  EXor outbursts - moderate-amplitude disk accretion events observed in Class I
and Class II protostellar sources - have time scales and amplitudes that are
consistent with the viscous accumulation and release of gas in the inner disk
near the dead zone boundary. We suggest that outbursts are indirectly triggered
by stellar dynamo cycles, via poloidal magnetic flux that diffuses radially
outward through the disk. Interior to the dead zone the strength of the net
field modulates the efficiency of angular momentum transport by the
magnetorotational instability. In the dead zone changes in the polarity of the
net field may lead to stronger outbursts because of the dominant role of the
Hall effect in this region of the disk. At the level of simple estimates we
show that changes to kG-strength stellar fields could stimulate disk outbursts
on 0.1 AU scales, though this optimistic conclusion depends upon the uncertain
efficiency of net flux transport through the inner disk. The model predicts a
close association between observational tracers of stellar magnetic activity
and EXor events.
