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Supra-thermal neutral atoms such as H and He have been detected with the particle instrument CELIAS/HSTOF onboard SOHO since 1996. While the observed energetic neutral hydrogen (ENH) fluxes are well distinct from background ion fluxes, the helium (ENHe) fluxes have to be corrected for the local He ion and alpha particle fluxes in the same energy region. The latter ion background originates from well known sources such as solar particle events or co-rotating interaction regions and the ion particle flux and spectrum is a function of time. The resulting H/He ratio in the supra-thermal energy region is lower than expected from the anomalous cosmic ray abundances. Heavier neutral particles are not identified in the CELIAS/HSTOF dataset. We will discuss the neutral energetic flux measurements in view of the H/He ratio and potential source regions from the heliosheath region and beyond. [ABSTRACT FROM AUTHOR]

In the years 2004 and 2007, the instruments onboard the Voyager 1&2 spacecraft delivered unprecedented data on the structure of the solar wind termination shock. This shock has been assumed to be responsible for the acceleration of the anomalous component of cosmic rays (ACRs) from a pick-up ion seed population derived from the interstellar neutral gas penetrating the heliosphere. In this article, we present model results on the evolution of solar wind turbulence and suprathermal tails with heliocentric distance to support the hypothesis that stochastic acceleration all over the heliosphere but in particular in the heliosheath is an important process to energize the ACRs if not the most important one. [ABSTRACT FROM AUTHOR]

Based on observations near Earth’s orbit with the CELIAS/STOF sensor on board SOHO and with the ACE magnetometer, and on the model of Zank et al. we calculate the evolution of transport parameters of energetic particles over heliocentric distance. From the calculated transport parameters we estimate the spatial distribution of termination shock energetic particles (TSPs) and of the Anomalous Cosmic Rays (ACRs) near the termination shock and in the heliosheath. In agreement with Voyager 1 observations we find that the TSP/ACR pressure is of the order of a significant fraction of the dynamical pressure of the neutral gas component of the interstellar medium. Therefore, the solar wind termination shock should be a cosmic-ray mediated shock. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Energetic hydrogen and helium atoms have been identified and their fluxes monitored by the High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 since 1996. The potential sources of energetic neutral atoms (ENAs) in the heliosphere are co-rotating interaction regions (CIRs), solar energetic particle events, pre-accelerated pickup ions as well as low-energy (up to few hundred keV) anomalous cosmic ray (ACR) ions in the outer heliosphere, close to and beyond the solar wind termination shock. The ENAs, neutralized via charge transfer reactions, can penetrate into the inner solar system, unaffected by the interplanetary magnetic field. The observed ENA fluxes in the apex direction of the heliosphere set limits on potential theories of the dominant sources of the energetic neutral atoms and we discuss implications of these ENA observations on modeling of the heliosheath. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]