Abstract
Abstract. We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in ExB drift, the asymmetry (Ia) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE ExB drift and Ia. The larger ExB drift (>20m/s) and smaller |Ia| (<0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller ExB drift and larger |Ia| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller ExB drifts are presented under the quiet (Kp<3) and disturbed (Kp≥3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in ExB drifts are mainly caused by the decrease in the eastward electric field.
Highlights
Equatorial spread F (ESF) takes its name from the rangedispersed ionogram at stations near the dip equator (Booker and Well, 1938)
The seasonal variation in ESF (Fig. 4) should be compared with that in the total bottomside spread F (BSSF) of Whalen (2002), who analyzed the ionograms at Huancayo (12◦ S, 75.3◦ W) during 1958
It is remarkable that this work is a first attempt to examine the seasonal variation in the equatorial ionization anomaly (EIA) asymmetry
Summary
Equatorial spread F (ESF) takes its name from the rangedispersed ionogram at stations near the dip equator (Booker and Well, 1938). It is known that there are some candidate mechanisms helping the ESF development These are (1) the pre-reversal enhancement (PRE) in upward E×B drift and associated uplifting of the F-layer (Fejer et al, 1999; Whalen, 2002), (2) a small or null transequatorial component of the thermospheric winds and associated symmetry in the equatorial ionization anomaly (EIA) (Maruyama and Matuura, 1984; Maruyama, 1988; Mendillo et al, 2000, 2001), (3) a simultaneous decay of the E region conductivity at both ends of the field line (Tsunoda, 1985; Stephan et al, 2002), and (4) a sharp gradient at the bottomside of the F-layer (Kelley, 1989). The minimum virtual height of the F-layer (h F) and the latitudinal distribution of the vertical total electron content (VTEC) are used to derive the PRE E×B drift and the asymmetry of EIA, respectively We utilize these data to study the dependence of ESF on the PRE E×B drift velocity, the asymmetry of EIA, and the magnetic activity (Kp)
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