Observations And Modeling Studies Of Solar Wind-Magnetosphere-Ionospheric Electric Field Penetration And Shielding

The solar wind-magnetosphere- ionosphere electric field penetration and shielding process has long been one of the most important topics. However, the penetration and shielding phenomenon is transient and of global scale, thus it is difficult to observe and simulate the process, which leads to dispute on some key questions as follows.1. The mechanism of solar wind electric field penetrating to magnetosphere;2. The time scale of penetration process;3. The shielding electric field development;4. How much the solar wind electric field can penetrate to low latitude region, i.e., penetration efficiency;5. The local time dependence of electric field penetration.To study these topic, two methods, observational studies and numerical simulations, are employed in this dissertation. The most results are novel or unique, which are expected to extend our knowledges of solar wind-magnetosphere-ionospere coupling processes. The main works are listed as follows:(1) Unusually long-lasting multiple electric field penetration event during November 11-16, 2003. The striking feature of this event is that the equatorial electric field exhibited impulsive perturbations in association with north-south oscillations of IMF Bz during 125 hours. My analysis results suggested that these perturbations are caused by the penetration of the solar wind electric field to equatorial ionosphere through magnetic reconnection on the dayside magnetosphere. Chapter 2 will present the observations and possible theoretical explanations.(2) The magnetospheric reconfiguration during an overshielding event on September 15, 2005. Chapter 3 reports a detailed analysis of an event in which the solar wind dynamic pressure underwent a sharp reduction, which caused magnetic changes in the ring current and plasma sheet as well as overshielding-type ionospheric electric fields in the dayside equatorial region. The event is analyzed in terms of a combination of observational data that is unique, as far as I know, and is interpreted in light of existing theory.These observations are important to question 1-4, however, the present observational instruments are not helpful to question 5, and thus the numerical simulations are required.(3) Based on an ionospheric dynamo model, I have modified its high latitude boundary condition, and involved the solar wind-magnetospheric dynamo effect. The new model is employed to study the global features of electric field penetration and shielding.(3a) The effect of geomagnetic field configuration on ionospheric electric field distribution. With same neutral wind model and conductance model, three geomagnetic models, centered dipole, tilted dipole and international geomagnetic reference field (IGRF), are respectively employed to calculate the ionospheric electric field. Chapter 5 will show how the geomagnetic field configuration affects ionospheric electric field distribution.(3b) The local time dependence of penetration efficiency. Chapter 6 suggests that:(1) Without considering transpolar potential saturation, equatorial electric field increment linearly responds to IEF increment, and neutral wind do not affect the penetration efficiency; (2) Assuming constant magnetic reconnection length L=2.6 Re, the local time dependence of penetration efficiency can be described as follows: it is about 10% between 9 and 23LT, however, increase rapidly from 2% up to 30% and decrease quickly to 2% between 0 and 7LT. All above characteristics are basically consistent with observations.(4) Because the magnetic reconnection on the dayside magnetosphere is the most important coupling mechanism of solar wind electric field to magnetosphere-ionosphere system, this dissertation also includes a case study of multiple flux transfer event on the dayside magnetosphere. Using Double Star TC1 observations, three methods, Maximum/Minimum Variance Analysis (MVA), deHoffmann-Teller (dHT) and Grad-Shafranov Reconstrution, are applied to this event. The results will be presented in chapter 7.In summary, the works shown in this dissertation, including case studies and numerical simulation, are expected to extend the present view to solar wind-equatorial ionospheric electric field penetration. Specially for the part of case studies, the magnetospheric reconfiguration in association with overshielding event verified a theory which had been proposed for many years, as well as the unusually long-lasting multiple penetration of solar wind electric field to equatorial ionospheric raise new challenge to the traditional theory.