  Recent observations suggest that the level of pulse-to-pulse intensity
modulation observed in a given radio pulsar may depend on its period and period
derivative. Such a ``modulation index relationship'' (MIR) may be an important
tool for determining the physical processes behind the radio emission. In the
context of sparking gap models, the exact functional form of the MIR depends on
the physical processes occurring on the surface of the neutron star in the
region known as the ``vacuum gap.'' Several possible vacuum gap models are
studied here in order to determine the expected MIR for a given model. Current
observations are consistent with two of the four models studied: the curvature
radiation driven vacuum gap and the curvature radiation driven near threshold
vacuum gap (CR-NTVG). It is shown that the inverse Compton scattering driven
vacuum gap models are not supported by the current data. Given that the current
data best supports the CR-NTVG model, its possible that all pulsars have strong
(approx 10^13 G) surface magnetic fields.
