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 <0.001 TECU and can measure TEC gradients with a precision of about 0.0002 TECU/km. 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 (Fukao et al. 1998; Yamamoto et al. 2005).

💡 Research Summary

This paper demonstrates that the Very Large Array (VLA) operating at very‑high‑frequency (VHF) can be used as a highly sensitive instrument for probing nighttime, mid‑latitude ionospheric total electron content (TEC) fluctuations. Using a long, continuous observation of the bright cosmic source Cygnus A, the authors show that the VLA can detect TEC variations with amplitudes below 0.001 TECU and measure TEC gradients with a precision of roughly 0.0002 TECU km⁻¹—orders of magnitude finer than typical GPS‑based techniques.

Two complementary spectral‑analysis methods are developed. The first method applies a two‑dimensional Fourier transform to the array‑wide TEC gradient field and tracks individual wave modes whose wavelengths are comparable to or larger than half the array size (≈20 km). This approach successfully isolates and characterises a series of medium‑scale traveling ionospheric disturbances (MSTIDs) that appear intermittently throughout the night. The MSTIDs generally propagate east‑west with phase speeds of 150–200 km h⁻¹ and wavelengths of 100–300 km. Their occurrence times, locations, and propagation directions are cross‑validated against concurrent GPS TEC maps, confirming the reliability of the VLA measurements. In addition to the MSTIDs, the method routinely detects smaller‑scale waves (≈5–20 km) that often share the same propagation direction as the larger disturbances.

The second method is statistical in nature. It reduces the wavelength detection limit to about 5 km and groups waves that travel in similar directions into “wave families.” By clustering these families, the authors obtain average properties—wavelength, phase speed, and azimuth—for each group. Two distinct families emerge. The first coincides temporally with the MSTIDs and exhibits characteristics consistent with quasi‑periodic (QP) echoes generated within sporadic‑E (Es) layers by the E‑F coupling instability. These waves have wavelengths of 5–15 km, phase speeds of 30–80 km h⁻¹, and propagate in the same direction as the accompanying MSTIDs. The second family appears to be driven primarily by neutral wind shear, mirroring the predominant QP echo populations reported in the Kyushu Sporadic‑E experiments (Fukao et al., 1998; Yamamoto et al., 2005). Their propagation directions vary with the background wind field, and they display a broader range of phase speeds (20–100 km h⁻¹) and wavelengths (5–20 km).

By combining the two techniques, the study bridges the gap between individual wave tracking and ensemble statistical description, allowing simultaneous investigation of ionospheric structures from a few kilometres up to several hundred kilometres. The VLA’s ability to resolve fine‑scale TEC gradients provides a unique window onto small‑scale phenomena—such as QP echoes—that are often invisible to GPS networks. Moreover, the detection of two separate QP‑echo populations suggests that both E‑F coupling processes and neutral wind dynamics play significant roles in shaping the small‑scale ionospheric morphology during nighttime conditions.

Overall, the work establishes the VLA VHF system as a powerful, high‑resolution tool for ionospheric research. It demonstrates that the array can not only complement traditional GPS observations but also extend the observable parameter space to include sub‑10 km structures with unprecedented sensitivity. These capabilities open new avenues for studying the coupling between sporadic‑E layers, medium‑scale traveling disturbances, and the neutral atmosphere, ultimately improving our understanding of ionospheric dynamics and informing more accurate space‑weather models.