A POSSIBLE GENERATION MECHANISM FOR THE IBEX RIBBON FROM OUTSIDE THE HELIOSPHERE

The brightest and most surprising feature in the first all-sky maps of energetic neutral atom (ENA) emissions (0.2-6 keV) produced by the Interstellar Boundary Explorer (IBEX) is an almost circular ribbon of a ∼1400 opening angle, centered at (l, b) = (330, 550), covering the part of the celestial sphere with the lowest column densities of the Local Interstellar Cloud (LIC). We propose a novel interpretation of the IBEX results based on the idea of ENA produced by charge exchange between the neutral H atoms at the nearby edge of the LIC and the hot protons of the Local Bubble (LB). These ENAs can reach the Sun's vicinity because of very low column density of the intervening LIC material. We show that a plane-parallel or slightly curved interface layer of contact between the LIC H atoms (n H = 0.2 cm-3, T = 6000-7000 K) and the LB protons (n p = 0.005 cm-3, T ∼ 106 K), together with an indirect contribution coming from multiply scattered ENAs from the LB, may be able to explain both the shape of the ribbon and the observed intensities, provided that the edge is <(500-2000) AU away, the LIC proton density is (correspondingly) < (0.04-0.01) cm-3, and the LB contains ∼1% of non-thermal protons over the IBEX energy range. If this model is correct, then IBEX, for the first time, has imaged in ENAs a celestial object beyond the confines of the heliosphere and can directly diagnose the plasma conditions in the LB.