  The magnetic fields of young stars set their coronal properties and control
their spin evolution via the star-disc interaction and outflows. Using 14
magnetic maps of 10 classical T Tauri stars (CTTSs) we investigate their closed
X-ray emitting coronae, their open wind-bearing magnetic fields, and the
geometry of magnetospheric accretion flows. The magnetic fields of all the
CTTSs are multipolar. Stars with simpler (more dipolar) large-scale magnetic
fields have stronger fields, are slower rotators, and have larger X-ray
emitting coronae compared to stars with more complex large-scale magnetic
fields. The field complexity controls the distribution of open and closed field
regions across the stellar surface, and strongly influences the location and
shapes of accretion hot spots. However, the higher order field components are
of secondary importance in determining the total unsigned open magnetic flux,
which depends mainly on the strength of the dipole component and the stellar
surface area. Likewise, the dipole component alone provides an adequate
approximation of the disc truncation radius. For some stars, the pressure of
the hot coronal plasma dominates the stellar magnetic pressure and forces open
the closed field inside the disc truncation radius. This is significant as
accretion models generally assume that the magnetic field has a closed geometry
out to the inner disc edge.
