A new technique for spectral analysis of ionospheric TEC fluctuations observed with the very large array VHF system: From QP echoes to MSTIDs

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 ≤ 10⁻³ TECU and can measure TEC gradients with a precision of about 2 × 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 (∼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 (Es) 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 Es 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 (SEEK). 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.