Cosmic Ray Anisotropy and Interstellar Spectra

Various measurements with ground- and space-based detectors have revealed the astrophysical relevance of a correct understanding of heliospheric physics. Most important for the proposed project are those observations related to outer bundary of the heliosphere. First, the determination of the local interstellar spectra of cosmic rays is improved continuously with the Voyager spacecraft. Second, the Voyagers and, particularly, the IBEX mission have increased our quantitative knowledge about the local interstellar magnetic field, plasma, and neutral gas surrounding the heliosphere. Both have triggered a refined understanding of cosmic ray modulation. Third, the hypothesis that the observed cosmic ray anisotropy in the low-TeV range is partly caused by the heliospheric structure, has recently gained new support from measurements with several terrestrial large area particle detectors and telescopes, as well as from theoretical considerations regarding, for example, magnetic reconnection in the heliotail. In order to judge on the various suggestions for the correct physical description of the outer heliosphere and its astrophysical relevance, we will develop quantitative models accounting for the large-scale structure of the heliosphere and its interstellar vicinity, particularly including the so-called heliosheath and heliotail as well as the local interstellar magnetic field. Consequently, the main objectives of the present proposal are defined as follows: We will extend our previous modelling of the large-scale heliosphere embedded in the interstellar medium by self- consistently including magnetic fields - especially with respect to the highly elongated heliospheric tail - and we will study the corresponding cosmic ray flux by solving both the transport equation and the equation of motion for these particles. Thus, we can test whether the heliotail indeed affects the anisotropy of the cosmic ray flux in the low-TeV range. Furthermore, this model allows us to study the effect of outer heliospheric structures on the local interstellar cosmic ray spectra. For these applications we will introduce new, so-called logically rectangular grids into heliospheric modelling since the presently used ones have severe shortcomings. These new grids have the advantages of being locally adjustable to the problem`s geometry, avoiding both coordinate singularities and strongly differing cell sizes. The project results will consist of a modeling of our direct astrophysical environment, that will allow, for the first time, a quantitative interpretation of potential heliospheric signatures in astrophysical observations.

The so called heliosphere is a region around the Sun, where the solar wind a super- sonic plasma outflow from the Sun pushes a cavity into the interstellar medium. The present project mostly investigated the physics of the outer heliosphere and its interstellar environment. The focus was on the dynamical motion of the gas in this region and also on the transport of energetic particles in this environment. Within this project, we developed both analytical and computational models for the description of the large-scale structure with regard to the gas dynamics. In this context, it was particularly important to correctly model the interaction of the different gas phases. This included, e.g., interstellar neutral gas and the ionised solar wind. For this, we compiled the relevant numerical modelling tools and the relevant physical description to investigate the consequences of these interactions. As an example, we found new physical hints to strengthen the assumption that there might be a bow shock in the interstellar medium upstream of the heliosphere. Apart from that, we used our numerical models to verify new analytical models, developed within the project, for the gas structure of the heliosphere and to model the dynamics of the solar wind including shock waves and turbulence. We also took into account recent developments regarding numerical grids. In the future, this will allow us to use numerical grids that are adapted to the physical setup. For the modelling of energetic particle transport we focused on the outer helio- sphere. There, we investigated changes in the distribution function of these particles. We found not only a possible decrease in the flux of the particles, but we also in- vestigated corresponding anisotropies in the distribution function.