Where do field lines go in the quiet magnetosphere?

The state of our knowledge is reviewed concerning the global pattern of geomagnetic field lines. Sources of information on that pattern include (1) magnetic field models, derived directly from magnetic data or indirectly from general observed properties and from physics, (2) the tracing of magnetospheric features, e.g., polar cusps or the inner edge of the plasma sheet, (3) matching of magnetic flux, and (4) analysis of magnetic fields. Field line structure inside about 8 Earth radii is known fairly well, but beyond that, especially in the tail, the situation becomes rather uncertain and variable. Two particularly difficult problems are (1) the linkage between open field lines and the interplanetary field and (2) the field line structure of the “quiescent” magnetosphere following periods of prolonged northward Bz.