  The scale radius of dark matter halos is a critical parameter for specifying
the density distribution of dark matter, and is therefore a fundamental
parameter for modeling galaxies. We develop here a novel, observationally
motivated probe to quantitatively infer its value. We demonstrate that
disturbances in the extended atomic hydrogen gas disks of galaxies can be used
to infer the scale radius of dark matter halos. Our primary metric is the phase
of the $m=1$ mode of the disturbance in the outskirts of the gas disk, which we
take to be produced by a tidal interaction. We apply the method to the
Whirlpool Galaxy, which has an optically visible satellite. We explore
potential degeneracies due to orbital inclination and initial conditions and
find our results to be relatively insensitive to these considerations. Our
method of tracing the dark potential well through observed disturbances in
outer gas disks is complementary to gravitational lensing.
