Analysis Of Controlling Factors Leading To The Development Of R-T Instability In Low-latitude/Equatorial Ionosphere

Low latitude-equatorial ionosphere is an important part of the Earth space environment, where the electrodynamic processes are very complicated and inconstant. Moreover, it can cause the reflection, scatter, refraction and absorption of the received radio waves propagating through the ionosphere and strongly affect the satellite-ground and long-distance ground-ground radio wave communications. Specially, there exist irregularities with different scales in the low latitude-equatorial ionosphere, which can cause random temporal and spatial fluctuations of the amplitude and phase of the trans-ionosphere propagating radio waves, i.e., ionospheric scintillation. Therefore, the generation mechanisms of the irregularities have been widely concerned and have become one of the important topics in space weather research.After the extensive studies over the years, we have made great progress in the understanding on the generation mechanisms of ionospheric plasma bubble and plume type equatorial F layer irregularities. However, there are still many problems to be resolved. For example, various observations show that there exist spatial gradient of electric fields and thermospheric neutral winds in the F region, but whether and to what extent they could affect the growth of the R-T instability resulting in the generation of the equatorial irregularities has not been thoroughly investigated. Observation results also show that the day-to-day variations of the irregularities are frequent and significant, and that their occurrences are clearly dependent on solar activities, seasons and longitudes. More thorough studies are needed to explain such variations theoretically. Beside, the plasma bubbles and plumes are often simultaneously observed on the bottom and topside of the F region, which also need more theoretical explanations. In consideration of these problems, this thesis analyzes the linear growth of R-T instability in order to further understand the controlling factors of the generation and evolvement of the equatorial F layer irregularities, and to facilitate the understanding of the generation mechanisms of the irregularities.Based on the general conception, the basic structure and the morphological characteristics of ionosphere, this thesis summarizes the previous experimental and theoretical results as well as the unresolved problems of the equatorial F-region irregularities, including the basic mechanisms, the occurrence rate, the duration of ESF and various factors which could affect the occurrence of ESF. Under local, flux tube integration and dipole descriptions, the theoretical analysis of the factors controlling the generation and evolvement of the general R-T instability is carried out; the description of the linear growth rate of the general R-T instability considering the background conductivities, electric fields, neutral winds, and their spatial gradients is derived; then a qualitative analysis on how the background conductivities, electric fields, neutral winds, and their spatial gradients affect the general R-T instability processes is conducted; finally, we make a quantitative estimation on the degrees of the above mentioned effects by numerical calculations.The major work of this thesis can be concluded as follows:1. In equatorial plane, give the local linear growth rate of generalized R-T instability, and do statistic analysis about linear growth rate and ionospheric F peak height. The results show that there is a good correlation between the F peak height and the growth rate, the peak height could be one of factors controlling the R-T instability.2. Investigate the effects of electric couple of the E region and the F region and E regions finite conductivity and zonal gradient of flux tube integrated Pedersen conductivity during the development of R-T instability using flux tube integrate method.3. Give the three-dimensional linear growth rate description of generalized R-T instability, concluding background conductivity, electric fields and neutral winds and their gradients. Based on the growth rate, analyze the controlling effects of the gradients and the degree to the growth rate they could.The work of this thesis is the first attempt to consider the controlling effects and the degree of the spatial gradient of background conductivity, electric fields and neutral winds to the generalized R-T instability. The innovative results list as follows:1. Confirm that background conductivity gradients always inhibit the growth of the R-T instability.2. Find out that the spatial gradients of the electric field vector and the neutral wind vector can promote/restrain the linear growth of the generalized R-T instability remarkably. Whether it promotes or restrains the linear growth of the R-T instability depends on the polarity of the electric field and the neutral wind vectors and their gradients and the degrees of the effects depend on the gradient magnitude. Numerical results show that, for the typical background ionospheric condition, the spatial gradients of the electric field and neutral wind vectors can make the growth rate of the R-T instability change over 100%.3. Results of the qualitative analysis and numerical calculation both show that the linear growth rate could be significant positive above the F-region peak where the electron density gradient is parallel to the gravity due to the spatial gradients of the background electric fields and neutral winds,, implying that the general R-T instability could be initiated not only on the bottom-side the F region, but also above the F-region peak.