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Abstract:
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Interplanetary electron beams, produced by solar flares, are unstable in the
solar wind and generate Langmuir waves at the local plasma frequency, fp. These waves
are then converted into the so-called type III radio bursts which are freely propagating
electromagnetic emissions at fp or its harmonic. The type IIIs are therefore observed as
drifting emissions from high to low frequencies, in the kilometric wavelengths range. Since
the first theoretical explanation by Ginzburg and Zhelezniakov (1958a), several refined
models have attempted to describe in details the physical processes at the origin of type
III bursts. The mechanisms of “Bump-on-tail” instabilities, Langmuir waves generations,
conversion of these Langmuir waves into radio emissions throughout nonlinear wave-wave
interactions etc, have been studied in detail. Of particular interest from the observational
point of view are the so called in situ type III bursts for which the electron beam, at
the origin of the emission and traveling along open interplanetary magnetic field lines, is
observed directly in situ by a spacecraft, together with the local Langmuir waves and the
resulting radio emissions. Until now only a few of these in situ type IIIs have been reported
in the literature.
The first research study performed in this thesis was to examine the first 16 years of
radio, waves and particles data recorded by the Wind spacecraft in the Solar Wind and
to look for in situ type IIIs. Applying rigorous and careful criteria, this examination has
yielded to a data set of 36 high-quality events. With such a numerous data set, which is
statistically representative of the studied phenomenon, it is now possible to constrain obser-
vationally and with a better confidence the type III generation models. After having built
our statistical dataset, we have studied, for each of the events, the precise shapes of the
Langmuir wave power distributions, observed in the spectral domain. We have fitted these
observed distributions by a Pearson’s system of probability distributions and have shown
that the probability distributions of the logarithm of the Langmuir waves power spectral
density belong to three “main” types of Pearson’s probability distributions: type I, type
IV and type VI. In addition we have modeled the effects of the instrumental integration
time of the Wind radio receivers on the observed Langmuir wave power distributions. By
combining our observations with our models we have shown that it was not possible to con-
clude definitively, that the distribution of the Langmuir waves energy in the real temporal
domain is lognormal, as it is predicted in some theories as the Stochastic Growth Theory
by Robinson (1992).
In the last part of the thesis, we have shown how our high-quality data set of 36 in situ
type III events can be used for further studies that could allow to constrain the theoretical
models even better. For instance we have investigated the correlation between the Langmuir
waves power and the energy of impulsive electron or with the power of the radio emissions
themselves. |