Tropical E region field aligned irregularities: Simultaneous observations of continuous and quasiperiodic echoes

The antenna beam at 13 degrees N off zenith direction of the Indian mesosph ere-stratosphere-troposphere (MST) radar at Gadanki (13.5 degrees N, 79.2 d egrees E) is orthogonal to geomagnetic field lines at 110 km altitude. This beam was used to observe E region field-aligned irregularities during July -August 1994, February and June 1996, and July-September 1997 with a range resolution of 600 m. On a few occasions the radar was operated at 150 m ran ge resolution also. Observations showed continuous and multiple echo region s during nighttime. However, out of the 15 nights of observations, quasiper iodic (QP) echoes were also detected on the nights of July 20-21, 1994, and September 3-4, 1997. In this paper, major features of these quasiperiodic echoes in the form of height-time maps of echo intensity are presented. On July 20-21, 1994, quasiperiodic echoes with a periodicity of about 2-4 min from the altitude between 102 and 116 km appeared as tilted striations in t he height-time-intensity maps. The rate of descent of altitude associated w ith these tilted striations was in the range of 40-60 m s(-1). The fluctuat ions in the Doppler velocity along the length of striations resembled a wav e structure with vertical wavelength of the order of 18-20 km. Observations on September 3-4, 1997, suggest that these high-frequency waves initially modulate the E-s layer up to an altitude as high as 120 km and produce the quasiperiodic variation in the echoes. As time progresses, the period of QP echoes as well as the echo expanse decreases, and finally the normal conti nuous layer emerges.