  The interaction between a YSO stellar magnetic field and its protostellar
disc can result in stellar accretional flows and outflows from the inner disc
rim. Gas flows with a velocity component perpendicular to disc midplane subject
particles to centrifugal acceleration away from the protostar, resulting in
particles being catapulted across the face of the disc. The ejected material
can produce a "dust fan", which may be dense enough to mimic the appearance of
a "puffed-up" inner disc rim. We derive analytic equations for the time
dependent disc toroidal field, the disc magnetic twist, the size of the stable
toroidal disc region, the jet speed and the disc region of maximal jet flow
speed. We show how the observed infrared variability of the pre-transition disc
system LRLL~31 can be modelled by a dust ejecta fan from the inner-most regions
of the disc whose height is partially dependent on the jet flow speed. The
greater the jet flow speed, the higher is the potential dust fan scale height.
An increase in mass accretion onto the star tends to increase the height and
optical depth of the dust ejection fan, increasing the amount of 1--8~$\mu$m
radiation. The subsequent shadow reduces the amount of light falling on the
outer disc and decreases the 8-- 40~$\mu$m radiation. A decrease in the
accretion rate reverses this scenario, thereby producing the observed "see-saw"
infrared variability.
