Kilometric irregularities in the E and F regions of the daytime equatorial ionosphere observed by a high resolution HF radar

Abstract

During the International Equatorial Electrojet Year, high resolution vertical HF coherent radar experiments were performed in the Ivory Coast (9°24′62″N, 5°37′38″W) at frequencies in the range 1–8 MHz. Kilometric scale irregularities were observed in the E and F regions of the daytime equatorial ionosphere. These irregularities constitute a large sporadic E layer within the altitude range 100 to 150 km, larger than the width of the equatorial electrojet. The higher part of this irregular region is detached in the morning and evening forming an intermediate region at 150–160 km. These 150 km echoes overtakes the E region electrojet echoes at 100–120 km at noon. The echolocation shows that the reflections arise from the vertical direction up to 150 km with a significant contribution of reflections from East West directions above 110 km. The F1 region irregularities are characterized by time and height scales significantly larger than the E region irregularities scales. As in the E region, the F1 region reflections arise from the vertical and from East West directions when the reflection distance increases. The variations of the Doppler frequency versus the observation distance at several sounding frequencies are well explained by a horizontal motion of the irregular plasma flow both in the E and F regions with velocities of 100 m/s and 130 m/s respectively.

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

“…They produce also an increase of the echo width of 40 km at 250 km to 15 km at 440 km corresponding to 10±12 km at 180±300 km after ionogram inversion. The period of observation of these irregularities is correlated with that of the irregularities in the electrojet region (Blanc et al, 1996). Similar amplitude¯uctuations are never observed in the evening, when the E-region irregularities disappear or in the morning before they appear.…”

“…Echolocation measurements showed that echoes on these irregularities arise both from vertical and oblique re¯ections in the east-west direction. The use of several radar frequencies allowed to locate them close to the re¯ection level as the Doppler frequency shift was proportional to the radar frequency (Blanc et al, 1996). These irregularities are di cult to observe on the ionograms as they just appear as an increase in the echo width of few tens of kilometers.…”

“…The origin of these¯uctuations can be related to electron densities irregularities near the plasma frequency re¯ection level, producing partial re¯ections just below the re¯ection level or oblique re¯ections. Irregularities have also been analyzed during this same radar campaign with a high time and height resolution mode adapted to the study of small-scale structures (Blanc et al, 1996). Irregularities are characterized by time variations of less than 1s and by a kilometric heightscale in the E region and by larger time-and heightscales in the F region.…”

Equatorial electrojet at African longitudes: first results from magnetic measurements

“…An HF Radar was operated by the CEA (Commisariat aÁ l'Energie Atomique) during three months in 1993 and three other months in 1994 at Korhogo (Blanc et al, 1996). It allowed the study of the E-and F-layer density pro®les in the ionosphere, virtual re¯ection heights, echo power spectra, and consequently Doppler shifts, diurnal drift, plasma instabilities, and irregular structures (spread F, plume, small-scale inhomogeneities) by echo location.…”

HF Doppler radar observations of sporadic E at an Indian low latitude station, Visakhapatnam

“…The QP echoes were reported from many radars locations (Yamamoto et al, , 1992Woodman et al, 1991;Tsunoda et al, 1994Tsunoda et al, , 1995, which were reported to be usually located in the valley region between the E-and F-regions and extend from 177 km to 215 km (Tsunoda et al, 1995) and the slope of the range rate was about −100 m/s and always negative. The QP echoes first reported by Balsley (1964) as 150 km echoes were studied close to the magnetic equator in great detail (Royrvik and Miller, 1981;Royrvik, 1982;Kudeki and Fawcett, 1993;Hysell et al, 1997;Chau and Woodman, 2004;Chau, 2004;Kudeki et al, 1998;Euklund, 2000, 2004;Blank et al, 1996;de Paula and Hysell, 2004). There were reports from the Indian MST radar at Gadanki outside the equatorial electrojet (Geomag.…”