Comparison of gravity waves detected in the lower ionosphere by narrowband VLF measurements and in the F layer by a continuous Doppler sounding system

Gravity waves (GWs) are an important class of atmospheric waves that can propagate from the troposphere up to the upper atmosphere, where they can contribute significantly to the dynamical changes in the ionosphere. The aim of the present contribution is to gain a deeper understanding of the coupling between the lower and the middle ionosphere via gravity waves by the concurrent analysis of narrowband VLF measurements (characterizing GWs in the E layer) carried out at the Tihany Geophysical Observatory (TGO), Hungary and the multi-point and multi-frequency continuous Doppler sounding system (characterizing GWs in the F layer) operated by the Institute of Atmospheric Physics, CAS in the Czech Republic. The signal from the German VLF transmitter (call sign: DHO, frequency: 23.4 kHz) detected at the TGO have been considered for the present study, since the Doppler system located in the western part of the Czech Republic is close to the midpoint of the DHO-TGO wave propagation path. The inferred gravity wave activities have also been compared with the lightning locations provided by the World Wide Lightning Location Network (WWLLN) in order to identify the possible source areas of the upward propagating gravity waves. 

The preliminary analysis was carried out for the summer months of 2021. From the narrowband VLF measurements, the day-to-day variation of the average GW activity for each night was determined using the Wavelet transform. In the case of the Doppler system, the estimated Doppler shifts and the spreadF proxy were used to characterize the nighttime GW activity. Our results show low correlation (~0.04) between GW activity inferred from the VLF and Doppler measurements. On the other hand, for both the VLF and Doppler measurements, we could identify certain areas where the large number of lightning strokes was associated with enhanced GW activity in the corresponding ionospheric measurement. However, the areas found for the two measurements do not overlap, which may indicate that the VLF measurement is sensitive to a different area where the Doppler system is located. This work has been supported by the PITHIA-NRF Trans-National Access (TNA) programme.