  Polstar is a proposed NASA MIDEX mission that would feature a high resolution
UV spectropolarimeter capable of measure all four Stokes parameters onboard a
60cm telescope. The mission would pioneer the field of time-domain UV
spectropolarimetry. Time domain UV spectropolarimetry offers the best resource
to determine the geometry and physical conditions of protoplanetary disks from
the stellar surface to <5 AU. We detail two key objectives that a dedicated
time domain UV spectropolarimetry survey, such as that enabled by Polstar,
could achieve: 1) Test the hypothesis that magneto-accretion operating in young
planet-forming disks around lower-mass stars transitions to boundary layer
accretion in planet-forming disks around higher mass stars; and 2) Discriminate
whether transient events in the innermost regions of planet-forming disks of
intermediate mass stars are caused by inner disk mis-alignments or from stellar
or disk emissions.
