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

Abstract

We present and discuss common-volume dual-frequency radar observations of 150-km echoes made at 30 and 53 MHz from Gadanki by employing identical transmitter power and antenna beam width. These controlled experiments reveal that 150-km echoes are stronger, broader in spectral width, and more frequent in their occurrence at 30 MHz than at 53 MHz. Echo intensities at 30 and 53 MHz are found to be consistent with the rocket-borne observations of wave number spectrum of meter-scale irregularities in the 150-km region. The frequency dependence of spectral widths of the echoes while is found to be somewhat similar to that of electrojet type-1 echoes it is opposite to that of electrojet type-2 echoes and also to that of incoherent scattering in direction perpendicular to magnetic field. Spectral widths of the echoes observed by both radars are found to be independent of SNR. A detailed comparison of these observations with those made earlier suggests that instrument functions play an important role in manifesting SNR-dependent/independent spectral width property of the 150-km echoes. The narrow spectral properties and frequency dependence of echo intensity and spectral width clearly suggest that the irregularities responsible for the radar backscatter are linked with weak plasma turbulence and the meter-scale irregularities responsible for radar echoes ought to be growing with direct injection of energy at the meter scale itself to overcome damping. We surmise the potential role of photoelectrons and atmospheric gravity waves in the underline plasma instability process.

Simulating the Upper Hybrid Instability as a Cause for 150 km Echoes

“…Just off perpendicular to B , ion spectra reveal broad frequency enhancements and significant power in the ion line, especially for longer wavelengths. This is consistent with observations of 150 km echoes, which have been show to have more backscattered power for 30 MHz radars than 50 MHz (A. Patra et al., 2020; A. K. Patra et al., 2020). It also provides some evidence for the lack of observations of 150 km echoes at ALTAIR.…”

“…There are two populations of 150 km echoes, Naturally Enhanced Incoherent Scatter (NEIS), or Type‐A, which have a typical signal‐to‐noise ratio (SNR) of around 5 dB, and Field Aligned Irregularities (FAIs), or Type‐B, which can have a SNR of around 20 dB (Chau & Kudeki, 2013; A. Patra et al., 2020; A. K. Patra et al., 2020). NEIS accounts for the majority of observations of 150 km echoes and is primarily observed by Jicamarca Radio Observatory (JRO) as it is the most sensitive (Chau & Kudeki, 2013).…”

The Photoelectron‐Driven Upper Hybrid Instability as the Cause of 150‐km Echoes

“…The kinetic simulations in Oppenheim and Dimant (2016) showed that this mode conversion does occur in a photoelectron‐driven instability, but the exact mechanism is unknown. The weak turbulence theory may explain this mode conversion, and a wavenumber matching condition in weak turbulence theory could also explain the lack of observations at the higher frequency ALTAIR radar (Forme, 1999; Nicholson, 1983), as well as concurrent observations at 30 and 50 MHz from the Gadanki radar (Patra et al, 2020).…”

The Generation of 150 km Echoes Through Nonlinear Wave Mode Coupling

“…Observationally, 150‐km echoes have only been observed by radars with transmit frequencies in the range of 30–50 MHz (Kudeki & Fawcett, 1993; Patra, Chaitanya, Rao, & Kamaraj, 2020). Notably, the ALTAIR radar (150 MHz) has never observed 150‐km echoes despite its favorable location at the equator (Chau et al., 2023).…”