We are undertaking a large-scale , Micro-Arcsecond Scintillation-Induced Variability ( MASIV ) survey of the northern sky , \delta > 0 ^ { \circ } , at 4.9Â GHz with the VLA .
Our objective is to construct a sample of 100 to 150 scintillating extragalactic sources with which to examine both the microarcsecond structure and the parent populations of these sources , and to probe the turbulent interstellar medium responsible for the scintillation .
We report on our first epoch of observations which revealed variability on timescales ranging from hours to days in 85 of 710 compact flat-spectrum sources .
The number of highly variable sources , those with RMS flux density variations greater than 4 % of the mean , increases with decreasing source flux density but rapid , large amplitude variables such as J1819+3845 are very rare .
When compared with a model for the scintillation due to irregularities in a 500Â pc thick electron layer , our preliminary results indicate maximum brightness temperatures \sim 10 ^ { 12 } Â K , similar to those obtained from VLBI surveys even though interstellar scintillation is not subject to the same angular resolution limit .