Information about the HMF can be obtained through a variety of indirect means, as discussed in Sections 2.2 and 3, but the bulk of our understanding comes from spacecraft-borne magnetometers, which make in situ observations of the of HMF. The first observations of the near-Earth solar wind were made by the Mariner spacecraft in the early 1960s. Subsequent spacecraft in near-Earth space have provided a reasonably complete record of the near-Earth heliospheric magnetic field since 1965. The OMNI dataset (see Section 5.3) collates the near-Earth solar wind measurements from numerous spacecraft. A full review of all heliospheric spacecraft is beyond the scope of this review, but there are a number which bear particular note as they form the basis of much of the discussion in the rest of the paper. Pioneer 10 and 11 (Smith et al., 1975), launched in the early 1970s, were the first spacecraft to explore beyond 1 AU. While contact has been lost, Pioneer 10 was tracked to nearly 80 AU. Voyager 1 and 2 (Behannon et al., 1977) were launched in 1977. Both have scientific instruments still operating. Voyager 1 crossed the termination shock in 2004 at 94.5 AU and recently became the first spacecraft to cross the heliopause at 121.6 AU and enter interstellar space. Voyager 2, following behind, crossed the termination shock at 84 AU in 2007. See Section 2.6 for further detail. Helios 1 and 2 (Scearce et al., 1975), launched in 1974 and 1976, explored the inner heliosphere in the ecliptic plane between 0.3 and 1 AU from the Sun. Ulysses (Balogh et al., 1992), launched in 1990 into an approximately 6-year orbit of the Sun inclined at 80.2° to the solar equator, with perihelion at 1.3 AU and aphelion at 5.4 AU. It was the first spacecraft to explore the 3-dimensional structure of the heliosphere over a large latitude range. Operations ceased in 2009 after nearly 3 orbits. Finally, STEREO (Acuña et al., 2008), launched in 2006, consists of two spacecraft at 1 AU separating in solar longitude ahead of and behind the Earth. They carry instrumentation aimed at obtaining stereoscopic views of the Sun and making multi-point in-situ measurements of the solar wind and HMF.
There have been a number of excellent reviews of the HMF (e.g., Balogh and Erdős, 2013; Zurbuchen, 2007), particularly focussed on the three-dimensional structure revealed by the Ulysses spacecraft (Smith, 2008). Here, we hope to incorporate observations from the most recent solar cycle and put it in context of the long-term evolution of the HMF. Recent models of HMF evolution will also be discussed. Section 2 introduces the steady-state heliosphere, an approximation most valid when the solar corona is slowly evolving over a solar rotation period, such as times close to solar minimum. Section 3 briefly discusses particle probes of the HMF, as these underpin our understanding of transient HMF structures summarised in Section 4. Section 5 discusses the evolution of the HMF over the solar cycle, including the long-term variation of the HMF inferred from proxy data.