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Zusammenfassung:
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Interplanetary dust grains contain important information about the
Solar System. Analyzing these particles is an important aspect of the
heliosphere study. Dust impacts have been observed using radio and
wave instruments onboard the spacecraft since the 1980s. The interac-
tion between the impact-generated plasma cloud and antenna – space-
craft system elements generates the characteristic signal waveform.
The present work focuses on the detection and interpretation of the
dust generated signals from radio instruments onboard various space-
craft orbiting at 1 AU.
In the first part of the thesis, we aim to develop a model which links
the observed electric signals to the dust impact properties. We propose
a new model which takes into account the effect of impact - ionization
- charge collection and electrostatic-influence. Our model provides an
analytical expression for the pulse. It allows us to measure the amount
of total ion charge, the fraction of escaping charge, the rise timescale,
and the relaxation timescale. The proposed model is simple and conve-
nient for large data fitting. To validate the model, we use the Time Do-
main Sampler (TDS) subsystem of the STEREO/WAVES instrument,
which generates high-cadence time series of voltage pulses for each
monopole. Since the beginning of the STEREO mission in 2007, we
have collected all the dust events detected by S/WAVES/TDS simul-
taneously on all three monopoles at 1 AU. Our study confirms that
the rise time vastly exceeds the spacecraft’s short timescale of elec-
tron collection by the spacecraft. Aside from electron dynamics, we
also obtained interesting results regarding the cloud’s electron tem-
perature. The presented model provides an effective tool for analyzing
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dust waveforms, and is applicable for different space missions which
investigate the distribution of dust particles, e.g., Solar Orbiter and
Parker Solar Probe.
In the second part of the thesis, we focus on the interstellar dust
(ISD). Interplanetary and interstellar dust are the two main dust pop-
ulations at 1 AU. Our objective is to search for interstellar dust by
analyzing the data sets collected by STEREO and Wind, starting from
the beginning of the missions. Between 2007 and 2012, while being at
the solar minimum with a solar dipole pointing southward, all three
spacecraft recorded ISD flux at 1 AU. However, before and after that
period, the disappearance of the interstellar component was noticeable.
The observed change of the impact rate suggests that the flux of inter-
stellar dust at 1 AU varies with the solar cycle. Each time the magnetic
dipole field changes its polarity during the solar cycle, small interstel-
lar grains experience focusing or defocusing. Consequently, the dust
grains are systematically deflected either towards, or away from the
solar magnetic equator plane by the solar wind magnetic field which
thus affects the dust dynamics and the total interstellar dust flux in
the inner heliosphere. Our study provides the first quantitative de-
scription of the time variation of ISD flux at 1 AU. |