Distribution of hydrogen and production of energetic neutral hydrogen in the heliosphere.

Abstract

We have completed an analysis of the measurements of hydrogen Lyman-alpha resonant emission of the interplanetary gas performed by the Ultraviolet Spectrometer (UVS) aboard spacecraft Voyager 1 and Voyager 2 at 2-25 AU. Assuming the very local interstellar medium has a temperature around 10⁴ K, and a relative motion of 20-25 km/s with respect to the sun, and taking into account solar gravitation, radiation pressure, and ionization processes, the "hot" model of distribution for hydrogen atoms is found consistent with the observations of the Voyagers when the density of interstellar neutral hydrogen is ∼ 0.32 ± 0.08 cm⁻³ for the data of Voyager 1, and ∼ 0.22 ± 0.06 cm⁻³ for the data of Voyager 2. The modeling hydrogen density in VLISM determined by the observations of both Voyagers beyond 15 AU is independent of the chosen parameters, only the instrumental calibrations of both Voyagers after Jupiter and Saturn encounters could cause the discrepancy of the resulting densities. The VLISM hydrogen density reported her is more reliable than those previous observations (in Table 1.3), since those results were based on measurements in the heliocentric distance < 15 AU where model dependence is significant. The interaction between this interplanetary neutral gas and the various populations of interplanetary ions produces energetic neutral atoms, the flux of which provides us a means to investigate the dynamics and structure of the heliosphere, such as the planetary magnetosphere, the corotation interaction regions, the solar flare events, the quiet-time space, and the termination shock. Simple modeling shows that energetic neutral hydrogen (ENH) from these proton populations have distinct signatures and that the detection of these particles can reveal energy, spatial, and temporal distributions of the protons in and out of the ecliptic plane, thus providing information on the acceleration and propagation of ions in three-dimensional interplanetary space, including the solar-wing termination shock.