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Abstract
Abstract. Plasma probe data from DMSP-F13, DMSP-F15 and DEMETER satellites were used to examine longitudinal structures in the topside equatorial ionosphere during fall equinox conditions of 2004 year. Since the launch of DEMETER satellite on 29 June 2004, all these satellites operate close together in the topside ionosphere. Here, data taken from Special Sensor-Ion, Electron and Scintillations (SSIES) instruments on board DMSP-F13, F15 and Instrument Analyser de Plasma (IAP) on DEMETER, are used. Longitudinal variations in the major ions at two altitudes (~730 km for DEMETER and ~840 km for DMSP) are studied to further describe the recently observed "wavenumber-four" (WN4) structures in the equatorial topside ionosphere. Different ion species H+, He+ and O+ have a rather complex longitudinal behavior. It is shown that WN4 is almost a regular feature in O+ the density distribution over all local times covered by these satellites. In the evening local time sector, H+ ions follow the O+ behavior within WN4 structures up to the pre-midnight hours. Near sunrise H+ and later in the daytime, He+ longitudinal variations are out of phase with respect to O+ ions and effectively reduce the effect of WN4 on total ion density distribution at altitudes 730–840 km. It is shown that both a WN4 E×B drift driver and local F-region winds must be considered to explain the observed ion composition variations.
Highlights
Longitudinal variations in the equatorial ionospheric anomaly (EIA) have been intensively studied following recent observations of wavenumber-four structures in the nighttime OI135.6-nm emission along the geomagnetic equator by the Far Ultraviolet (FUV) and Global Ultraviolet Imager (GUVI) instruments on board the IMAGE and TIMED satellites (Sagawa et al, 2005; Henderson et al, 2005)
Four longitudinal peaks in 135.6-nm brightness observed on both sides of the geomagnetic equator were found to be in near coincidence with four tidal maxima in atmospheric temperature at 115 km altitude predicted from the Global Scale Wave Model (GSWM) (Immel et al, 2006)
The French microsatellite DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) was launched on 29 June 2004 onto circular synchronous orbit (SSO) with 98◦ inclination and 710–730 km initial height (Cussac et al, 2006)
Summary
Longitudinal variations in the equatorial ionospheric anomaly (EIA) have been intensively studied following recent observations of wavenumber-four structures in the nighttime OI135.6-nm emission along the geomagnetic equator by the Far Ultraviolet (FUV) and Global Ultraviolet Imager (GUVI) instruments on board the IMAGE and TIMED satellites (Sagawa et al, 2005; Henderson et al, 2005). It is suggested that these observations result from upward propagating atmospheric tides reaching E-region heights, and modifying the dynamo electric field to produce four longitudinal peaks in the equatorial fountain effect (Immel et al, 2006; Hagan et al, 2007; Pedatella et al, 2008) In support to this assumption, longitudinal variations in the noontime current density in the equatorial electrojet derived from CHAMP, Ørsted and SAC-C magnetometer data, show the same WN4 structures with apparent horizontal scale 10 000 km in the satellite frame of reference (England et al, 2006; Luhr et al, 2008).
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