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Non-Adiabatic Ion Acceleration caused by Electromagnetic Waves (Cluster and Double Star observations) and Geomagnetic Field Response to the Variations of the Solar Wind Dynamic Pressure

Abstract : This doctoral dissertation includes two main topics: one is about the nonadiabatic acceleration of ions in the near-Earth plasma sheet, the other is about the geomagnetic pulsations driven by the decrease of solar wind dynamic pressure. The nonadiabatic acceleration of plasma sheet ions is important to understand the formation of ring current and substorm energetic injections. In the first part, we present case studies of nonadiabatic acceleration of plasma sheet ions observed by Cluster and Double Star TC-1 in the near-Earth magnetotail (e.g. at (X, Y, Z)=(-7.7, 4.6, 3.0) RE in the 30 October 2006 event), much closer to the Earth than previously reported. We find that the ion energy flux variations, which are characterized by a decrease over 10 eV-20 keV and an increase over 28-70 keV, are caused by the ion nonadiabatic acceleration closely associated with strong electromagnetic field fluctuations around the H+ gyrofrequency. We also find that the ions after nonadiabatic acceleration have 'bunched gyrophases', which is the first report in the plasma sheet since the 'gyrophase bunching effect' was observed in the solar wind in 1980s. We interpret the ion energy flux variations and the bunched gyrophases by using a nadiabatic model. The analytic results and simulated spectrums are in good agreement with the observations. This analysis suggests that nonadiabatic acceleration associated with magnetic field fluctuations is an effective mechanism for ion energization in the near-Earth plasma sheet. The presented energy flux structures can be used as a proxy to identify this dynamic process. In the second part, we investigate the response of geomagnetic field to an impulse of solar wind dynamic pressure (Psw), which hits the magnetosphere on 24 August 2005. Using the high resolution geomagnetic field data from 15 ground stations and the data from Geotail, TC-1 and TC-2, we studied the geomagnetic pulsations at auroral latitudes driven by the sharp decrease of Psw in the trailing edge of the impulse. The results show that the sharp decrease of Psw can excite a global pulsation in the frequency range 4.3-11.6 mHz. The reversal of polarizations between two auroral latitude stations, larger Power Spectral Density (PSD) close to resonant latitude and increasing frequency with decreasing latitude indicate that the pulsations are associated with Field Line Resonance (FLR). The fundamental resonant frequency (the peak frequency of PSD between 4.3-5.8 mHz) is magnetic local time dependent and largest around magnetic local noon. This feature is due to the fact that the size of magnetospheric cavity is local time dependent and smallest at noon. A second harmonic wave at about 10 mHz is also observed, which is strongest in the daytime sector, and is heavily attenuated while moving to night side. The comparison between the PSDs of the pulsations driven by sharp increase and sharp decrease of Psw shows that the frequency of pulsations is inversely proportional to the size of the magnetopause. Since the FLR is excited by compressional cavity/waveguide waves, these results indicate that the resonant frequency in the magnetospheric cavity/waveguide is decided not only by solar wind parameters but also by magnetic local time of observation point.
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Submitted on : Friday, July 12, 2013 - 2:28:21 PM
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Zhiqiang Wang. Non-Adiabatic Ion Acceleration caused by Electromagnetic Waves (Cluster and Double Star observations) and Geomagnetic Field Response to the Variations of the Solar Wind Dynamic Pressure. Earth and Planetary Astrophysics [astro-ph.EP]. Université Paul Sabatier - Toulouse III, 2013. English. ⟨tel-00843936⟩

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