Role of equatorial ionization anomaly in the initiation of equatorial spread F

Abstract

A comparative study is made of the changes in the latitudinal structure of the F region electron density at fixed altitudes in the Indian equatorial region on days with and without postsunset onset of equatorial spread F, using (N‐h) profile data of Ahmedabad (dip latitude 18.6°N), Waltair (dip latitude 10.6°N), and Kodaikanal (dip latitude 1.5°N). It is found that on spread F days the ratio of the electron density in the altitude region 270 ‐ 300 km between Ahmedabad and Waltair showed a sudden enhancement starting at 1700 LT by a factor of 8 to 30 (at 1900 LT) from a near‐constant value of about 2 during the daytime. No such enhancement of the density ratio was evidenced on days without spread F. The enhancement of the electron density ratio prior to the onset of spread F is interpreted as an intensification of the northern crest of the equatorial anomaly, with the ionization in the bottomside F region as far north as 9° from the dip equator participating in the crest intensification process. The rapid intensification of the ionization anomaly engenders a similar augmentation of the neutral anomaly around sunset hours. This in turn creates a localized cell of altitude dependent equatorward neutral wind that aids further intensification of the crests of both the anomalies and augmentation of ionization in the magnetic field line tube passing through the height of maximum plasma density of the F2 layer over the equator. The net result of these coupling processes is a weakening of the ambient transequatorial wind (particularly during northern winter months) and reduction of the north‐south asymmetry of the ionization anomaly crests, a condition favorable for the onset of spread F (Maruyama and Matuura, 1984). The present study thus indicates a significant role of the anomaly intensification at altitudes much below the F region peak and associated neutral dynamics in the initiation of spread F.

Characteristics of equatorial ionization anomaly (EIA) in relation to transionospheric satellite links around the northern crest in the Indian longitude sector

“…Similarly from Fig. 2b There are several studies relating the equatorward TEC gradient with the occurrence of post-sunset scintillations (Raghavarao et al, 1988;Ray et al, 2006). This is perhaps for the first time that efforts are being made to correlate occurrence of post-sunset scintillations with the poleward TEC gradient in the Indian longitude sector.…”

“…The impact of a developed equatorial ionization anomaly (EIA) in the afternoon hours on the subsequent development of equatorial ionization density irregularities during postsunset hours was suggested by Raghavarao et al (1988) and validated by several workers (Valladares et al, 2001(Valladares et al, , 2004Ray et al, 2006;Rama Rao et al, 2006;Ram et al, 2006;Nava et al, 2007). However the majority of the validation exercise was performed at locations existing between the magnetic equator and the northern and southern crests of the EIA.…”

On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years

“…Maruyamma and Matuura (1984) suggested a mechanism wherein strong meridional winds create a N-S asymmetry in the EIA, thus producing substantial changes in E-region integrated conductivities, which, in turn, control the post-sunset F-region height rise and the triggering of ESF. Raghavarao et al (1988) experimentally showed the intensification of the crest-to-trough ratio of the EIA well in advance of the onset of ESF linking the EIA and the ESF. Following this, Sridharan et al (1994) demonstrated the existence of a precursor in the OI 630 nm day glow that represented the EIA strength which facilitated the prediction of ESF at least 2 h prior to its actual occurrence.…”

Observational evidence for the plausible linkage of Equatorial Electrojet (EEJ) electric field variations with the post sunset F-region electrodynamics

“…By a careful analysis of ground based ionospheric data, a close linkage between EIA and ESF was shown by Raghavarao et al (1988), Alex et al (1989) and Jayachandran et al (1997). The basic mechanism responsible for the generation of ESF is the Collisional Rayleigh-Taylor (CRT) plasma instability.…”

“…It is the background electrodynamic and neutral dynamic conditions that basically dictate the occurrence/non-occurrence of ESF on a particular day. Earlier studies have tried to make an attempt to forecast the occurrence of ESF (Raghavarao et al, 1988;Sridharan et al, 1994;Mendillo et al, 2001;Thampi et al, 2006, and the references therein). These studies imply that the background ionospheric and thermospheric conditions during the daytime play a very crucial role in the generation of ESF.…”