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We examine the long-term evolution of the intra-hour variable quasar,J1819+3845, whose variations have been attributed to interstellar scintillationby extremely local turbulent plasma, located only 1-3pc from Earth. Thevariations in this source ceased some time between June 2006 and February 2007.The evolution of the source spectrum and the long-term lightcurve, and thepersistent compactness of the source VLBI structure indicates that thecessation of rapid variability was associated with the passage of thescattering material out of the line of sight to the quasar. We present ananalysis of the linear polarization variations and their relation to totalintensity variations. The proper motion of polarized features in the quasar jetis found to be subluminal. Systematic time delays between Stokes I, Q and U, incombination 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 ofthe nearest objects to the Solar System.  We determine the physical properties of this scattering material. Theelectron density in the scattering region is extremely high with respect to thewarm 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 theturbulence in AU and $\Delta L = 100 \Delta L_{100}$ AU is the depth of thescattering region. If this plasma is in pressure balance with the localmagnetic field, one expects a ~2 rad/m^2 rotation measure change associatedwith the passage of this material past the quasar. We examine the rotationmeasures of sources and the diffuse polarized emission in the surroundingregion. We place a limit of 10 rad/m^2 on the RM change. The variability ofsources near J1819+3845 is used to deduce that the screen must therefore beeither very small (~100 AU) or patchy.

The intra-hour variable quasar J1819$+$3845: 13-year evolution, jet polarization structure and interstellar scattering screen properties