The Three-Dimensional Structure of the Inner Heliosphere

In this review we summarize our current knowledge regarding the three-dimensional structure of the quasi-steady, large-scale inner heliosphere. This understanding is based on the interpretation of a wide array of remote and in situ measurements, in conjunction with sophisticated numerical models. Observations by the Ulysses spacecraft, in particular, have provided an unprecedented set of measurements for more than 18 years, and observations by the STEREO spacecraft promise no less. Global MHD models of the solar corona and heliosphere have matured to the point that a wide range of measurements can now be reproduced with reasonable fidelity. In the absence of transient effects, this structure is dominated by corotating interaction regions which can be understood - to a large extent - from the consequence of solar rotation on a spatially-variable velocity profile near the Sun, leading to parcels of plasma with different plasma and magnetic properties becoming radially aligned. This interaction is one of the principal dynamic processes that shape the structure of the interplanetary medium. To illustrate some of these phenomena, we discuss the structural features of the current solar minimum, which has, thus far, displayed a number of distinct characteristics in relation to recent previous minima of the space age.