Observation of coherent echoes with narrow spectra near 150 km altitude during daytime away from the dip equator

Abstract

We present the rare observation of backscatter echoes perpendicular to the geomagnetic field from the 150 km altitude vicinity over a region situated away from the dip equator, namely, at Gadanki (13.5°N, 79.2°E, 6.4° dip latitude). A layer of weak echoes, having Signal-to-Noise Ratio (SNR) of the order of -12 to -3 dB, was seen drifting downward from 157 km at 0930 LT to about 148 km at 1028 LT on July 12, 1997. The Doppler velocity as well as Doppler Width was in the range 10-20 ms^-1. While there was no quasi-periodicity as such in terms of echo occurrence, the echo properties were in other respects very similar to 150 km echoes seen in equatorial regions. This included the descending phase with time during the morning hours. These constitute the first observations of daytime 150 km echoes away from the dip-equator and only add to the mystery regarding the origin of these structures.