We have used a relatively long , contiguous VHF observation of a bright cosmic radio source ( Cygnus A ) with the Very Large Array ( VLA ) through the nighttime , midlatitude ionosphere to demonstrate the phenomena observable with this instrument .
In a companion paper , we showed that the VLA can detect fluctuations in total electron content ( TEC ) with amplitudes of \leq 10 ^ { -3 } TECU and can measure TEC gradients with a precision of about 2 \times 10 ^ { -4 } TECU km ^ { -1 } .
We detail two complementary techniques for producing spectral analysis of these TEC gradient measurements .
The first is able to track individual waves with wavelengths of about half the size of the array ( \sim 20 km ) or more .
This technique was successful in detecting and characterizing many medium-scale traveling ionospheric disturbances ( MSTIDs ) seen intermittently throughout the night and has been partially validated using concurrent GPS measurements .
Smaller waves are also seen with this technique at nearly all times , many of which move in similar directions as the detected MSTIDs .
The second technique allows for the detection and statistical description of the properties of groups of waves moving in similar directions with wavelengths as small as 5 km .
Combining the results of both spectral techniques , we found a class of intermediate and small scale waves which are likely the quasi-periodic ( QP ) echoes that have been observed to occur within sporadic- E ( E _ { s } ) layers .
We find two distinct populations of these waves .
The members of one population are coincident in time with MSTIDs and are consistent with being generated within E _ { s } layers by the E – F coupling instability .
The other population seems more influenced by the neutral wind , similar to the predominant types of QP echoes found by the Sporadic- E Experiments over Kyushu \citep [ SEEK ] [ ] fuk98 , yam05 .
We have also found that the spectra of background ( i.e. , isotropic ) fluctuations can be interpreted as the sum of two turbulent components with maximum scales of about 300 km and 10 km .