  Halo currents generated during disruptions on Alcator C-Mod have been
measured with Langmuir "rail" probes. These rail probes are embedded in a lower
outboard divertor module in a closely-spaced vertical (poloidal) array. The
dense array provides detailed resolution of the spatial dependence (~1 cm
spacing) of the halo current distribution in the plasma scrape-off region with
high time resolution (400 kHz digitization rate). As the plasma limits on the
outboard divertor plate, the contact point is clearly discernible in the halo
current data (as an inversion of current) and moves vertically down the
divertor plate on many disruptions. These data are consistent with filament
reconstructions of the plasma boundary, from which the edge safety factor of
the disrupting plasma can be calculated. Additionally, the halo current
"footprint" on the divertor plate is obtained and related to the halo flux
width. The voltage driving halo current and the effective resistance of the
plasma region through which the halo current flows to reach the probes are also
investigated. Estimations of the sheath resistance and halo region resistivity
and temperature are given. This information could prove useful for modeling
halo current dynamics.
