The propagation of energetic charged particles in the heliospheric magnetic field is one of the fundamental problems in heliophysics. In particular, the structure of the heliospheric magnetic field remains an unsolved problem and is discussed as a controversial topic. The first successful analytic approach to the structure of the heliospheric magnetic field was the Parker field. However, the measurements of the Ulysses spacecraft at high latitudes revealed the possible need for refinements of the existing magnetic field model during solar minimum. Among other reasons, this led to the development of the Fisk field. This approach is highly debated and could not be ruled out with magnetic field measurements so far. A promising method to trace this magnetic field structure is to model the propagation of electrons in the energy range of a few MeV. Employing three-dimensional and time-dependent simulations of the propagation of energetic electrons, this work shows that the influence of a Fisk-type field on the particle transport in the heliosphere leads to characteristic variations of the electron intensities on the timescale of a solar rotation. For the first time it is shown that the Ulysses count rates of 2.5-7 MeV electrons contain the imprint of a Fisk-type heliospheric magnetic field structure. From a comparison of simulation results and the Ulysses count rates, realistic parameters for the Fisk theory are derived. Furthermore, these parameters are used to investigate the modeled relative amplitudes of protons and electrons, including the effects of drifts.

With the Interstellar Boundary Explorer (IBEX) the measurement of energetic neutral atoms (ENAs) will enable a remote sensing of the heliospheric boundary region where the solar wind plasma flow meets the interstellar medium. Together with Voyager 1 (& 2) in (and at least near) the healiosheath there will be a very fortunate instrument configuration in space that will allow us to obtain valuable information about the large-scale structure and dynamics of the heliosphere. Following ongoing theoretical discussions we have computed all-sky maps of the differential flux of ENAs produced from charge exchange with solar wind protons in a three-dimensionally structured and time-varying heliosheath. We employ a three-dimensional, time-dependent hydrodynamic model heliosphere to compute the production of ENAs by a generalisation of previous two-dimensional approaches. We find three-dimensionally structured all-sky ENA flux maps that are time-varying due to the solar activity cycle. While the study adds to the complexity of the task to correctly interpret the forthcoming measurements to be made with IBEX, it underlines the potential of this mission to significantly increase our understanding of the structure and dynamics of the heliosphere.

The local interstellar spectrum (LIS) is one of the most important but unknown parameters used in all modeling efforts to describe the modulation of Galactic cosmic rays on their way from the galaxy through a possible bow shock, heliosheath, and heliosphere toward the Earth. Because it has not been measured thus far, several LIS models derived from numerical simulations or data on Earth were developed. A new method to determine the LIS was introduced when the Voyager spacecraft crossed the termination shock and entered the heliosheath. Webber & Higbie derived a new LIS, which is lower than all previous LIS models over the entire energy range, on the basis of these measurements. Numerical simulations by Scherer et al. showed that particles already in the outer heliosheath (OHS) are modulated, suggesting that the LIS by Webber & Higbie is a heliopause spectrum (HPS) rather than the "true" LIS. By using the same simplified simulation model, we estimate the diffusion coefficient in the OHS to be consistent with several 10 26 to 10 27 cm 2 s −1 for all LIS models under consideration by mapping them to this HPS and conclude that the Voyager measurements will not be able to determine the LIS in the near future. We then discuss the circumstances under which the terrestrial archive can be used to at least exclude LIS models, unless one awaits a dedicated mission like e.g., the Interstellar Probe.

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