methodology should aim to:
• optimise the design and configuration of scientific spacecrafts and their embedded plasma instruments
• enhance the in-flight data analysis of plasma measurements by providing quantitative corrections that will better represent the undisturbed environment.
1.7 Plan and sum up
A simple summary of these spacecraft/plasma interactions overview consists in saying that a spacecraft disturbs its near environment which in return alters the satellite. In this way, the near spacecraft environment results in a combination of a natural ambient environment and a spacecraft generated environment. The first depends on the vehicle position on its orbit and the Solar activity at a given time, the second depends on the satellite structure and on the first environment, through highly synergistic interactions that cannot be considered as linear. This final spacecraft environment (which differs from the natural ambient environment) is the one measured by the mission instruments, and should thus be considered as biased, as for its consecutive measurements.
In Chapter2, we will first describe the Solar wind (section 2), flowing over all the scientific satellites simulated in this work. The chapter will start with a reminder of a few concepts of plasma physics: defining the particle distribution functions, plasma scales and the magnetic field effect on plasmas, before presenting the Solar wind characteristics, with an introduction of its interaction with a well known space travelling object: the Earth. Some observations and examples of Solar wind measurements will finally be illustrated. Note that the Table A.1 in section AppendixA.1displays the various physical and geophysical constants used in this work. Chapter 3 will detail the various plasma interactions with space vehicles, starting with a review of what are the equilibrium electrostatic potentials and consecutive currents balance. The main natural and spacecraft generated particles interactions with a space probe will also be explained, with a presentation of the several analytical methods existing to model those phe- nomena. Other effects of a spacecraft presence in a plasma will be introduced, such as wakes and potential barriers. The idea is to enumerate the various interactions that can disturb space plasma measurements, and thus present a non exhaustive list of the main issues to be consid- ered by numerical modellers, satellite and plasma instrument designers and plasma physicists processing low energy plasma experiment outputs. This is why this section continues with a description of the measurement biases intrinsically linked to the particle detectors themselves. It finally ends with a brief presentation of other perturbing issues related to spacecraft technical design (particle emitters, Solar arrays structures, outgassing, etc).
From this state of knowledge the Spacecraft Plasma Interaction System (SPIS), the numerical tool used for all computer simulations performed in this work will be presented (Chapter4) with a tutorial on the good numerical practices to properly and efficiently model spacecraft/plasma interactions, from the very beginning of the study to the analysis of the simulation results. The presentation will directly jump to the first applications of those methods, by presenting two parametric studies, whose corresponding published articles are joined in the appendixA.4 and A.5. Those papers focus on particular issues linked to the spacecraft plasma interactions: the photoelectron and secondary electron sheaths, the associated potential barrier and its effect on
a simple conducting satellite equilibrium currents and potential. The first paper deals with a simulation set at the Solar Probe Plus satellite at its closest perihelion (at 0.044 AU from the Sun) with a sensitivity study of several physical and numerical parameters on final steady probe potential and currents. The second extends the simplified Solar Probe Plus case to ten heliocentric distances, from 0.044 to 1 AU, in order to observe the evolution of the sheaths, potential barriers and satellite equilibrium potential and currents. The chapter ends with a discussion of the effects of the simulated phenomena on low energy plasma measurements.
Chapter5 takes on the numerical particle instruments concept and simulations. First defin- ing the scientist’s needs, it presents the principle of numerical measurement and the methods. The next sections show a simulation campaign of a single particle detector instrument immersed in a plasma, with step-by-step increments in the complexity of the environment aiming at get- ting closer to a realistic simulation of satellite/plasma interactions. At each step the simulated plasma measurements are presented and compared with theoretical results. The differences between analytical theory and numerical results are evaluated and discussed.
Chapter6enters into more sophisticated and completed SPIS simulations, with detailed ge- ometries of two spacecraft, Solar Orbiter and Cluster, immersed in their natural environment, coupled with their own particle analysers, EAS and LEEA (section 1.2.1). After the presen- tation of the simulations inputs (geometry, environments, detectors), simulation results for the satellites and the near environment will be displayed and discussed, before presenting the nu- merical particle measurement outputs and their analysis. The chapter ends with a discussion of the scientific applications and engineering considerations.
Finally, Chapter 7 presents a conclusion of this thesis, and some perspective on possible future work.
Chapter 2
The Solar wind plasma
Contents2.1 Plasma physics . . . 19
2.1.1 Distribution functions . . . 19
2.1.2 Plasma scales: Debye length and plasma frequency . . . 22
2.1.3 Magnetic field. . . 23