Descending ion layer property in the Gadanki radar observations of 150 km echoes and its implication to the echoing phenomenon

Abstract

Gadanki radar observations of the daytime 150 km echoes displaying descending features, resembling the descending ion layer behavior, are presented. The descending pattern is intriguing since it indicates the possible role of density gradient associated with the ion layer in generating the irregularities responsible for 150 km radar echoes, not envisioned before. Unusually strong SNR and narrow spectral features of these echoes clearly indicate the role of sharp density gradients in the echoing process. Given the fact that Gadanki is located at magnetically low latitude, it is argued that these descending echoing layers are produced by interchange instability on the gradient of daytime descending ion layer formed by meridional wind shear associated with tidal/gravity waves quite similar to that observed during nighttime. The observations reported here are first of its kind illustrating the role of density gradient layers and are important in furthering our understanding on the puzzling 150 km echoing phenomenon.

First Dual‐Frequency Radar Observations of 150‐km Echoes From Gadanki and Implications

“…Corresponding spectral width distributions are shown in Figures 3c and 3d. We also note that the SNR-dependent spectral properties are the dominant population at 53-MHz observations while echoes having SNR-independent spectral properties are only a minority population, consistent with earlier observations made from Gadanki (Patra, 2011;Patra & Pavan Chaitanya, 2016). We also note that the SNR-dependent spectral properties are the dominant population at 53-MHz observations while echoes having SNR-independent spectral properties are only a minority population, consistent with earlier observations made from Gadanki (Patra, 2011;Patra & Pavan Chaitanya, 2016).…”

“…Given that earlier observations at~50 MHz already revealed two categories of echoes, that is, SNR-dependent and SNR-independent spectral characteristics (Chau & Kudeki, 2013;Patra, 2011), we have attempted to examine such features at 30 MHz and also to compare them with those measured simultaneously from the same ionospheric volume. In Figure 3, we show the relationship between SNR and spectral width and the distribution of spectral width observed by the two radars.…”

Common‐Volume Dual‐Frequency Radar Observations of 150‐km Echoes and Implications

“…It is worthwhile to mention that the work of Oppenheim and Dimant (2016) is the only one to date that has proposed a tangible photoelectron path for the enhancement of plasma waves accounting for 10-25-dB enhancement in the electron density fluctuations and both narrow and wide spectral features associated with the 150-km echoes. It is also not clear how the type-A echoes (spectral width dependent on SNR) and type-B echoes (spectral width independent of SNR) (Chau & Kudeki, 2013;Patra, 2011), which were attributed to naturally enhanced incoherent scattering (NEIS) and FAI, respectively, by Chau and Kudeki (2013), could be reconciled. It is also not clear how the type-A echoes (spectral width dependent on SNR) and type-B echoes (spectral width independent of SNR) (Chau & Kudeki, 2013;Patra, 2011), which were attributed to naturally enhanced incoherent scattering (NEIS) and FAI, respectively, by Chau and Kudeki (2013), could be reconciled.…”

Photoelectron‐induced waves: A likely source of 150 km radar echoes and enhanced electron modes

“…Kudeki and Fawcett [] proposed that a wind‐driven gravity wave interchange instability underlies these waves and the periodicity in the vertical structure supports that idea. Patra [] argued that descending ion layers, possibly with a high concentration of metallic ions, could generate these gradients. However, the gradients and the winds do not appear consistently strong enough to account for the regularity of 150 km echoes.…”