  We study the effect of gravitational focusing of the earth on dark matter. We
find that the effect can produce a detectable diurnal modulation in the dark
matter signal for part of the parameter space which for high dark matter masses
is larger than the diurnal modulation induced by the fluctuations in the flux
of dark matter particles due to the rotation of the earth around its own axis.
The two sources of diurnal modulation have different phases and can be
distinguished from each other. We demonstrate that the diurnal modulation can
potentially check the self-consistency of experiments that observe annual
modulated signals that can be attributed to dark matter. Failing to discover a
daily varying signal can result conclusively to the falsification of the
hypothesis that the annual modulation is due to dark matter. We also suggest
that null result experiments should check for a daily modulation of their
rejected background signal with specific phases. A potential discovery could
mean that dark matter collisions have been vetoed out.
