  High-velocity clouds (HVCs) are interstellar clouds of atomic hydrogen that
do not partake of the Galactic rotation and have velocities of a several
hundred kilometers per second. A considerable number of these clouds are
falling down towards the Galactic disk. HVCs form large and massive complexes,
so their collisions with the disk must release a great amount of energy into
the interstellar medium. The cloud-disk interaction produces two shocks, one
propagates through the cloud and the other through the disk; the properties of
these shocks depend mainly on the cloud velocity and the disk-cloud density
ratio. In this work we study the conditions necessary for these shocks to
accelerate particles by diffusive shock acceleration and the produced
non-thermal radiation. We analyze particle acceleration in both the cloud and
disk shocks. Solving a time-dependent 2-D transport equation for both
relativistic electrons and protons we obtain particle distributions and
non-thermal spectral energy distributions. In a shocked cloud significant
synchrotron radio emission is produced along with soft gamma rays. In the case
of acceleration in the shocked disk, the non-thermal radiation is stronger; the
gamma rays, of leptonic origin, might be detectable with current instruments. A
large number of protons are injected into the Galactic interstellar medium, and
locally exceed the cosmic-ray background. We conclude that under adequate
conditions the contribution from HVC-disk collisions to the galactic population
of relativistic particles and the associated extended non-thermal radiation
might be important.
