Abstract
Abstract. The zonal structure of radar backscatter plumes associated with Equatorial Spread F (ESF), probably modulated by atmospheric gravity waves, has been investigated with the Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (0.20° S, 100.32° E; dip latitude 10.1° S) and the FM-CW ionospheric sounders on the same magnetic meridian as the EAR. The occurrence locations and zonal distances of the ESF plumes were determined with multi-beam observations with the EAR. The ESF plumes drifted eastward while keeping distances of several hundred to a thousand kilometers. Comparing the occurrence of the plumes and the F-layer uplift measured by the FM-CW sounders, plumes were initiated within the scanned area around sunset only, when the F-layer altitude rapidly increased. Therefore, the PreReversal Enhancement (PRE) is considered as having a zonal variation with the scales mentioned above, and this variation causes day-to-day variability, which has been studied for a long time. Modulation of the underlying E-region conductivity by gravity waves, which causes inhomogeneous sporadic-E layers, for example, is a likely mechanism to determine the scale of the PRE.
Highlights
Field-aligned irregularities or plasma plumes associated with Equatorial Spread F (ESF) have been studied intensively with a coherent backscatter technique after the discovery of upward–developing plasma plumes with the JicamarcaVHF radar (Woodman and LaHoz, 1976)
We have shown three important aspects of the initiation of ESF plumes as follows: (1) ESF plumes or 3-m scale irregularities are initiated exclusively around sunset (Yokoyama et al, 2004), (2) groups of ESF plumes drift eastward while keeping distances of several hundred to a thousand kilometers between them, and (3) plumes are initiated within the scanned area around sunset only when the F-layer altitude over the longitudinal sector of the Equatorial Atmosphere Radar (EAR) rapidly increases in the evening
From the spatial distribution of occurrence locations of ESF plumes estimated with multi-beam observations of the EAR and the relationship between plume occurrence and F-layer uplift, we have concluded that the intensity of PreReversal Enhancement (PRE) is not zonally uniform but has a zonal variation with a scale of several hundred to a thousand kilometers
Summary
VHF radar (Woodman and LaHoz, 1976). The most important phenomenon related to the ESF is a sharp enhancement of the eastward electric field in the evening equatorial F-region that increases both the electrodynamic (direct) and gravitational (indirect) terms of the growth rate of the Rayleigh-Taylor instability. This enhanced electric field raises the F-layer to higher altitudes, an effect which is well known as the PreReversal Enhancement (PRE) While the seasonal and solar-cycle dependence of the PRE has been well investigated (e.g. Fejer et al, 1979), the day-to-day variability of the PRE is still an unresolved problem for the prediction of the ESF occurrence (Abdu, 2001; Tsunoda, 2005)
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