  The extreme, intra-hour and > 10% rms flux density scintillation observed in
AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has
been attributed to scattering in highly turbulent, nearby regions in the
interstellar medium. Such behavior has been found to be rare. We searched for
rapid scintillators among 128 flat spectrum AGNs and analyzed their properties
to determine the origin of such rapid and large amplitude radio scintillation.
The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two
hour intervals over 11 days. We detected six rapid scintillators with
characteristic time-scales of < 2 hours, none of which have rms variations >
10%. We found strong lines of evidence linking rapid scintillation to the
presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200
muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity
of rapid and large amplitude scintillators to the requirement of additional
constraints, including large source compact fractions. J1819+3845 was found to
display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11
day variations, suggesting that the highly turbulent cloud responsible for its
extreme scintillation has moved away, with its scintillation now caused by a
more distant screen ~ 50 to 150 pc away.
