  We present evidence that the accretion of warm gas onto the Galaxy today is
at least as important as cold gas accretion. For more than a decade, the source
of the bright H-alpha emission (up to 750 mR) along the Magellanic Stream has
remained a mystery. We 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 order to light up much
of the Stream as observed, it must have a small angle of attack (~20 deg) to
the halo, and this may already find support in new HI observations. Another
prediction is that the Balmer ratio will be substantially enhanced due to the
slow shock. We find that 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 (>0.1 Msun/yr). The ablated material falls onto the Galaxy as a
warm drizzle; diffuse ionized gas at 10^4 K is an important constituent of
galactic accretion. We consider the stability of HI clouds falling towards the
Galactic disk and show that most of these must break down into smaller
fragments that become partially ionized. The Galactic halo is expected to have
huge numbers of smaller neutral and ionized fragments. When the ionized
component is accounted for, the rate of gas accretion is ~0.4 Msun/yr, roughly
twice the rate deduced from HI observations alone.
