Effects in the inner heliosphere caused by changing conditions in the galactic environment

Abstract

Interstellar neutral gas flows through the inner heliosphere because of the Sun's relative motion with respect to the surrounding local interstellar medium. For contemporary medium conditions, interstellar He constitutes the largest neutral gas contributor in interplanetary space at Earth's orbit. This neutral gas is the source of a small, but noticeable population of pickup ions in the solar wind, which subsequently is injected into ion acceleration processes much more efficiently than solar wind ions. In fact, He+ has been found to be the third most abundant energetic ion species at 1 AU after H+ and He2+. During its history, the solar system must have encountered a variety of interstellar environments, including hot and dilute bubbles and much denser clouds, and will continue to do so in the future. We have studied the neutral gas and its secondary products in the inner heliosphere, such as pickup ions and energetic particles, for conditions that range from that of a hot bubble to dense clouds with densities up to 100 times the current value, allowing for variation in temperature and bulk flow velocity. We have used multi-fluid models of the global heliosphere and kinetic models of the He flow, to derive the spatial distribution of interstellar neutral H and He in the inner heliosphere. With this limitation the termination shock is at 8 AU, an d the Earth remains in the supersonic solar wind. Except for the case when the Sun is immersed in a hot bubble with no neutral gas, interstellar He maintains its dominant role in the inner heliosphere. However, even for the highest densities, the interstellar gas generally does not yet affect the solar wind dynamically at 1 AU, except for the region of the He focusing cone on the downwind side of the interstellar flow. Because dense clouds usually are also cold, a rather narrow cone with a drastically increased density develops, which acts like a huge and stationary comet in the sense that the solar wind would be decelerated to approximately the interstellar flow speed at 1 AU. Under such conditions energetic particles generated from interstellar pickup ions would dominate the energetic particle population at least during solar minimum. © 2006 Springer. All Rights Reserved.