Naturally enhanced ion-line spectra around the equatorial 150-km region

Abstract

Abstract. For many years strong radar echoes coming from 140-170 km altitudes at low latitudes have been associated to the existence of field-aligned irregularities (FAIs) (the so called 150-km echoes). In this work, we present frequency spectra as well as angular distribution of 150-km echoes. When the 150-km region is observed with beams perpendicular to the magnetic field (B) the observed radar spectra are very narrow with spectral widths between 3-12 m/s. On the other hand, when few-degrees off-perpendicular beams are used, the radar spectra are wide with spectral widths comparable to those expected from ion-acoustic waves at these altitudes (>1000 m/s). Moreover the off-perpendicular spectral width increases with increasing altitude. The strength of the received echoes is one to two orders of magnitude stronger than the expected level of waves in thermal equilibrium at these altitudes. Such enhancement is not due to an increase in electron density. Except for the enhancement in power, the spectra characteristics of off-perpendicular and perpendicular echoes are in reasonable agreement with expected incoherent scatter spectra at these angles and altitudes. 150-km echoes are usually observed in narrow layers (2 to 5). Bistatic common volume observations as well as observations made few kilometers apart show that, for most of the layers, there is very high correlation on power fluctuations without a noticeable time separation between simultaneous echoes observed with Off-perpendicular and Perpendicular beams. However, in one of the central layers, the echoes are the strongest in the perpendicular beam and absent or very weak in the offperpendicular beams, suggesting that they are generated by a plasma instability. Our results indicate that most echoes around 150-km region are not as aspect sensitive as originally thought, and they come from waves that have been enhanced above waves in thermal equilibrium.

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

“…The off-perpendicular echoes were found to be weaker (by 20-30 dB), and Doppler spectra were broader (as high as 1,000 m/s) than their perpendicular counterpart (e.g., Chau et al, 2009). The perpendicular echoes have narrow spectral features (<15 m/s) and can be as strong as 50 dB above noise.…”

A review of Mesosphere–Stratosphere–Troposphere (MST) radar developments and studies, circa 1997–2008

“…A great deal of effort has been expended to explain these echoes, but the explanation remains elusive. Sample publications include Chau et al (2009), Chau (2004, Chau and Kudeki (2006b), Choudhary et al (2004), Ecklund (2007, 2008), Nicolls et al (2009), Patra and, Patra et al (2008), and references therein.…”

On the characteristics of 150-km echoes observed in the Brazilian longitude sector by the 30 MHz São Luís radar

“…We must clarify that the comparisons made in this paper refer to Jicamarca observations made with the JULIA radar mode, with antenna beams pointing perpendicular to the magnetic field. New offperpendicular observations made at Jicamarca (Chau, 2004;Chau et al, 2009) also show echoes around 150-km altitude with spectra as wide as the ion line of the incoherent scatter spectrum (spectral width larger than 1000 m s −1 ). Finally, Patra and Rao (2006) also found that most spectral widths of off-equatorial 150-km echoes fall within 2 and 8 m s −1 with a maximum occurrence around 4 m s −1 .…”

“…Future work includes a more detailed analysis of the changes in echo intensity as a function of season and solar flux and how this variability relates to the new findings of Chau et al (2009). Their results indicates that some of 150-km echoes might not be produced by field-aligned irregularities, but by naturally enhanced ion acoustic waves.…”

“…However, it has been hypothesized that the ionospheric irregularities causing 150-km echoes could be generated by a gravity wave driven interchange instability (Kudeki and Fawcett, 1993) or by polarization electric fields generated off magnetic equator by a sporadic E layer instability (Tsunoda, 1994). More recent results indicate that 150-km echoes can be related to naturally enhanced ion acoustic waves (Chau et al, 2009).…”