The intra-hour variable quasar J1819$+$3845: 13-year evolution, jet
  polarization structure and interstellar scattering screen properties
  We examine the long-term evolution of the intra-hour variable quasar,
J1819+3845, whose variations have been attributed to interstellar scintillation
by extremely local turbulent plasma, located only 1-3pc from Earth. The
variations in this source ceased some time between June 2006 and February 2007.
The evolution of the source spectrum and the long-term lightcurve, and the
persistent compactness of the source VLBI structure indicates that the
cessation of rapid variability was associated with the passage of the
scattering material out of the line of sight to the quasar. We present an
analysis of the linear polarization variations and their relation to total
intensity variations. The proper motion of polarized features in the quasar jet
is found to be subluminal. Systematic time delays between Stokes I, Q and U, in
combination with the structure of the source obtained from 8.4GHz VLBI data,
confirm the estimate of the screen distance: 1-2pc, making the screen one of
the nearest objects to the Solar System.
  We determine the physical properties of this scattering material. The
electron density in the scattering region is extremely high with respect to the
warm ionized ISM, with an estimated density of $n_e \sim 97 \, l_0^{1/3}
{\Delta L}_{100}^{-1/2}$cm$^{-3}$, where $l_0$ is the outer scale of the
turbulence in AU and $\Delta L = 100 \Delta L_{100}$ AU is the depth of the
scattering region. If this plasma is in pressure balance with the local
magnetic field, one expects a ~2 rad/m^2 rotation measure change associated
with the passage of this material past the quasar. We examine the rotation
measures of sources and the diffuse polarized emission in the surrounding
region. We place a limit of 10 rad/m^2 on the RM change. The variability of
sources near J1819+3845 is used to deduce that the screen must therefore be
either very small (~100 AU) or patchy.
