  Since its discovery in 1996, the source of the bright H-alpha emission (up to
750 mR) along the Magellanic Stream has remained a mystery. There is no
evidence of ionising stars within the HI stream, and the extended hot halo is
far too tenuous to drive strong shocks into the clouds. We now present a
hydrodynamical model that explains the known properties of the H-alpha emission
and provides new insights on the lifetime of the Stream clouds. The upstream
clouds are gradually disrupted due to their interaction with the hot halo gas.
The clouds that follow plough into gas ablated from the upstream clouds,
leading to shock ionisation at the leading edges of the downstream clouds.
Since the following clouds also experience ablation, and weaker H-alpha
(100-200 mR) is quite extensive, a disruptive cascade must be operating along
much of the Stream. In our model, the clouds are evolving on timescales of
100-200 Myr, such that the Stream must be replenished by the Magellanic Clouds
at a fairly constant rate. The ablated material falls onto the Galaxy as a warm
drizzle which suggests that diffuse ionized gas at 10**4 K may be an important
constituent of galactic accretion. The observed HI emission provides a new
constraint on the rate of disruption of the Stream and, consequently, the
infall rate of metal-poor gas onto the Galaxy. When the ionized component of
the Stream is fully accounted for, the rate of gas accretion is 0.4 Msun/yr,
roughly twice the rate deduced from HI observations alone.
