  Radio emission propagating over an Earth-pulsar line of sight provides a
unique probe of the intervening ionized interstellar medium (ISM). Variations
in the integrated electron column density along this line of sight, or
dispersion measure (DM), have been observed since shortly after the discovery
of pulsars. As early as 2006, frequency-dependent dispersion measures have been
observed and attributed to several possible causes. Ray-path averaging over
different effective light-cone volumes through the turbulent ISM contributes to
this effect as will DM misestimation due to radio propagation across compact
lensing structures such as those caused by "extreme scattering events". We
present methods to assess the variations in frequency-dependent dispersion
measures due to the turbulent ISM versus these compact lensing structures along
the line of sight. We analyze recent Low-Frequency Array (LOFAR) observations
of PSR J2219+4754 to test the underlying physical mechanism of the observed
frequency-dependent DM. Previous analyses have indicated the presence of strong
lensing due to compact overdensities halfway between the Earth and pulsar.
Instead we find the frequency dependence of the DM timeseries for PSR
J2219+4754 is consistent with being due solely to ISM turbulence and there is
no evidence for any extreme scattering event or small-scale lensing structure.
The data show possible deviations from a uniform turbulent medium, suggesting
that there may be an enhanced scattering screen near one of the two ends of the
line of sight. We present this analysis as an example of the power of
low-frequency observations to distinguish the underlying mechanisms in
frequency-dependent propagation effects.
