Radar Probing of the Sun

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: This thesis is dedicated to the theory of solar radar experiments. The Sun exhibits a variety of interesting and complicated physical phenomena, examined mainly through analysis of its radiation. Active solar probing by radar provides an alternative possibility to study the Sun. This concept was tested originally in the 1960's by solar radar experiments at El Campo, Texas, but due to an insufficient level of technology at that time the experimental results were of a poor quality and thus difficult to interpret. Recently, the space weather program has stimulated interest in this topic. New experimental proposals require further development of the theory of solar radar experiments to meet the current knowledge about the Sun and the modern level of technology.Three important elements of solar radar experiments are addressed in this thesis: i) generation of wave turbulence and radiation in the solar corona, ii) propagation of the radar signal to the reflection point, and iii) reflection (scattering) of the incident radar signal from the Sun.It is believed that the radio emission of solar type II and III bursts occurs due to conversion of Langmuir waves, generated by electron beams, into electromagnetic radiation (plasma emission mechanism). The radar signal propagating through the emission source region can get scattered by the Langmuir turbulence and finally deliver the observer insights of the physics of this turbulence. Such process of scattering is considered in this thesis in the weak turbulence limit by means of the wave-kinetic theory. Scattering frequency shifts, scattering cross-sections, efficiency of scattering (the coefficient of absorption due to scattering), optical depths, and the spectra of the scattered signal are estimated.Type II solar radio bursts are known to be associated with the electron beams accelerated by interplanetary shocks. From their dynamic spectra the properties of the shocks and regions in the vicinity of the shock are usually inferred by assuming a plasma emission mechanism. In situ observations of the source region of type II burst, presented in this thesis, suggest that an additional emission mechanism may be present. This mechanism is related to energetic particles crossing the shock front, known in electrodynamics as transition radiation.Plasma density fluctuations are known to scatter radio waves and thus broadening their angular dispersion. In the thesis this process is studied in the solar wind and terrestrial electron and ion foreshocks on the basis of in situ observations of density fluctuations. It is shown that the angular broadening of the radar signal is negligible in this regions.The results of this thesis can be applied for the preparation of future solar radar experiments and interpretation of experimental data.

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