We analyze deterministic and random temporal variations in dispersion measure
(DM) from the full three-dimensional velocities of pulsars with respect to the
solar system, combined with electron-density variations on a wide range of
length scales. Previous treatments have largely ignored the pulsar's changing
distance while favoring interpretations involving the change in sky position
from transverse motion. Linear trends in pulsar DMs seen over 5-10~year
timescales may signify sizable DM gradients in the interstellar medium (ISM)
sampled by the changing direction of the line of sight to the pulsar. We show
that motions parallel to the line of sight can also account for linear trends,
for the apparent excess of DM variance over that extrapolated from
scintillation measurements, and for the apparent non-Kolmogorov scalings of DM
structure functions inferred in some cases. Pulsar motions through atomic gas
may produce bow-shock ionized gas that also contributes to DM variations. We
discuss possible causes of periodic or quasi-periodic changes in DM, including
seasonal changes in the ionosphere, annual variation of the solar elongation
angle, structure in the heliosphere-ISM boundary, and substructure in the ISM.
We assess the solar cycle's role on the amplitude of ionospheric and solar-wind
variations. Interstellar refraction can produce cyclic timing variations from
the error in transforming arrival times to the solar system barycenter. We
apply our methods to DM time series and DM gradient measurements in the
literature and assess consistency with a Kolmogorov medium. Finally, we discuss
the implications of DM modeling in precision pulsar timing experiments.Comment: 24 pages, 17 figures, published in Ap
