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For any plasma to leave the Sun and reach space, it must travel along the magnetic field. Any study of how the Sun exerts its influence in space must, therefore, address the connection of the solar magnetic field far into the solar system.

The large scale structure of the interplanetary magnetic field (IMF) is well known (e.g Mariani and Neubauer, 1990; Smith, 2008): the Sun's rotation winds the field into the Parker spiral; compression and rarefaction of the plasma in co-rotating interaction regions (CIRs) produces increases and decreases of the field strength; the polarity of the solar source field is reflected in that of the IMF; and the field is pervaded by waves and turbulence over a wide range of scales. Over the Sun's 22 years magnetic cycle, the IMF reflects the changing character of the solar field, from approximate dipole to a much more complex, multi-pole structure. 

However, the mapping between solar and interplanetary fields is only known on relatively large scales and in a crude manner. Observations of the Sun's surface photospheric magnetic field combined with coronal observations or MHD models of the corona, make it possible to estimate the mapping between the lower corona and the “source surface” at several solar radii, but many simple questions remain about how the Sun's magnetic field opens into space (e.g. Antiochos et al., 2007), particularly with regard to the emergence of new coronal holes and the long-range connectivity of active regions, as well as how the IMF disconnects from the Sun. Distant observations by Ulysses over the Sun's poles have helped to constrain such mappings (e.g. Hoeksema, 1995; Forsyth et al., 1997) but Solar Orbiter, being much closer to the Sun and hence eliminating many of the uncertainties caused by local stream-stream interactions, will dramatically improve the precision with which this can be constrained. 

Beyond the source surface, MHD models must be used. These models will be greatly constrained by Solar Orbiter magnetic field data, with the important consequence of improving the systematic prediction ability of such models throughout the heliosphere. This connection is also essential for many elements of the Solar Orbiter science objectives of linking solar and interplanetary phenomena. 


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