Observations of Ducted and Nonducted Fast Z mode Echoes from IMAGE: Measurement of Z mode Duct Parameters

We present observations of ducted and nonducted fast Z mode echoes from the radio sounding at 50-1000 kHz on the IMAGE satellite. The Radio Plasma Imager (RPI) on IMAGE has recorded ~2000 cases of ducted and nonducted fast Z mode echoes in the altitude range of ~800-10,000 km, invariant latitude range of 30 ^o - 70 ^o, and at all MLT. About ~1000 cases are ducted echoes found inside the Z mode cavities (a minimum exhibited in Z mode cutoff frequency (f_z) within which a narrow range of frequencies can be trapped). About 5% of the ducted fast Z mode echoes were reflected from the conjugate hemispheres. These last mentioned echoes were detected when the satellite was within 20 ^o of geomagnetic equator. Most of the ducted fast Z mode echoes were observed for f_pe/f_ce range of 0.25 - 4, where (f_pe and f_ce are plasma- and gyro-frequency respectively. The lower and upper cutoffs of the Z mode echoes increase and the bandwidth of Z mode echoes decreases with f_pe/f_ce ratio in accordance with the cold plasma wave theory. We have used ray tracing simulations to study the ducted and nonducted propagation of Z mode waves radiated from IMAGE. In absence of ducts, we obtain four non-ducted fast Z mode echoes, each reflecting from locations where f = f_z. Three of the echoes retrace their paths after reflection and one forms a loop. In contrast with non-ducted Z mode echoes, the ducted fast Z mode echoes are characterized by multiple traces that show a definite relationship in their time-delay (tg) vs frequency (f) curves. We provide a new method to measure the Z mode duct properties at mid latitudes. Analysis of the ducted fast Z mode echoes observed on 10 Jan 2005 (L = 2.7, Altitude = ~1750 km, MLT = 17.2) shows that a duct of half-width ΔL = 0.06 L (~78 km at 1750 km) and density depletions of ~45%, covering the altitude range ~1300 - 10000 km are required to trap Z mode waves and to reproduce observed time-delays. We believe our results demonstrate that Z mode echoes observed on the IMAGE satellite will provide measurements of Z mode ducts at mid latitudes.