources inside the termination shock at less than 100 AU to others some 1000 AU away - outside the heliosphere in the local interstellar medium. Here we 1) summarize the observational aspects of the Ribbon as established over IBEX’s first five full years of observations, 2) summarize the primary current ideas of what physical phenomena may be producing this feature, and 3) briefly examine the respective strengths and weaknesses of the various interpretations.