Description
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Alfvén/ion-cyclotron waves are important normal modes in a plasma. Their existence can explain several observed features in space plasmas such as temperature anisotropies and preferred heating of certain ion species in the solar corona and the solar wind. The properties of this wave mode and different aspects of the spectral transfer of wave energy in wavenumber space are analyzed. The way Alfvén/ion-cyclotron waves shape a particle distribution function is treated with kinetic methods in an analytical model, and an additional observational effect in particle measurements is found. The nature of the important Alfvén/ion-cyclotron mode is discussed in a multi-fluid analysis. A derived system of differential equations is used to analyze the dispersion properties of weakly-compressive high-frequency waves, which are superposed on a low-frequency Alfvén/ion-cyclotron wave. In a certain parameter range, a wave instability is found, which can be interpreted as a linear spectral transfer process due to the nonuniform background of the low-frequency wave. As a particular example for a nonlinear spectral transfer process, the parametric decay is treated in a two-dimensional numerical study using a hybrid code. The same code is then used to analyze the spectral transfer from an initially broad turbulent spectrum on low-wavenumber MHD scales into the dispersive kinetic regime, where dissipation of certain wave modes is found. Also here compressive fluctuations grow during the cascade and make a significant contribution to the turbulent spectrum on small scales. The thesis is completed with a general introduction to waves and turbulence in space plasmas and a basic presentation of the applied mathematical methods.
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These properties characterize a plasma at first glance [...]. More than 99% of the visible matter in the universe is in the plasma state. Also, most mass of the solar system is in this state because the Sun, as the dominating mass, is a plasma ball in itself. Its outer atmosphere above the photospheric surface is called the solar corona. It can be seen with the naked eye during a solar eclipse as an extended and structured glow around the occulted Sun. Since the 1940s, it has been already known from spectroscopic analyses [...] that the temperature of the corona exceeds several million degrees Kelvin, whereas the solar photosphere, the main source of the solar light emission, has a temperature of only 5700 K. The reason for this increase in the plasma temperature with height, coinciding with a strong decrease in density, is still an open question and is known as the coronal heating problem.
