Quiet and disturbed time dynamics of low latitude F-region

Abstract

Preface : The ionosphere is partially ionized part of the atmosphere that extends from 50km to 800 kms. The dynamics of the equatorial ionosphere is different as compared to mid/high latitude of ionosphere, due to the horizontal magnetic field present at the equator. This horizontal magnetic field along with east/ west electric field creates the peculiar phenomena. Equatorial spread - F (ESF) irregularities is one of the phenomena. These ESF irregularities (spans scale size of few cms to hundreds of kilometer) are electron density irregularities that are generated in the equatorial F-region in the post sunset hours, through the nonlinear evolution of Rayleigh-Taylor plasma instability. This is a nighttime equatorial F-region phenomenon. The study of ESF irregularities is important as its presence in the ionosphere can severely affect trans-ionospheric radio communications. The fast moving small scale ESF irregularities causes more degradation to the incoming radio signal. Thus, the knowledge about spatial scales and drift of ESF irregularities are important. Equatorial plasma bubbles (EPBs) and dynamics of associated ESF irregularities of different spatial scales are being studied using different techniques for the last few decades. But its study still forms an important element of ionospheric research due to the uncertainties involved in the day-to day prediction of occurrence of these EPBs. For quiet and disturbed days, we are not only interested in the occurrence EPBs but we are also keen to know the level of degradation caused to incoming radio signal by these freshly generated EPBs, which is closely related to the strength and spatial scales of the ESF irregularities. The evolution and structuring of EPBs can be considerably different on quiet and disturbed days. However, we have little knowledge about the structuring and evolution of freshly generated ESF irregularities. When we think of freshly generated EPBs one should have some method or technique to confirm that the observed signatures are associated with ESF irregularities formed locally. In present work we have used long-term observations of amplitude scintillation on 251 MHz recorded by spaced receiver at Indian dip equatorial station Tirunelveli. In addition we used ionosonde data from close by station Trivandrum and some ionospheric models. As we are using 251 MHz signal we get information about intermediate scale (100m - few kms) irregularities. By using spaced receiver analysis technique, we get strength of scintillations (S4), drift speed of ground scintillation pattern (V0), the random velocity of the irregularities of ground scintillation pattern (VC) and maximum cross - correlation between intensity variations recorded by two receivers (CI ). By using the CI parameter one can identify the freshly generated (or) drifted ESF irregularities. Here, our focus mainly on the freshly generated ESF (F-ESF) irregularities during quiet and disturbed days. We have investigated seasonal and solar flux dependence of occurrence of freshly generated ESF irregularities during both quiet and disturbed days in early and later phases of their evolution and the degradation caused by these irregularities to the incoming radio signal by taking S4 index as a proxy. F-ESF associated with geomagnetic activity causes more degradation to incoming radio signal. In order to understand moderate - strong scintillations associated with F-ESF during geomagnetic activity, we have used the theoretical model given by the Engavale & Bhattacharyya, 2005. This theoretical model indicates, ESF irregularities placed at higher altitude produce higher S4 when power spectral index and density fluctuations are high. In addition, the duration of active phase of freshly generated ESF irregularities are estimated and its variation with the solar flux and the geomagnetic activity have been studied. And also we have checked the effect of the magnetic activity on active phase duration of F-ESF. As a case study, the geomagnetic storm of 17 March 2015 and associated effects observed at low latitude F-region are examined in detail. We also report the difference in the structuring of ESF irregularities due to presence of earthquake linked electric fields in the equatorial F-region.