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

• Published in: Annales geophysicae (1988) Vol. 27; no. 3; pp. 933 - 942
• Main Authors: CAU, J. L, WOODMAN, R. F, MILLA, M. A, KUDEKI, E
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Göttingen Copernicus 02.03.2009; Copernicus GmbH; Copernicus Publications
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Magnetic fields; Physics of the ionosphere; Physics of the magnetosphere; magnetic field; Ionosphere Equatorial ionosphere; Field aligned irregularity; Thermal wave; Thermal equilibrium; Ionospheric irregularities; correlation; Plasma instability; fluctuations; Incoherent scattering; Angular distribution; Frequency spectrum; Low latitude; electron density; Ionospheric irregularity; Radar echo; Instruments and techniques; Ion acoustic wave; Power spectrum; Ionosphere (Equatorial ionospher Ionospheric irregularitie Instruments and techniques)
• ISSN: 0992-7689; 1432-0576; 1432-0576
• DOI: 10.5194/angeo-27-933-2009

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 off-perpendicular 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.