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Abstract:
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The quasi-thermal noise spectroscopy is an accurate method of determination of
density and temperature in space plasmas. When an electric antenna is immersed
into a plasma, it is able to measure electrostatic uctuations caused by the thermal
motion of plasma particles. These uctuations are detected as the power spectral
density at the antenna terminals, observing a spectrum at frequencies comparable
to the electron plasma frequency for both electrons and protons, since the proton
signal is strongly Doppler-shifted towards higher frequencies due to the solar wind
drift velocity. Beside inducing the uctuating electric eld, some of the electrons
are impacting the antenna surface, causing disturbances of the antenna electric potential.
The signal caused by this population is directly proportional to the ux of
plasma electrons impacting the antenna and is dominant if the antenna has a large
surface area. In this work, we use the orbit limited theory to calculate the incoming
particle ux for a non-thermal plasma described by velocity distribution function,
commonly measured in the solar wind. The increase in the particle collection by
cylindrical and spherical objects is quanti ed and presented as a function of the
surface electrostatic potential and the fraction of supra-thermal particles. Including
these results into the theory has turned out to be absolutely necessary for accurate
measurements of the plasma parameters whenever the shot noise is the dominant
component in the power spectrum. This is the case for STEREO because the impact
noise is overwhelming on this probe, due to the presence of short and thick
antennas. The comprehensive study of data on this mission is motivated by the fact
that the electron analyzers are malfunctioning since launch and no information on
thermal electrons is available. Results obtained are veri ed by comparing with the
results from Wind, showing a good match between the values measured by the two
spacecraft. Uncertainties of the measurements are determined by the uncertainties
of the instruments used and are estimated to be around 40%. The nal outcome of
this work will be establishing a database of the electron moments in both STEREO
A and B that will be covering the entire duration of the mission. In the second part
of the thesis, we use the kinetic approach to expand the theory of the quasi-thermal
noise to plasmas where electron-neutral collisions play a dominant role. This technique
is able to measure the electron density, temperature and the collision frequency
as independent parameters using the wide frequency range both below and above
the plasma frequency, if the ratio of the collisional to plasma frequency is not smaller
than 0.1. The results presented here have can be potentially applied in laboratory
plasmas and unmagnetized ionospheres, while at the ionosphere of Earth their use
is limited to low frequencies due to the presence of the magnetic field. |