  Extreme scattering events (ESEs) in the interstellar medium (ISM) were first
observed in regular flux measurements of compact extragalactic sources. They
are characterized by a flux variation over a period of weeks, suggesting the
passage of a "diverging plasma lens" across the line of sight. Modeling the
refraction of such a lens indicates that the structure size must be of order AU
and the electron density of order 10s of cm^{-3}. Similar structures have been
observed in measurements of pulsar intensity scintillation and group delay.
Here we report observations of two ESEs showing increases in both intensity
scintillation and dispersion made with the Parkes Pulsar Timing Array (PPTA).
These allow us to make more complete models of the ESE, including an estimate
of the "outer-scale" of the turbulence in the plasma lens. These observations
show clearly that the ESE structure is fully turbulent on an AU scale. They
provide some support for the idea that the structures are extended along the
line of sight, such as would be the case for a scattering shell. The dispersion
measurements also show a variety of AU scale structures which would not be
called ESEs, yet involve electron density variations typical of ESEs and likely
have the same origin.
